Assessing causality in the association between child adiposity and physical activity levels: a Mendelian randomization analysis

Four-year longitudinal associations of physical activity, waist circumference, and blood pressure in UK adolescents

BACKGROUND

This study assessed the specific influence of physical activity (PA) and waist circumference (WC) on the 4-year growth trajectory of blood pressure in UK high-school students.

METHODS

Four-year longitudinal monitoring of 1501 adolescents was conducted as part of the EoEHHS. Measurements were taken in Grades (G)7, 9, and 11.

RESULTS

Systolic (SBP) and diastolic blood pressure (DBP) increased over the 4-year period (SBP G7 114.6 ± 8.9 mmHg, G9 118.1 ± 9.7 mmHg, G11 122.8 ± 7.8 mmHg; DBP G7 66.7 ± 6.6 mmHg, G9 68.0 ± 6.4 mmHg, G11 70.0 ± 5.2 mmHg). Baseline WC predicted baseline and growth in SBP, but the strongest contribution to SBP came from changes in WC (β = 0.084, p = 0.002). Baseline PAQ-A score (β = -0.822, p = 0.020) and changes in PAQ-A score (β = -0.650, p = 0.019) were associated with smaller increases in DBP over the 4-year measurement period.

CONCLUSIONS

Baseline and change in WC predicted the growth trajectory of SBP, while baseline and change in PA predicted the growth trajectory of DBP. PA and WC have a prognostic value in predicting changes in blood pressure in adolescents. Increasing PA during adolescence could slow the rise in DBP over time. This is meaningful for future hypertension and CVD risk reduction into adulthood.

IMPACT

Hypertension in adolescents is a growing health problem that is often overlooked. Baseline and changes in waist circumference over a 4-year period predicted development of systolic blood pressure, while baseline and changes in physical activity predicted development of diastolic blood pressure. Physical activity and waist circumference have a prognostic value in predicting changes in blood pressure in adolescents and could be valuable in planning programmes to prevent hypertension in similar communities and reduce the risk of future adult hypertension.

Exploring the causal relationship between inflammatory cytokines and immunoinflammatory dermatoses: a Mendelian randomization study

Objectives

Previous studies have shown that the onset and progression of several immunoinflammatory dermatoses are closely related to specific immune-inflammatory responses. To further assess the causal relationship between 41 inflammatory cytokines and immunoinflammatory dermatoses, we used a Mendelian randomization method.

Methods

Mendelian two-sample randomization utilized inflammatory cytokines from a GWAS abstract containing 8,293 healthy participants as well as psoriasis (4,510 cases and 212,242 controls), atopic dermatitis (7,024 cases and 198,740 controls), and vitiligo (131 cases and 207,482 controls). The causal relationship between exposure and outcome was explored primarily using inverse variance weighting. In addition, multiple sensitivity analyses, including MR-Egger, weighted median, simple model, weighted model, and MR-PRESSO, were simultaneously applied to enhance the final results.

Results

The results showed that in clinical practice, IL-4 and IL-1RA were suggestive indicators of atopic dermatitis risk (OR = 0.878, 95% CI = 0.78-0.99, p = 0.036; OR = 0.902, 95% CI = 0.82-1.00, p = 0.045). SCGF-b was a suggestive indicator of psoriasis risk (OR = 1.095, 95% CI = 1.01-1.18, p = 0.023). IL-4 is a suggestive indicator of vitiligo risk (OR = 2.948, 95% CI = 1.28-6.79, p = 0.011).

Conclusion

Our findings suggest that circulating inflammatory cytokines may play a crucial role in the pathogenesis of chronic skin inflammation. IL-4 and IL-1RA may have inhibitory roles in the risk of developing atopic dermatitis, while SCGF-b may have a promoting role in the risk of developing psoriasis. Furthermore, IL-4 may contribute to the risk of developing vitiligo. These results provide insights into further understanding the mechanisms of chronic skin inflammation and offer new targets and strategies for the prevention and treatment of related diseases.

Mendelian randomization for nephrologists

Confounding is a major limitation of observational studies. Mendelian randomization (MR) is a powerful study design that uses genetic variants as instrumental variables to enable examination of the causal effect of an exposure on an outcome in observational data. With the emergence of large-scale genome-wide association studies in nephrology over the past decade, MR has become a popular method to establish causal inferences. However, MR is a complex and challenging methodology that requires careful consideration to ensure robust results. This review article aims to summarize the basic concepts of MR, its application and relevance in nephrology, and the methodological challenges and limitations as well as discuss the current guidelines for design and reporting. With reference to a clinically relevant example of examining the causal relationship between the estimated glomerular filtration rate and cancer, this review outlines the key steps to conducting an MR study, including the key considerations and potential pitfalls at each step. These include defining the clinical question, selecting the data sources, identifying and refining appropriate genetic variants by considering linkage disequilibrium and associations with potential confounders, harmonization of variants across data sets, validation of the genetic instrument by assessing its strength, estimation of the causal effects, confirming the validity of the findings, and interpreting and reporting results.

Distilling causality between physical activity traits and obesity via Mendelian randomization

BACKGROUND

Whether obesity is a cause or consequence of low physical activity levels and more sedentary time has not yet been fully elucidated. Better instrumental variables and a more thorough consideration of potential confounding variables that may influence the causal inference between physical activity and obesity are needed.

METHODS

Leveraging results from our recent genome-wide association study for leisure time moderate-to-vigorous intensity (MV) physical activity and screen time, we here disentangle the causal relationships between physical activity, sedentary behavior, education-defined by years of schooling-and body mass index (BMI), using multiple univariable and multivariable Mendelian Randomization (MR) approaches.

RESULTS

Univariable MR analyses suggest bidirectional causal effects of physical activity and sedentary behavior with BMI. However, multivariable MR analyses that take years of schooling into account suggest that more MV physical activity causes a lower BMI, and a higher BMI causes more screen time, but not vice versa. In addition, more years of schooling causes higher levels of MV physical activity, less screen time, and lower BMI.

CONCLUSIONS

In conclusion, our results highlight the beneficial effect of education on improved health and suggest that a more physically active lifestyle leads to lower BMI, while sedentary behavior is a consequence of higher BMI.

Health behaviours associated with healthy body composition among Aboriginal adolescents in Australia in the 'Next Generation: Youth Well-being study'

This study described the distribution of healthy body composition among Aboriginal adolescents in Australia aged 10-24 years and examined associations with health behaviours and self-rated health. Data were cross-sectional from the 'Next Generation: Youth Well-being study' baseline (N = 1294). We used robust Poisson regression to quantify associations of self-reported health behaviours (physical activity, screen time, sleep, consumption of vegetables, fruit, soft drinks and fast food, and tobacco smoking and alcohol) and self-rated health to healthy body mass index (BMI) and waist/height ratio (WHtR). Overall, 48% of participants had healthy BMI and 64% healthy WHtR, with healthy body composition more common among younger adolescents. Higher physical activity was associated with healthy body composition (5-7 days last week vs none; adjusted prevalence ratio (aPR) healthy BMI 1.31 [95% CI 1.05-1.64], and healthy WHtR 1.30 [1.10-1.54]), as was recommended sleep duration (vs not; aPR healthy BMI 1.56 [1.19-2.05], and healthy WHtR 1.37 [1.13-1.67]). There was a trend for higher proportion of healthy body composition with more frequent fast food consumption. Healthy body composition was also associated with higher self-rated health ('very good/excellent' vs 'poor/fair'; aPR healthy BMI 1.87 [1.45-2.42], and healthy WHtR 1.71 [1.40-2.10]). Culturally appropriate community health interventions with a focus on physical activity and sleep may hold promise for improving body composition among Aboriginal adolescents.

The causal association between systemic inflammatory regulators and primary ovarian insufficiency: a bidirectional mendelian randomization study

BACKGROUND

Recent studies have suggested a potential link between systemic inflammatory regulators and primary ovarian insufficiency (POI); however, a causal relationship between them remains unclear. In this study, we explored the causal link between systemic inflammatory regulators and POI risk using a bidirectional, two-sample Mendelian randomization (MR) strategy.

RESULTS

This approach utilized the most extensive genome-wide association study involving 41 systemic inflammatory regulators in a sample of 8,293 Finnish individuals and POI data from the FinnGen consortium (254 cases vs. 118,228 controls). The inverse variance weighting approach served as a primary MR method, and four additional MR techniques (Maximum Likelihood, MR-Egger, Weighted Median, and constrained maximum likelihood and model averaging Bayesian information criterion ) were applied to support and validate results. Cochran's Q statistics were used to assess the heterogeneity of instrumental variables, whereas the MR-Egger and MR Pleiotropy Residual Sum and Outlier tests detected horizontal pleiotropy. The MR Steiger test evaluated the strength of a causal association. Our findings suggest that lower levels of vascular endothelial growth factor (odds ratio [OR] = 0.73, 95% confidence interval [CI]: 0.54-0.99, P = 0.046) and interleukin-10 (OR = 0.54, 95% CI: 0.33-0.85, P = 0.021) are associated with an increased risk of POI. Reverse MR analysis revealed no significant effect of POI on the expression of these 41 systemic inflammatory regulators. No notable heterogeneity or horizontal pleiotropy was observed in the instrumental variables.

CONCLUSIONS

This study revealed a causal association between 41 systemic inflammatory regulators and POI, demonstrating that decreased levels of VEGF and IL-10 are linked to an elevated risk of POI. Further investigations are necessary to assess the potential of these biomarkers as early predictors, preventive strategies, and therapeutic targets for POI.

Causal effects of systemic inflammatory regulators on chronic kidney diseases and renal function: a bidirectional Mendelian randomization study

Background

While targeted systemic inflammatory modulators show promise in preventing chronic kidney disease (CKD) progression, the causal link between specific inflammatory factors and CKD remains uncertain.

Methods

Using a genome-wide association study of 41 serum cytokines from 8,293 Finnish individuals, we conducted a bidirectional two-sample Mendelian randomization (MR) analysis. In addition, we genetically predicted causal associations between inflammatory factors and 5 phenotypes, including CKD, estimated glomerular filtration rate (eGFR), dialysis, rapid progression of CKD, and rapid decline in eGFR. Inverse variance weighting (IVW) served as the primary MR method, while MR-Egger, weighted median, and MR-pleiotropy residual sum and outlier (MR-PRESSO) were utilized for sensitivity analysis. Cochrane's Q test for heterogeneity. Leave-one-out method ensured stability of MR results, and Bonferroni correction assessed causal relationship strength.

Results

Seventeen cytokines were associated with diverse renal outcomes. Among them, after Bonferroni correction test, higher tumor necrosis factor alpha levels were associated with a rapid decrease in eGFR (OR = 1.064, 95% CI 1.028 - 1.103, P = 0.001), higher interleukin-4 levels were associated with an increase in eGFR (β = 0.003, 95% CI 0.001 - 0.005, P = 0.002), and higher growth regulated oncogene alpha (GROα) levels were associated with an increased risk of CKD (OR=1.035, 95% CI 1.012 - 1.058, P = 0.003). In contrast, genetic susceptibility to CKD was associated with an increase in GROa, and a decrease in eGFR may lead to an increase in stem cell factor. We did not find the presence of horizontal pleiotropy during the analysis.

Conclusion

We discovered causally related inflammatory factors that contribute to the initiation and progression of CKD at the genetic prediction level.

Short and medium-term effects of the COVID-19 lockdowns on child and parent accelerometer-measured physical activity and sedentary time: a natural experiment

BACKGROUND

The COVID-19 pandemic has resulted in marked impacts on children's physical activity, with large reductions in moderate-to-vigorous physical activity (MVPA) reported during lockdowns. Previous evidence showed children's activity levels were lower and sedentary time higher immediately post-COVID lockdown, while there was little change in parental physical activity. We need to know if these patterns persist.

METHODS

Active-6 is a natural experiment using repeated cross-sectional data conducted in two waves. Accelerometer data were collected on 393 children aged 10-11 and their parents from 23 schools in Wave 1 (June 2021-December 2021), and 436 children and parents from 27 schools in Wave 2 (January 2022-July 2022). These were compared to a pre-COVID-19 comparator group (March 2017-May 2018) of 1,296 children and parents in the same schools. Mean minutes of accelerometer-measured MVPA and sedentary time were derived for week- and weekend-days and compared across waves via linear multilevel models. We also analysed the date of data collection as a time series, to explore temporal patterns via generalised additive mixed models.

RESULTS

There was no difference in children's mean MVPA in Wave 2 (weekdays: -2.3 min; 95% CI: -5.9, 1.3 and weekends: 0.6 min; 95% CI: -3.5, 4.6) when compared to the pre-COVID-19 data. Sedentary time remained higher than pre-pandemic by 13.2 min (95% CI:5.3, 21.1) on weekdays. Differences compared to pre-COVID-19 changed over time, with children's MVPA decreasing over winter, coinciding with COVID-19 outbreaks, and only returning to pre-pandemic levels towards May/June 2022. Parents' sedentary time and weekday MVPA was similar to pre-COVID-19 levels, with MVPA higher than pre-pandemic by 7.7 min (95% CI: 1.4, 14.0) on weekends.

CONCLUSION

After an initial drop, children's MVPA returned to pre-pandemic levels by July 2022, while sedentary time remained higher. Parents' MVPA remained higher, especially at weekends. The recovery in physical activity is precarious and potentially susceptible to future COVID-19 outbreaks or changes in provision, and so robust measures to protect against future disruptions are needed. Furthermore, many children are still inactive, with only 41% meeting UK physical activity guidelines, and so there is still a need to increase children's physical activity.

Association of physical activity with metabolic profile from adolescence to adulthood

OBJECTIVE

Physical activity benefits cardiometabolic health, but little is known about its detailed links with serum lipoproteins, amino acids, and glucose metabolism at young age. We therefore studied the association of physical activity with a comprehensive metabolic profile measured repeatedly in adolescence.

METHODS

The cohort is derived from the longitudinal Special Turku Coronary Risk Factor Intervention Project. At ages 13, 15, 17, and 19 years, data on physical activity were collected by a questionnaire, and circulating metabolic measures were quantified by nuclear magnetic resonance metabolomics from repeatedly assessed serum samples (age 13: n = 503, 15: n = 472, 17: n = 466, and 19: n = 361).

RESULTS

Leisure-time physical activity (LTPA;MET h/wk) was directly associated with concentrations of polyunsaturated fatty acids, and inversely with the ratio of monounsaturated fatty acids to total fatty acids (-0.006SD; [-0.008, -0.003]; p < 0.0001). LTPA was inversely associated with very-low-density lipoprotein (VLDL) particle concentration (-0.003SD; [-0.005, -0.001]; p = 0.002) and VLDL particle size (-0.005SD; [-0.007, -0.003]; p < 0.0001). LTPA showed direct association with the particle concentration and size of high-density lipoprotein (HDL), and HDL cholesterol concentration (0.004SD; [0.002, 0.006]; p < 0.0001). Inverse associations of LTPA with triglyceride and total lipid concentrations in large to small sized VLDL subclasses were found. Weaker associations were seen for other metabolic measures including inverse associations with concentrations of lactate, isoleucine, glycoprotein acetylation, and a direct association with creatinine concentration. The results remained after adjusting for body mass index and proportions of energy intakes from macronutrients.

CONCLUSIONS

Physical activity during adolescence is beneficially associated with the metabolic profile including novel markers. The results support recommendations on physical activity during adolescence to promote health and possibly reduce future disease risks.

Systemic inflammatory regulators and proliferative diabetic retinopathy: A bidirectional Mendelian randomization study

Background

Increasing evidence shows that systemic inflammation is an embedded mechanism of proliferative diabetic retinopathy (PDR). However, the specific systemic inflammatory factors involved in this process remained obscure. The study aimed to identify the upstream and downstream systemic regulators of PDR by using Mendelian randomization (MR) analyses.

Methods

We performed a bidirectional two-sample MR analysis implementing the results from genome-wide association studies for 41 serum cytokines from 8,293 Finnish individuals, and PDR from FinnGen consortium (2,025 cases vs. 284,826 controls) and eight cohorts of European ancestry (398 cases vs. 2,848 controls), respectively. The inverse-variance-weighted method was adopted as the main MR method, and four additional MR methods (MR-Egger, weighted-median, MR-pleiotropy residual sum and outlier (MR-PRESSO), and MR-Steiger filtering methods) were used for the sensitivity analyses. Results from FinnGen and eight cohorts were pooled into a meta-analysis.

Results

Our results showed that genetically predicted higher stem cell growth factor-β (SCGFb) and interleukin-8 were positively associated with an elevated risk of PDR, with a combined effect of one standard deviation (SD) increase in SCGFb and interleukin-8 causing 11.8% [95% confidence interval (CI): 0.6%, 24.2%]) and 21.4% [95% CI: 3.8%, 41.9%]) higher risk of PDR, respectively. In contrast, genetically predisposition to PDR showed a positive association with the increased levels of growth-regulated oncogene-α (GROa), stromal cell-derived factor-1 alpha (SDF1a), monocyte chemotactic protein-3 (MCP3), granulocyte colony-stimulating factor (GCSF), interleukin-12p70, and interleukin-2 receptor subunit alpha (IL-2ra).

Conclusions

Our MR study identified two upstream regulators and six downstream effectors of PDR, providing opportunities for new therapeutic exploitation of PDR onset. Nonetheless, these nominal associations of systemic inflammatory regulators and PDR require validation in larger cohorts.

Serum 25-hydroxyvitamin D levels and the risk of non-alcoholic fatty liver: A two-sample Mendelian randomization study

BACKGROUND

Accumulated studies have shown that low expression of 25-hydroxyvitamin D [25(OH)D] was significantly associated with the risk of non-alcoholic fatty liver disease (NAFLD). However, the exact causality is still unknown. The aim of this study was to investigate whether levels of 25(OH)D are associated with risk of NAFLD, using a two-sample Mendelian randomization (MR).

METHODS

Data from a recent large vitamin D genome-wide association study (GWAS) on 417,580 Europeans were utilized, and the largest published histology-based NAFLD GWAS study (1,483 cases and 17,781 healthy controls) for genetic variants predicted to cause NAFLD were searched. All genetic datasets for the MR analyses were obtained using publicly available summary statistics based on individuals of European ancestry from the MR-Base and NHGRI-EBI GWAS Catalog database. Inverse-variance weighted (IVW) MR approach was used to estimate causal effects in the main analysis, complemented by 4 additional methods to control for pleiotropy. Sensitivity analyses were conducted to verify whether heterogeneity and pleiotropy can bias the MR results.

RESULTS

The MR analysis did not provide strong evidence for the causal association of circulating 25(OH)D with NAFLD by IVW method (OR = 0.746, 95%CI 0.517-1.078; P = 0.119). The results were consistent using four other MR methods. Sensitivity analysis using all different analytical approaches yielded similar results. There was no evidence for pleiotropy (MR-Egger intercept: -0.0003758, P = 0.970). The replication process also showed consistent results using IVW method (P = 0.710).

CONCLUSION

This study indicates that serum 25(OH)D levels did not possess an obvious effect on the risk of NAFLD. The associations in previous studies may be due to residual confounding or reverse causation.

Genetic Pathways Underlying Individual Differences in Regular Physical Activity

Twin and family studies show a strong contribution of genetic factors to physical activity (PA) assessed by either self-report or accelerometers. PA heritability is around 43% across the lifespan. Genome-wide association studies have implied biological pathways related to exercise ability and enjoyment. A polygenic score based on genetic variants influencing PA could help improve the success of intervention programs.

Effect of adiposity and physical fitness on cardiometabolic risk factors in adolescents: A 2-year longitudinal study

Although risk factors for cardiometabolic diseases begin to present in young people, the association between physical fitness and adiposity with traditional and novel risk factors for cardiometabolic diseases across adolescence remains relatively unknown. Following ethical approval, fifty-two adolescents (age 11.6 ± 0.6 years; 32 girls) were recruited for a 2-years longitudinal study. Adiposity was assessed based on sum of skinfolds, waist circumference and body mass index, and physical fitness as distance run on the multi-stage fitness test (MSFT). Risk factors for cardiometabolic diseases (pro- and anti-inflammatory cytokines, plasma insulin, Homeostatic Model Assessment of Insulin Resistance - HOMA-IR, blood pressure) were measured following an overnight fast. Relationships between independent and response variables were analysed using multi-level modelling (final combined models were created using the stepwise backward elimination method). Plasma insulin concentration and HOMA-IR were positively associated with adiposity and inversely associated with distance run on the MSFT (all p < 0.05). The final combined models for plasma insulin concentration and HOMA-IR contained main effects for age, skinfolds and distance run on the MSFT. Levels of the anti-inflammatory cytokine IL-10 were inversely related to the sum of skinfolds (p = 0.046), whereas there was a trend for levels of the pro-inflammatory cytokine TNF-α to be positively related to the sum of skinfolds (p = 0.056). Adiposity and physical fitness are important, independent, determinants of metabolic health in adolescents. Furthermore, adiposity influences levels of pro- and anti-inflammatory cytokines in adolescence, with greater adiposity associated with a poorer inflammatory profile. The present study demonstrates an independent effect of physical fitness on metabolic health longitudinally across adolescence. It is therefore recommended that future work develops therapeutic interventions that reduce adiposity and enhance physical fitness in adolescents, to promote lifelong health.

Initiation of New Glucose-Lowering Therapies May Act to Reduce Physical Activity Levels: Pooled Analysis From Three Randomized Trials

OBJECTIVE

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) and glucagon-like peptide 1 receptor agonists (GLP-1RA) reduce body weight and improve cardiometabolic health, but their effect on physical activity is unknown.

RESEARCH DESIGN AND METHODS

We pooled data (n = 148) from three randomized trials to investigate the effect of empagliflozin (SGLT2i) and liraglutide (GLP-1RA), in comparison with sitagliptin (dipeptidyl peptidase 4 inhibitor) and dietary therapies, on accelerometer-assessed physical activity.

RESULTS

Liraglutide (mean -1,144 steps/day; 95% CI -2,069 to -220), empagliflozin (-1,132 steps/day; -1,739, -524), and sitagliptin (-852 steps/day; -1,625, -78) resulted in reduced total daily physical activity after 6 months (P < 0.01 vs. control). Moderate- to vigorous-intensity physical activity was also reduced. Dietary interventions led to no change or an increase in physical activity.

CONCLUSIONS

The initiation of all glucose-lowering therapies was associated with reduced physical activity, warranting further investigation.

Prenatal and postnatal correlates of moderate-to-vigorous physical activity in midlife: evidence from the 1970 British Cohort Study

Background

It is hypothesised that lifelong physical activity behaviours are established in early life, however there is minimal, and contradictory, evidence examining prenatal and postnatal factors in relation to adulthood physical activity. We investigated associations between prospectively ascertained prenatal/postnatal factors and device-measured moderate-to-vigorous physical activity (MVPA) in midlife.

Methods

Analyses included 5011 participants from the 1970 British Cohort Study, a birth cohort study of individuals born within the same week. At birth, the following factors were ascertained: socioeconomic position (SEP), maternal age, number of previous pregnancies, maternal smoking, maternal diabetes, gestational age, birth weight, breastfeeding status and infant health concerns. MVPA was captured at age 46 with a thigh-worn accelerometer device following a 24-hour protocol over 7 days.

Results

In sex-adjusted models, lower SEP (−6.7 min/day (95% CI: −9.0 to –4.4) in those with a partly or unskilled paternal occupation), younger maternal age (0.4 min/day (0.2 to 0.5) per additional year of maternal age), maternal smoking during pregnancy (−2.5 min/day (−4.0 to –1.0)) and post-term gestational age (−7.4 min/day (−11.5 to –3.4); boys only) were associated with lower MVPA at age 46. In the mutually adjusted model, associations did not change but there was some evidence that birth weight may also be associated with MVPA levels.

Conclusions

SEP, maternal age, maternal smoking, post-term birth in boys and birth weight were associated with MVPA in midlife, indicating that midlife physical activity behaviours may be partially established at birth. Early interventions in disadvantaged environments may have a positive impact on physical activity throughout the life course.

Relations between physical activity, sedentary time, and body fat from childhood to adolescence: Do they differ by sex?

BACKGROUND

Efforts to reveal the direction of influence between physical activity (PA), sedentary time (ST) and body fat in youth have produced inconsistent results, possibly due to a lack of adjustment for confounders and other factors. Sex-specific associations have rarely been studied.

METHODS

A sample from two Norwegian birth cohorts (n = 809) were followed biennially over five waves from the age of 6-14 years. Physical activity and ST were recorded by accelerometers, and body fat was assessed by bioelectrical impedance measurements.

RESULTS

By applying a dynamic panel model (DPM) that adjusts for all time-invariant confounding factors, it was found that among boys, increased fat mass index (FMI) at ages 8, 10 and 12 years predicted decreased PA two years later (8-10 years: B = -0.67, (95% CI: -1.1, -0.24); 10-12 years: B = -0.33, (95% CI: -0.61, -0.05); 12-14 years: B = -0.29, (95% CI: -0.52, -0.06)). Regarding the opposite direction of influence, more PA at age 12 forecasted reduced FMI at age 14 (B = -0.16, (95% CI: -0.24, -0.07)), whereas increased FMI predicted increased ST across all time points in boys only (6-8 years: B = 0.23, (95% CI:0.02.43); 8-10 years: B = 0.23, (95% CI:.08.39); 10-12 years: B = 0.13, (95% CI:.03.23); 12-14 years: B = 0.17, (95% CI:.07, 26)). The revealed relationships were significantly stronger in boys compared to the (absent) relations in girls. Sensitivity analyses examining moderate to vigorous PA (MVPA) rather than total PA were in accordance with the main findings.

CONCLUSIONS

In boys, increased FMI predicted reduced PA and increased ST two years later from childhood to adolescence. The opposite direction of influence was evident from only ages 12-14. There were no prospective relationships between FMI and PA or ST among girls.

Accelerometer-measured physical activity and sedentary time among children and their parents in the UK before and after COVID-19 lockdowns: a natural experiment

BACKGROUND

Restrictions due to the coronavirus disease 2019 (COVID-19) pandemic reduced physical activity provision for both children and their parents. Recent studies have reported decreases in physical activity levels during lockdown restrictions, but these were largely reliant on self-report methods, with data collected via unrepresentative self-report surveys. The post-pandemic impacts on children's activity levels remain unknown. A key question is how active children become once lockdown restrictions are lifted.

METHODS

Active-6 is a repeated cross-sectional natural experiment. Accelerometer data from 1296 children aged 10-11 and their parents were collected in 50 schools in the Greater Bristol area, UK in March 2017-May 2018 (pre-COVID-19 comparator group), and compared to 393 children aged 10-11 and parents in 23 of the same schools, collected in May-December 2021. Mean minutes of accelerometer-measured moderate-to-vigorous physical activity (MVPA) were derived for weekdays and weekend and compared pre- and post-lockdown via linear multilevel models.

RESULTS

After adjusting for seasonality, accelerometer wear time and child/parent demographics, children's mean weekday and weekend MVPA were 7.7 min (95% CI: 3.5 to 11.9) and 6.9 min (95% CI: 0.9 to 12.9) lower in 2021 than in 2018, respectively, while sedentary time was higher by 25.4 min (95% CI: 15.8 to 35.0) and 14.0 min (95% CI: 1.5 to 26.5). There was no evidence that differences varied by child gender or household education. There was no significant difference in parents' MVPA or sedentary time, either on weekdays or weekends.

CONCLUSIONS

Children's MVPA was lower by 7-8 min/day in 2021 once restrictions were lifted than before the pandemic for all groups, on both weekdays and weekends. Previous research has shown that there is an undesirable age-related decline in children's physical activity. The 8-min difference reported here would be broadly comparable to the decline that would have previously been expected to occur over a three-year period. Parents' physical activity was similar to pre-pandemic levels. Our results suggest that despite easing of restrictions, children's activity levels have not returned to pre-pandemic levels. There is an urgent need to understand why these changes have occurred and how long they are maintained.

Mendelian randomization suggests a bidirectional, causal relationship between physical inactivity and adiposity

Physical inactivity and increased sedentary time are associated with excess weight gain in observational studies. However, some longitudinal studies indicate reverse causality where weight gain leads to physical inactivity and increased sedentary time. As observational studies suffer from reverse causality, it is challenging to assess the true causal directions. Here, we assess the bidirectional causality between physical inactivity, sedentary time, and adiposity by bidirectional Mendelian randomization analysis. We used results from genome-wide association studies for accelerometer-based physical activity and sedentary time in 91,105 individuals and for body mass index (BMI) in 806,834 individuals. We implemented Mendelian randomization using CAUSE method that accounts for pleiotropy and sample overlap using full genome-wide data. We also applied inverse variance-weighted, MR-Egger, weighted median, and weighted mode methods using genome-wide significant variants only. We found evidence of bidirectional causality between sedentary time and BMI: longer sedentary time was causal for higher BMI [beta (95% CI) from CAUSE method: 0.11 (0.02, 0.2), p = 0.02], and higher BMI was causal for longer sedentary time (0.13 (0.08, 0.17), p = 6.3 x 10^-4). Our analyses suggest that higher moderate and vigorous physical activity are causal for lower BMI (moderate: –0.18 (-0.3,–0.05), p = 0.006; vigorous: –0.16 (-0.24,–0.08), p = 3.8 × 10^-4), but indicate that the association between higher BMI and lower levels of physical activity is due to horizontal pleiotropy. The bidirectional, causal relationship between sedentary time and BMI suggests that decreasing sedentary time is beneficial for weight management, but also that targeting adiposity may lead to additional health benefits by reducing sedentary time.

Associations of lipoprotein particle profile and objectively measured physical activity and sedentary time in schoolchildren: a prospective cohort study

Background

Our understanding of the mechanisms through which physical activity might benefit lipoprotein metabolism is inadequate. Here we characterise the continuous associations between physical activity of different intensities, sedentary time, and a comprehensive lipoprotein particle profile.

Methods

Our cohort included 762 fifth grade (mean [SD] age = 10.0 [0.3] y) Norwegian schoolchildren (49.6% girls) measured on two separate occasions across one school year. We used targeted proton nuclear magnetic resonance (1H NMR) spectroscopy to produce 57 lipoprotein measures from fasted blood serum samples. The children wore accelerometers for seven consecutive days to record time spent in light-, moderate-, and vigorous-intensity physical activity, and sedentary time. We used separate multivariable linear regression models to analyse associations between the device-measured activity variables—modelled both prospectively (baseline value) and as change scores (follow-up minus baseline value)—and each lipoprotein measure at follow-up.

Results

Higher baseline levels of moderate-intensity and vigorous-intensity physical activity were associated with a favourable lipoprotein particle profile at follow-up. The strongest associations were with the larger subclasses of triglyceride-rich lipoproteins. Sedentary time was associated with an unfavourable lipoprotein particle profile, the pattern of associations being the inverse of those in the moderate-intensity and vigorous-intensity physical activity analyses. The associations with light-intensity physical activity were more modest; those of the change models were weak.

Conclusion

We provide evidence of a prospective association between time spent active or sedentary and lipoprotein metabolism in schoolchildren. Change in activity levels across the school year is of limited influence in our young, healthy cohort.

Trial registration

ClinicalTrials.gov, #NCT02132494. Registered 7th April 2014

Association of sedentary and physical activity behaviours with body composition: a genome-wide association and Mendelian randomisation study

Objectives

Studies suggest that body composition can be independently improved through physical activity (PA). We performed a Mendelian randomisation (MR) study to test the incremental benefits of sedentary behaviour and various PA exposures on body composition outcomes as assessed by anthropometric indices, lean body mass (kg), body fat (%) and visceral adipose tissue (VAT) (kg).

Methods

Genetic instruments were identified for both self-reported and accelerometer-measured sedentary behaviour and PA. Outcomes included anthropometric and dual-energy X-ray absorptiometry measures of adiposity, extracted from the UK Biobank and the largest available consortia. Multivariable MR (MVMR) included educational attainment as a covariate to address potential confounding. Sensitivity analyses were evaluated for weak instrument bias and pleiotropic effects.

Results

We did not identify consistent associations between genetically predicted self-reported and accelerometer-measured sedentary behaviour and body composition outcomes. All analyses for self-reported moderate PA were null for body composition outcomes. Genetically predicted PA at higher intensities was protective against VAT in MR and MVMR analyses of both accelerometer-measured vigorous PA (MVMR β=-0.15, 95% CI: -0.24 to -0.07, p<0.001) and self-reported participation in strenuous sports or other exercises (MVMR β=-0.27, 95% CI: -0.52 to -0.01, p=0.034) was robust across several sensitivity analyses.

Conclusions

We did not identify evidence of a causal relationship between genetically predicted PA and body composition, with the exception of a putatively protective effect of higher-intensity PA on VAT. Protective effects of PA against VAT may support prior evidence of biological pathways through which PA decreases risk of downstream cardiometabolic diseases.

Welch-weighted Egger regression reduces false positives due to correlated pleiotropy in Mendelian randomization

Modern population-scale biobanks contain simultaneous measurements of many phenotypes, providing unprecedented opportunity to study the relationship between biomarkers and disease. However, inferring causal effects from observational data is notoriously challenging. Mendelian randomization (MR) has recently received increased attention as a class of methods for estimating causal effects using genetic associations. However, standard methods result in pervasive false positives when two traits share a heritable, unobserved common cause. This is the problem of correlated pleiotropy. Here, we introduce a flexible framework for simulating traits with a common genetic confounder that generalizes recently proposed models, as well as a simple approach we call Welch-weighted Egger regression (WWER) for estimating causal effects. We show in comprehensive simulations that our method substantially reduces false positives due to correlated pleiotropy while being fast enough to apply to hundreds of phenotypes. We apply our method first to a subset of the UK Biobank consisting of blood traits and inflammatory disease, and then to a broader set of 411 heritable phenotypes. We detect many effects with strong literature support, as well as numerous behavioral effects that appear to stem from physician advice given to people at high risk for disease. We conclude that WWER is a powerful tool for exploratory data analysis in ever-growing databases of genotypes and phenotypes.

The carbohydrate-insulin model: a physiological perspective on the obesity pandemic

According to a commonly held view, the obesity pandemic is caused by overconsumption of modern, highly palatable, energy-dense processed foods, exacerbated by a sedentary lifestyle. However, obesity rates remain at historic highs, despite a persistent focus on eating less and moving more, as guided by the energy balance model (EBM). This public health failure may arise from a fundamental limitation of the EBM itself. Conceptualizing obesity as a disorder of energy balance restates a principle of physics without considering the biological mechanisms that promote weight gain. An alternative paradigm, the carbohydrate-insulin model (CIM), proposes a reversal of causal direction. According to the CIM, increasing fat deposition in the body-resulting from the hormonal responses to a high-glycemic-load diet-drives positive energy balance. The CIM provides a conceptual framework with testable hypotheses for how various modifiable factors influence energy balance and fat storage. Rigorous research is needed to compare the validity of these 2 models, which have substantially different implications for obesity management, and to generate new models that best encompass the evidence.

The genetic case for cardiorespiratory fitness as a clinical vital sign and the routine prescription of physical activity in healthcare

BACKGROUND

Cardiorespiratory fitness (CRF) and physical activity (PA) are well-established predictors of morbidity and all-cause mortality. However, CRF is not routinely measured and PA not routinely prescribed as part of standard healthcare. The American Heart Association (AHA) recently presented a scientific case for the inclusion of CRF as a clinical vital sign based on epidemiological and clinical observation. Here, we leverage genetic data in the UK Biobank (UKB) to strengthen the case for CRF as a vital sign and make a case for the prescription of PA.

METHODS

We derived two CRF measures from the heart rate data collected during a submaximal cycle ramp test: CRF-vo2max, an estimate of the participants' maximum volume of oxygen uptake, per kilogram of body weight, per minute; and CRF-slope, an estimate of the rate of increase of heart rate during exercise. Average PA over a 7-day period was derived from a wrist-worn activity tracker. After quality control, 70,783 participants had data on the two derived CRF measures, and 89,683 had PA data. We performed genome-wide association study (GWAS) analyses by sex, and post-GWAS techniques to understand genetic architecture of the traits and prioritise functional genes for follow-up.

RESULTS

We found strong evidence that genetic variants associated with CRF and PA influenced genetic expression in a relatively small set of genes in the heart, artery, lung, skeletal muscle and adipose tissue. These functionally relevant genes were enriched among genes known to be associated with coronary artery disease (CAD), type 2 diabetes (T2D) and Alzheimer's disease (three of the top 10 causes of death in high-income countries) as well as Parkinson's disease, pulmonary fibrosis, and blood pressure, heart rate, and respiratory phenotypes. Genetic variation associated with lower CRF and PA was also correlated with several disease risk factors (including greater body mass index, body fat and multiple obesity phenotypes); a typical T2D profile (including higher insulin resistance, higher fasting glucose, impaired beta-cell function, hyperglycaemia, hypertriglyceridemia); increased risk for CAD and T2D; and a shorter lifespan.

CONCLUSIONS

Genetics supports three decades of evidence for the inclusion of CRF as a clinical vital sign. Given the genetic, clinical and epidemiological evidence linking CRF and PA to increased morbidity and mortality, regular measurement of CRF as a marker of health and routine prescription of PA could be a prudent strategy to support public health.

The bidirectional associations between leisure time physical activity change and body mass index gain. The Tromsø Study 1974-2016

OBJECTIVES

To examine whether leisure time physical activity changes predict subsequent body mass index (BMI) changes, and conversely, whether BMI changes predict subsequent leisure time physical activity changes.

METHODS

This prospective cohort study included adults attending ≥3 consecutive Tromsø Study surveys (time: T1, T2, T3) during 1974-2016 (n = 10779). If participants attended >3 surveys, we used the three most recent surveys. We computed physical activity change (assessed by the Saltin-Grimby Physical Activity Level Scale) from T1 to T2, categorized as Persistently Inactive (n = 992), Persistently Active (n = 7314), Active to Inactive (n = 1167) and Inactive to Active (n = 1306). We computed BMI change from T2 to T3, which regressed on preceding physical activity changes using analyses of covariance. The reverse association (BMI change from T1 to T2 and physical activity change from T2 to T3; n = 4385) was assessed using multinomial regression.

RESULTS

Average BMI increase was 0.86 kg/m² (95% CI: 0.82-0.90) from T2 to T3. With adjustment for sex, birth year, education, smoking and BMI at T2, there was no association between physical activity change from T1 to T2 and BMI change from T2 to T3 (Persistently Inactive: 0.89 kg/m² (95% CI: 0.77-1.00), Persistently Active: 0.85 kg/m² (95% CI: 0.81-0.89), Active to Inactive: 0.90 kg/m² (95% CI: 0.79-1.00), Inactive to Active 0.85 kg/m² (95% CI: 0.75-0.95), p = 0.84). Conversely, increasing BMI was associated with Persistently Inactive (odds ratio (OR): 1.17, 95% CI: 1.08-1.27, p < 0.001) and changing from Active to Inactive (OR: 1.16, 95% CI: 1.07-1.25, p < 0.001) compared with being Persistently Active.

CONCLUSIONS

We found no association between leisure time physical activity changes and subsequent BMI changes, whereas BMI change predicted subsequent physical activity change. These findings indicate that BMI change predicts subsequent physical activity change at population level and not vice versa.

Physical activity behaviours in adolescence: current evidence and opportunities for intervention

Young people aged 10-24 years constitute 24% of the world's population; investing in their health could yield a triple benefit-eg, today, into adulthood, and for the next generation. However, in physical activity research, this life stage is poorly understood, with the evidence dominated by research in younger adolescents (aged 10-14 years), school settings, and high-income countries. Globally, 80% of adolescents are insufficiently active, and many adolescents engage in 2 h or more daily recreational screen time. In this Series paper, we present the most up-to-date global evidence on adolescent physical activity and discuss directions for identifying potential solutions to enhance physical activity in the adolescent population. Adolescent physical inactivity probably contributes to key global health problems, including cardiometabolic and mental health disorders, but the evidence is methodologically weak. Evidence-based solutions focus on three key components of the adolescent physical activity system: supportive schools, the social and digital environment, and multipurpose urban environments. Despite an increasing volume of research focused on adolescents, there are still important knowledge gaps, and efforts to improve adolescent physical activity surveillance, research, intervention implementation, and policy development are urgently needed.

Caution against examining the role of reverse causality in Mendelian Randomization

Recently, Mendelian Randomization (MR) has gained in popularity as a concept to assess the causal relationship between phenotypes in genetic association studies. An extension of standard MR methodology, the MR Steiger approach, has recently been developed to infer the causal direction between two phenotypes in prospective studies. Through simulation studies, we examined and quantified the ability of the MR Steiger approach to determine the causal direction between two phenotypes (i.e., effect direction). Through simulation studies, our results show that the MR Steiger approach may fail to correctly identify the direction of causality. This is true, especially in the presence of pleiotropy. We also applied the MR Steiger method to the COPDGene study, a case-control study of chronic obstructive pulmonary disease (COPD) in current and former smokers, to examine the role of smoking on lung function. We have created an R package on Github called reverseDirection which runs simulations for user-specified scenarios to examine when the MR Steiger approach can correctly determine the causal direction between two phenotypes in any user specified scenario. In summary, our results emphasize the importance of caution when the MR Steiger approach is used in to infer the direction of causality.

Evidence for shared genetics between physical activity, sedentary behaviour and adiposity-related traits

Observational, cross-sectional and longitudinal studies showed that physical activity and sedentary behaviour are associated with adiposity-related traits, apparently in a bidirectional manner. Physical activity is also suggested to suppress the genetic risk of adiposity. Since phenotypic associations with genetic variants are not subject to reverse causation or confounding, they may be used as tools to shed light on cause and effect in this complex interdependency. We review the evidence for shared genetics of physical activity and adiposity-related traits and for gene-by-physical activity interactions on adiposity-related traits in human studies. We outline limitations, challenges and opportunities in studying and understanding of these relationships. In summary, physical activity and sedentary behaviour are genetically correlated with body mass index and fat percentage but may not be correlated with lean body mass. Mendelian randomisation analyses show that physical activity and sedentary behaviour have bidirectional relationships with adiposity. Several studies suggest that physical activity suppresses genetic risk of adiposity. No studies have yet tested whether adiposity enhances genetic predisposition to sedentariness. The complexity of the comprehensive causal model makes the assessment of the single or combined components challenging. Substantial progress in this field may need long-term intervention studies.

Bi-directional prospective associations between sedentary time, physical activity and adiposity in 10-year old Norwegian children

There is an adverse cross-sectional association between sedentary time, physical activity (PA) and adiposity, but weak and inconsistent estimates raise question to the direction of associations. The present study aims to examine whether the prospective association between sedentary time, different PA intensities and indicators of adiposity is bi-directional. The Active Smarter Kids Study obtained data from 869 ten-year-old children with valid measurements for sedentary time, PA, and adiposity at baseline and follow-up. Time spent sedentary and PA was measured by accelerometry, adiposity was assessed by three different measures: body mass index (BMI), waist circumference (WC) and sum of four skinfolds (S4SF). Neither overall PA nor time spent sedentary predicted lower BMI or WC at follow-up, but the time spent in moderate-and-vigorous PA (MVPA) and vigorous PA (VPA) predicted lower S4SF at follow-up among boys (MVPA β - 0.066 [95% CI -0.105, -0.027] p = 0.001). Baseline BMI and WC predicted less overall PA, MVPA and VPA in boys. All adiposity measures predicted more time spent sedentary at follow-up in boys. The results suggest that overall PA and sedentary time do not predict future adiposity. Baseline adiposity may rather predict more sedentary time and less higher intensity activity.

Adult obesity and mid-life physical functioning in two British birth cohorts: investigating the mediating role of physical inactivity

Background

Associations between obesity and physical inactivity are bi-directional. Both are associated with physical functioning (PF, ability to perform physical tasks of daily living) but whether obesity influences PF via inactivity is unknown. We investigated whether mid-adult obesity trajectories were associated with subsequent PF and mediated by inactivity.

Methods

Body mass index (BMI; kg/m²) and inactivity were recorded at: 36, 43, 53 and 60–64 years in the 1946 Medical Research Council (MRC) National Survey of Health and Development (1946-NSHD; n = 2427), and at 33, 42 and 50 years in the 1958 National Child Development Study (1958-NCDS; n = 8674). Poor PF was defined as the lowest (gender and cohort-specific) 10% on the Short-form 36 Physical Component Summary subscale at 60–64 years (1946-NSHD) and 50 years (1958-NCDS). Estimated randomized-interventional-analogue natural direct (rNDE) and indirect (rNIE) effects of obesity trajectories on PF via inactivity are expressed as risk ratios [overall total effect (rTE) is rNDE multiplied by rNIE].

Results

In both cohorts, most individuals (∼68%) were never obese in adulthood, 16–30% became obese and ≤11% were always obese. In 1946-NSHD, rTE of incident obesity at 43 years (vs never) on poor PF was 2.32 (1.13, 3.51); at 53 years it was 1.53 (0.91, 2.15). rNIEs via inactivity were 1.02 (0.97, 1.07) and 1.02 (0.99, 1.04), respectively. Estimated rTE of persistent obesity from 36 years was 2.91 (1.14, 4.69), with rNIE of 1.03 (0.96, 1.10). In 1958-NCDS, patterns of association were similar, albeit weaker.

Conclusions

Longer duration of obesity was associated with increased risk of poor PF. Inactivity played a small mediating role. Findings reinforce the importance of preventing and delaying obesity onset to protect against poor PF.

Birthweight, lifetime obesity and physical functioning in mid-adulthood: a nationwide birth cohort study

BACKGROUND

Evidence is scant on long-term implications of childhood obesity and body mass index (BMI) gains over the life-course for poor physical functioning (PF). The objective was to establish whether (i) birthweight and BMI across the life-course, (ii) BMI gains at specific life-stages and (iii) age of obesity onset were associated with PF at 50 y.

METHODS

In the 1958 British birth cohort (n = 8674), BMI (kg/m2) was calculated using height and weight [measured (7, 11, 16, 33 and 45 y); self-reported (23 and 50 y)]. PF was assessed at 50 y using the validated PF subscale of the Short-form 36 survey; the bottom (gender-specific) 10% was classified as poor PF. Missing data were imputed via multiple imputation. Associations were examined using logistic regression, adjusting for health and social factors.

RESULTS

Birthweight was not associated with PF. At each adult age, odds of poor PF were highest for obese (vs normal), e.g. for 23 y obesity the odds ratio (OR)adjusted for poor PF was 2.28 (1.34, 3.91) and 2.67 (1.72, 4.14) in males and females respectively. BMI gains were associated with poor PF, e.g. for females, ORadjusted per standard deviation (SD) in BMI gain 16-23 y was 1.28 (1.13, 1.46); for BMI gains 45-50 y it was 1.36 (1.11, 1.65). Longer duration of obesity was associated with poor PF, e.g. in males, ORadjusted was 2.32 (1.26, 4.29) for childhood obesity onset and 1.50 (1.16, 1.96) for mid-adulthood onset (vs never obese, P-trend < 0.001).

CONCLUSION

Obesity, BMI gains, and earlier obesity onset were associated with poor PF in mid-adulthood, reinforcing the importance of preventing and delaying obesity onset.

Cross-sectional and prospective associations between aerobic fitness and lipoprotein particle profile in a cohort of Norwegian schoolchildren

BACKGROUND AND AIMS

The associations between aerobic fitness and traditional measures of lipid metabolism in children are uncertain. We investigated whether higher levels of aerobic fitness benefit lipoprotein metabolism by exploring associations with a comprehensive lipoprotein particle profile.

METHODS

In our prospective cohort study, we used targeted proton nuclear magnetic resonance (¹H NMR) spectroscopy to profile 57 measures of lipoprotein metabolism from fasting serum samples of 858 fifth-grade Norwegian schoolchildren (49.0% girls; mean age 10.0 years). Aerobic fitness was measured using an intermittent shuttle run aerobic fitness test. We used multiple linear regression adjusted for potential confounders to examine cross-sectional and prospective associations between aerobic fitness and lipoprotein particle profile.

RESULTS

Higher levels of aerobic fitness were associated with a favourable lipoprotein particle profile in the cross-sectional analysis, which included inverse associations with all measures of very low-density lipoprotein (VLDL) particles (e.g., -0.06 mmol·L^-1 or -0.23 SD units; 95% CI = -0.31, -0.16 for VLDL cholesterol concentration). In the prospective analysis, the favourable pattern of associations persisted, though the individual associations tended to be more consistent with those of the cross-sectional analysis for the VLDL subclass measures compared to the low-density lipoproteins and high-density lipoproteins. Adjustment for adiposity attenuated the associations in both cross-sectional and prospective models. Nevertheless, an independent effect of aerobic fitness remained for some measures.

CONCLUSIONS

Improving children's aerobic fitness levels should benefit lipoprotein metabolism, though a concomitant reduction in adiposity would likely potentiate this effect.

Childhood Obesity and Device-Measured Sedentary Behavior: An Instrumental Variable Analysis of 3,864 Mother-Offspring Pairs

OBJECTIVE

Intergenerational data on mother-offspring pairs were utilized in an instrumental variable analysis to examine the longitudinal association between BMI and sedentary behavior.

METHODS

The sample included 3,864 mother-offspring pairs from the 1970 British Cohort Study. Height and weight were recorded in mothers (age 31 [5.4] years) and offspring (age 10 years) and repeated in offspring during adulthood. Offspring provided objective data on sedentary behavior (7-day thigh-worn activPAL) in adulthood at age 46 to 47 years.

RESULTS

Maternal BMI, the instrumental variable, was associated with offspring BMI at age 10 (change per kg/m² , β = 0.11; 95% CI: 0.09 to 0.12), satisfying a key assumption of instrumental variable analyses. Offspring (change per kg/m² , β = 0.010; 95% CI: -0.02 to 0.03 h/d) and maternal BMI (β = 0.017; 95% CI: 0.001 to 0.03 h/d) was related to offspring sedentary time, suggestive of a causal impact of BMI on sedentary behavior (two-stage least squares analysis, β = 0.18 [SE 0.08], P = 0.015). For moderate-vigorous physical activity, there were associations with offspring BMI (β = -0.010; 95% CI: -0.017 to -0.004) and maternal BMI (β = -0.007; 95% CI: -0.010 to -0.003), with evidence for causality (two-stage least squares analysis, β = -0.060 [SE 0.02], P = 0.001).

CONCLUSIONS

There is strong evidence for a causal pathway linking childhood obesity to greater sedentary behavior.

Unhealthy Behaviours and Risk of Parkinson's Disease: A Mendelian Randomisation Study

BACKGROUND

Tobacco smoking and alcohol intake have been identified in observational studies as potentially protective factors against developing Parkinson's disease (PD); the impact of body mass index (BMI) on PD risk is debated. Whether such epidemiological associations are causal remains unclear. Mendelian randomsation (MR) uses genetic variants to explore the effects of exposures on outcomes; potentially reducing bias from residual confounding and reverse causation.

OBJECTIVE

Using MR, we examined relationships between PD risk and three unhealthy behaviours: tobacco smoking, alcohol intake, and higher BMI.

METHODS

19,924 PD cases and 2,413,087 controls were included in the analysis. We performed genome-wide association studies to identify single nucleotide polymorphisms associated with tobacco smoking, alcohol intake, and BMI. MR analysis of the relationship between each exposure and PD was undertaken using a split-sample design.

RESULTS

Ever-smoking reduced the risk of PD (OR 0.955; 95%confidence interval [CI] 0.921-0.991; p = 0.013). Higher daily alcohol intake increased the risk of PD (OR 1.125, 95%CI 1.025-1.235; p = 0.013) and a 1 kg/m2 higher BMI reduced the risk of PD (OR 0.988, 95%CI 0.979-0.997; p = 0.008). Sensitivity analyses did not suggest bias from horizontal pleiotropy or invalid instruments.

CONCLUSION

Using split-sample MR in over 2.4 million participants, we observed a protective effect of smoking on risk of PD. In contrast to observational data, alcohol consumption appeared to increase the risk of PD. Higher BMI had a protective effect on PD, but the effect was small.

Adiposity and High Blood Pressure during Childhood: A Prospective Analysis of the Role of Physical Activity Intensity and Sedentary Time in the GECKO Drenthe Cohort

Whereas in adults, physical inactivity is strongly related to obesity and hypertension, in young children the evidence is inconsistent and scarce. We examined the association between physical activity (PA) behaviours at 5–6 years of age and adiposity and blood pressure (BP) at 10–11 years in 947 children (51% boys) from the Groningen Expert Center for Kids with Obesity (GECKO) Drenthe cohort. Sedentary time (ST) and light, moderate, and vigorous PA were assessed using accelerometry (ActiGraph GT3X, wear time > 600 min/day, ≥3 days). Body mass index (BMI), waist circumference (WC), and systolic and diastolic BP were measured at 5–6 and 10–11 years of age and standardized as age- and sex-adjusted (and height-adjusted, for BP) z-scores. Adjusted linear and logistic regression models showed that most PA behaviours were not related to standardized BMI or WC, overweightness/obesity, abdominal overweightness/obesity, standardized systolic or diastolic BP, pulse pressure, or prehypertension at 10–11 years of age. Only if children spent more time in vigorous PA was WC slightly lower (B (95% CI) = −0.08 (−0.16, −0.01) SD, stdβ = −0.068) and the increase in WC over the years was less (B (95% CI) = −0.10 (−0.18, −0.01) SD; stdβ = −0.083). To conclude, at this very young age, PA behaviours are not a strong predictor for overweightness/obesity or hypertension later in childhood.

Do declines in occupational physical activity contribute to population gains in body mass index? Tromsø Study 1974-2016

OBJECTIVE

To examine whether occupational physical activity changes predict future body mass index (BMI) changes.

METHODS

This longitudinal cohort study included adult participants attending ≥3 consecutive Tromsø Study surveys (examinations 1, 2 and 3) from 1974 to 2016 (N=11 308). If a participant attended >3 surveys, the three most recent surveys were included. Occupational physical activity change (assessed by the Saltin-Grimby Physical Activity Level Scale) was computed from the first to the second examination, categorised into persistently inactive (n=3692), persistently active (n=5560), active to inactive (n=741) and inactive to active (n=1315). BMI change was calculated from the second to the third examination (height being fixed at the second examination) and regressed on preceding occupational physical activity changes using analysis of covariance adjusted for sex, birth year, smoking, education and BMI at examination 2.

RESULTS

Overall, BMI increased by 0.84 kg/m2 (95% CI 0.82 to 0.89). Following adjustments as described previously, we observed no differences in BMI increase between the occupational physical activity change groups (Persistently Inactive: 0.81 kg/m2, 95% CI 0.75 to 0.87; Persistently Active: 0.87 kg/m2, 95% CI 0.82 to 0.92; Active to Inactive: 0.81 kg/m2, 95% CI 0.67 to 0.94; Inactive to Active: 0.91 kg/m2, 95% CI 0.81 to 1.01; p=0.25).

CONCLUSION

We observed no prospective association between occupational physical activity changes and subsequent BMI changes. Our findings do not support the hypothesis that occupational physical activity declines contributed to population BMI gains over the past decades. Public health initiatives aimed at weight gain prevention may have greater success if focusing on other aspects than occupational physical activity.

Inferring causal direction between two traits in the presence of horizontal pleiotropy with GWAS summary data

Orienting the causal relationship between pairs of traits is a fundamental task in scientific research with significant implications in practice, such as in prioritizing molecular targets and modifiable risk factors for developing therapeutic and interventional strategies for complex diseases. A recent method, called Steiger’s method, using a single SNP as an instrument variable (IV) in the framework of Mendelian randomization (MR), has since been widely applied. We report the following new contributions. First, we propose a single SNP-based alternative, overcoming a severe limitation of Steiger’s method in simply assuming, instead of inferring, the existence of a causal relationship. We also clarify a condition necessary for the validity of the methods in the presence of hidden confounding. Second, to improve statistical power, we propose combining the results from multiple, and possibly correlated, SNPs as multiple instruments. Third, we develop three goodness-of-fit tests to check modeling assumptions, including those required for valid IVs. Fourth, by relaxing one of the three IV assumptions in MR, we propose several methods, including an Egger regression-like approach and its multivariable version (analogous to multivariable MR), to account for horizontal pleiotropy of the SNPs/IVs, which is often unavoidable in practice. All our methods can simultaneously infer both the existence and (if so) the direction of a causal relationship, largely expanding their applicability over that of Steiger’s method. Although we focus on uni-directional causal relationships, we also briefly discuss an extension to bi-directional relationships. Through extensive simulations and an application to infer the causal directions between low density lipoprotein (LDL) cholesterol, or high density lipoprotein (HDL) cholesterol, and coronary artery disease (CAD), we demonstrate the superior performance and advantage of our proposed methods over Steiger’s method and bi-directional MR. In particular, after accounting for horizontal pleiotropy, our method confirmed the well known causal direction from LDL to CAD, while other methods, including bi-directional MR, might fail.

In spite of its importance, due to technical challenges, orienting causal relationships between pairs of traits has been largely under-studied. Mendelian randomization (MR) Steiger’s method has become increasingly used in the last two years. Here we point out several limitations with MR Steiger’s method and propose alternative approaches. First, MR Steiger’s method is based on using only one single SNP as the instrument variable (IV), for which we propose a correlation ratio-based method, called Causal Direction-Ratio, or simply CD-Ratio. An advantage of CD-Ratio is its inference of both the existence and (if so) the direction of a causal relationship, in contrast to MR Steiger’s prior assumption of the existence and its poor performance if the assumption is violated. Furthermore, CD-Ratio can be extended to combine the results from multiple, possibly correlated, SNPs with improved statistical power. Second, we propose two methods, called CD-Egger and CD-GLS, for multiple and possibly correlated SNPs while allowing horizontal pleiotropy. Third, we propose three goodness-of-fit tests to check modeling assumptions for the three proposed methods. Finally, we introduce multivariable CD-Egger, analogous to multivariable MR, as a more robust approach, and an extension of CD-Ratio to cases with possibly bi-directional causal relationships. Our numerical studies demonstrated superior performance of our proposed methods over MR Steiger and bi-directional MR. Our proposed methods, along with freely available software, are expected to be useful in practice for causal inference.

Physical fitness development in relation to changes in body composition and physical activity in adolescence

The decline in adolescents' physical fitness (PF) in recent decades has raised concerns about current population's possible future challenges with health and physical functional capacity. This study explored the associations between body composition, physical activity, maturation, and PF development in adolescents. Furthermore, PF development of adolescents with low initial PF was assessed. A 2-year observational study was conducted between spring 2013 and 2015. Nine comprehensive schools and their 10- to 13-year-old students were invited to participate in the study (1778), and a total of 971 students (54.6%) agreed. Cardiorespiratory fitness (20-meter shuttle run), muscular fitness (push-ups), fundamental movement skills (5-leaps test), body composition (bioelectrical impedance analysis), moderate-to-vigorous physical activity (accelerometer), and pubertal status (self-assessment questionnaire) were measured at 1-year intervals. Latent growth curve modeling (LGM) was used to study PF development over time. Change in fat mass had the strongest and most coherent associations with PF development during adolescence. Fat-free mass, moderate-to-vigorous physical activity, and pubertal status were associated with PF development, although not systematically. Subgroup analyses showed that PF development in the low fitness group followed a similar pattern as the whole population. However, their PF remained significantly lower throughout the 2-year period. The findings suggest that fat accumulation is an essential detrimental factor for PF development during adolescence. Actions to prevent excessive fat accumulation might help to prevent future declines in functional capacity. Indications that low fitness levels sustain during adolescence highlight the relevance of detecting these individuals and providing interventions already before adolescence.

Is adiposity associated with objectively measured physical activity and sedentary behaviors in older adults?

BACKGROUND

Many older adults in the U.S. do not achieve the recommended amount of physical activity (PA) to fully realize a myriad of health benefits. Adiposity is one of those important correlates of PA and sedentary behaviors. However, the full extent to which adiposity is associated with PA and stationary time (STA) is uncertain. Therefore, we examined the association of adiposity with objectively measured PA and STA in black and white older adults.

METHODS

We conducted a cross-sectional study of older adults enrolled in the REasons for Geographic and Racial Differences in Stroke (REGARDS) Study 2003-2007 who participated in an ancillary accelerometer study 2009-2013. Assessment of body mass index (BMI) and waist circumference (WC) was completed during an in-home visit in the parent study. PA was measured by Actical™ accelerometers, which provided estimates of moderate-to-vigorous-intensity PA (MVPA), light-intensity PA (LPA), and STA for 4-7 consecutive days. Data from accelerometers were standardized to square root percentages of total wear time per day (SqrtMVPA%, SqrtLPA%, and SqrtSTA%). Interactions were tested for BMI and WC by race and sex, separately.

RESULTS

Data were available for 7873 participants (69.8 ± 8.7 yr, 54.2% women, 31.5% African American). In mixed linear regression models, significant interactions existed in BMI by race and sex for the SqrtMVPA%, WC by race and sex for the SqrtMVPA% and the SqrtLPA% model(p < 0.05). No interaction was significant for the logistic model of meeting the PA guideline or not. In subgroup analyses, BMI was inversely associated with SqrtMVPA%, SqrtLPA%, and positively related to SqrtSTA% in black women, white men and white women after adjustments. Similar patterns were observed between WC and SqrtMVPA%, SqrtLPA%, and SqrtSTA% in all groups, respectively. However, BMI was not associated with SqrtMVPA% in black men. Those with higher BMI or WC were less likely to meet the PA guideline in all groups.

CONCLUSIONS

Adiposity was inversely associated with higher levels of MVPA/LPA and positively associated with higher levels of STA among black and white older adults. Prevention efforts aimed at promoting weight control may be beneficial to prevent physical inactivity and sedentary lifestyle among older adults.

Age at puberty and accelerometer-measured physical activity: Findings from two independent UK cohorts

BACKGROUND

It is unclear if puberty timing influences future physical activity (PA).

AIM

To investigate the association of puberty timing with PA across adolescence and adulthood.

SUBJECTS AND METHODS

Data were from two British cohorts. Participants from an adolescent birth cohort (females = 2349, males = 1720) prospectively reported age at menarche and voice break and had PA recorded by Actigraph accelerometers at ages 14 years and 16 years. A cohort of middle-aged and older adults (40-70 years; females = 48,282; males = 36,112) recalled their age at puberty and had PA (mean acceleration; mg) measured by AxivityAX3 accelerometers.

RESULTS

After adjustment for age, education, smoking and BMI, per 1-year older age at menarche was associated with higher mean counts/minute at age 14 years (0.07 SD counts/minute; 95% CI = 0.04-0.11) with associations attenuated at age 16 years (0.02; -0.03-0.07). Differences in mean acceleration per older year at menarche were close to the null in women aged 40-49 years (0.02 mg; 0.01-0.03), 50-59 years (0.01; 0.00-0.02) and 60-70 years (0.01; 0.00-0.01). Age at voice break and PA associations were close to the null in both cohorts.

CONCLUSION

We found a positive association between puberty timing and PA in females which weakened at older ages and limited evidence of an association at any age in males.

Bidirectional 10-year associations of accelerometer-measured sedentary behavior and activity categories with weight among middle-aged adults

BACKGROUND

Although higher sedentary behavior (SB) with low light intensity (LPA) and moderate-to-vigorous intensity physical activity (MVPA) are thought to increase risk for obesity, other data suggest excess weight may precede these behaviors in the causal pathway. We aimed to investigate 10-year bidirectional associations between SB and activity with weight.

METHODS

Analysis included 886 CARDIA participants (aged 38-50 years, 62% female, 38% black) with weight and accelerometry ( ≥ 4 days with ≥ 10 h/day) collected in 2005-6 (ActiGraph 7164) and 2015-6 (ActiGraph wGT3X-BT). Accelerometer data were calibrated, harmonized, and expressed as counts per minute (cpm) and time-dependent intensity categories (min/day of SB, LPA, and MVPA; SB and MVPA were also separated into long-bout and short-bout categories). Linear regression models were constructed to estimate adjusted associations of baseline activity with 10-year change in weight and vice versa. When activity categories were the independent variables, standardized regression coefficients (βstd.) estimated associations of replacing SB with a one SD increase in other categories, adjusted for accelerometer wear time.

RESULTS

Over 10-years, weight increased by a mean 2.55 ± 8.05 kg and mean total activity decreased by 50 ± 153 cpm. In adjusted models, one SD higher baseline mean total activity (βstd. = -1.4 kg, p < 0.001), LPA (βstd. = -0.80 kg, p = 0.013), total MVPA (βstd. = -1.07 kg, p = 0.001), and long-bout MVPA (βstd. = -1.20 kg, p < 0.001) were associated with attenuated 10-year weight gain. Conversely, a one SD higher baseline weight was associated with unfavorable 10-year changes in daily activity profile including increases in SB (βstd. = 12.0 min, p < 0.001) and decreases in mean total activity (βstd. = 14.9 cpm, p = 0.004), LPA (βstd. = 8.9, p = 0.002), and MVPA (βstd. = 3.5 min, p = 0.001). Associations varied by race and gender.

CONCLUSIONS

Higher SB with lower activity and body weight were bidirectionally related. Interventions that work simultaneously to replace SB with LPA and long-bout MVPA while also using other methods to address excess weight may be optimal.

Active children are less adipose and insulin resistant in early adolescence; evidence from the Mysore Parthenon Cohort

Background

The aim of this study was to determine whether physical activity volume and intensity in mid-childhood and early adolescence were associated with cardiometabolic risk factors at 13.5 years.

Methods

Participants were recruited from the Mysore Parthenon observational birth cohort. At ages 6–10 and 11–13 years, volume and intensity of physical activity were assessed using AM7164 or GT1M actigraph accelerometers worn for ≥4 days, and expressed as mean counts per day and percentage time spent in light, moderate and vigorous physical activity according to criteria defined by Evenson et al. At 13.5 years, fasting blood samples were collected; lipids, glucose and insulin concentrations were measured and insulin resistance (HOMA) was calculated. Systolic and diastolic blood pressure were measured at the left arm using a Dinamap (Criticon). Anthropometry and bio-impedance analysis were used to assess body size and composition. Metabolic and anthropometric measures were combined to produce a metabolic syndrome risk score.

Results

At 6–10 years, boys and girls respectively spent a median (IQR) of 1.1 (0.5, 2.0) % and 0.8 (0.4, 1.3) % of recorded time vigorously active. At 11–13 years, boys and girls respectively spent a median (IQR) of 0.8 (0.4, 1.7) % and 0.3 (0.1, 0.6) % of time vigorously active. All of the physical activity parameters were positively correlated between the 6–10 year and the 11–13 year measurements indicating that physical activity tracked from childhood to early adolescence. There were no associations between physical activity at 6–10 years and individual 13.5 year risk factors but % time vigorously active was inversely associated with metabolic syndrome score (B = −0.40, 95% CI −0.75, 0.05). Volume of physical activity at 11–13 years was inversely associated with 13.5 year HOMA and fat percentage and vigorous physical activity was associated with HOMA, fat percentage, sum of skinfolds, waist circumference and total: HDL cholesterol ratio. Vigorous physical activity was inversely associated with metabolic syndrome score (B = −0.51, 95% CI −0.94, −0.08).

Conclusions

Volume and intensity of physical activity in early adolescence were negatively associated with metabolic and anthropometric risk factors. Interventions that aim to increase adolescent physical activity, especially vigorous, may prevent cardiometabolic disease in later life.

Association of BMI category with change in children's physical activity between ages 6 and 11 years: a longitudinal study

BACKGROUND/OBJECTIVES

To examine the association of body mass index (BMI) with change in children's physical activity and sedentary time between ages 6 and 11.

PARTICIPANTS

A total of 2132 children participated from 57 schools in Southwest England, from the B-PROACT1V study.

METHODS

Mean minutes of MVPA and sedentary time per day were derived from accelerometer-based measurements at ages 6, 9 and 11. Linear multilevel models examined the association of BMI categories with MVPA and sedentary time between 6 and 11, adjusting for seasonality, wear time, gender and household education. Differences in change over time were examined using interaction terms.

RESULTS

Average weekday MVPA decreased between ages 6 and 11 by 2.2 min/day/year (95% CI: 1.9 to 2.5), with a steeper decline at weekends. Average sedentary time increased at a rate of 12.9 min/day/year (95% CI: 12.2 to 13.6). There were no differences in mean levels of MVPA by BMI categories at age 6, but differences emerged as children aged, with the gap between children who were healthy weight and overweight increasing by 1.7 min/day (95% CI: 0.8-2.6) every year, and between healthy and obese by 2.0 min/day (95% CI: 0.9-3.1) each year. Children who were overweight/obese engaged in less average weekday sedentary time at age 6 than those of healthy weight, but the gap closed by age 11.

CONCLUSION

MVPA declines and sedentary time increases on average for all children between ages 6 and 11. While there are no differences in activity levels by BMI category at age 6, differences in MVPA emerge over time for those who are overweight and obese. Developing interventions that support children to retain activity levels as they approach older childhood, particularly those who are overweight/obese could improve public health.

Physical Activity in the Prevention of Weight Gain: the Impact of Measurement and Interpretation of Associations

PURPOSE OF REVIEW

To what extent do different methods of physical activity measurement and statistical analysis influence the reported associations between physical activity and weight gain?

RECENT FINDINGS

The obesity epidemic has led to a focus on lifestyle approaches to the prevention of weight gain. Physical activity is one such approach. A number of studies have reported beneficial associations between higher levels of physical activity and weight gain at the population level. However, limitations of physical activity measurement and analytical models in some of these studies are likely to have resulted in overestimation of the strength of association. Understanding the limitations of assessment methods and analytical models used in epidemiological research should facilitate more realistic appraisal for physical activity to prevent weight gain at the population level and inform approaches to future research.

Associations of physical activity and sedentary time with body composition in Brazilian young adults

The findings of studies on the association between physical activity and adiposity are not consistent, and most are cross-sectional and used only self-reported measures. The aims of this study were to evaluate: 1) independent and combined cross-sectional associations of objectively-measured physical activity and sedentary time with body composition outcomes at 30 years, and 2) prospective associations of changes in self-reported physical activity from 23 to 30 years with the same outcomes in participants from the 1982 Pelotas (Brazil) Birth Cohort. Body mass index, waist circumference, visceral abdominal fat, fat mass index, and android/gynoid fat ratio were the outcomes. 3,206 participants were analysed. In cross-sectional analyses, higher objectively-measured moderate-to-vigorous physical activity was associated with lower body mass index (β = 0.017, 95%CI: −0.026; −0.009), waist circumference (β = −0.043, 95%CI: −0.061; −0.025), visceral abdominal fat (β = −0.006, 95%CI: −0.009; −0.003), and fat mass index (β = −0.015, 95%CI: −0.021; −0.009), independent of sedentary time. Sedentary time was independently associated only with higher fat mass index (β = 0.003, 95%CI: 0.001; 0.005). In longitudinal analyses, using self-reported measure, adiposity was lower among those who were consistently active or who became active. Adiposity was similar among the “became inactive” and “consistently inactive” subjects. Our findings suggest metabolic benefits from engagement in physical activity throughout young adulthood, with stronger associations on concurrent levels.

Metabolomic correlates of central adiposity and earlier-life body mass index

BMI is correlated with circulating metabolites, but few studies discuss other adiposity measures, and little is known about metabolomic correlates of BMI from early life. We investigated associations between different adiposity measures, BMI from childhood through adulthood, and metabolites quantified from serum using ¹H NMR spectroscopy in 900 British men and women aged 60-64. We assessed BMI, waist-to-hip ratio (WHR), android-to-gynoid fat ratio (AGR), and BMI from childhood through adulthood. Linear regression with Bonferroni adjustment was performed to assess adiposity and metabolites. Of 233 metabolites, 168; 126; and 133 were associated with BMI, WHR, and AGR at age 60-64, respectively. Associations were strongest for HDL, particularly HDL particle size-e.g., there was 0.08 SD decrease in HDL diameter (95% CI: 0.07-0.10) with each unit increase in BMI. BMI-adjusted AGR or WHR were associated with 31 metabolites where there was no metabolome-wide association with BMI. We identified inverse associations between BMI at age 7 and glucose or glycoprotein at age 60-64 and relatively large LDL cholesteryl ester with postadolescent BMI gains. In summary, we identified metabolomic correlates of central adiposity and earlier-life BMI. These findings support opportunities to leverage metabolomics in early prevention of cardiovascular risk attributable to body fatness.

Searching for the causal effects of body mass index in over 300 000 participants in UK Biobank, using Mendelian randomization

Mendelian randomization (MR) has been used to estimate the causal effect of body mass index (BMI) on particular traits thought to be affected by BMI. However, BMI may also be a modifiable, causal risk factor for outcomes where there is no prior reason to suggest that a causal effect exists. We performed a MR phenome-wide association study (MR-pheWAS) to search for the causal effects of BMI in UK Biobank (n = 334 968), using the PHESANT open-source phenome scan tool. A subset of identified associations were followed up with a formal two-stage instrumental variable analysis in UK Biobank, to estimate the causal effect of BMI on these phenotypes. Of the 22 922 tests performed, our MR-pheWAS identified 587 associations below a stringent P value threshold corresponding to a 5% estimated false discovery rate. These included many previously identified causal effects, for instance, an adverse effect of higher BMI on risk of diabetes and hypertension. We also identified several novel effects, including protective effects of higher BMI on a set of psychosocial traits, identified initially in our preliminary MR-pheWAS in circa 115,000 UK Biobank participants and replicated in a different subset of circa 223,000 UK Biobank participants. Our comprehensive MR-pheWAS identified potential causal effects of BMI on a large and diverse set of phenotypes. This included both previously identified causal effects, and novel effects such as a protective effect of higher BMI on feelings of nervousness.

Associations between physical activity and asthma, eczema and obesity in children aged 12-16: an observational cohort study

OBJECTIVES

To compare the physical activity of adolescents with three common long-term conditions (asthma, eczema and obesity) with adolescents without these conditions.

DESIGN

Cross-sectional and longitudinal analyses of adolescents at ages 12, 14 and 16 in a large UK cohort study.

SETTING

The Avon Longitudinal Study of Parents and Children.

PARTICIPANTS

6473 adolescents with complete accelerometer data at at least one time point.

METHODS

Mean minutes of moderate to vigorous intensity physical activity (MVPA) and sedentary time per day were derived from accelerometer-based measurements at ages 12, 14 and 16. Obesity was defined at each time point from height and weight measurements. Parents reported doctor-assessed asthma or eczema. Cross-sectional and longitudinal regression models examined any differences in MVPA or sedentary time for adolescents with asthma, eczema or obesity compared with those without.

RESULTS

In longitudinal models, boys engaged in an average of 69.7 (95% CI 67.6 to 71.7) min MVPA at age 12, declining by 3.1 (95% CI 2.6 to 3.6) min/year while girls' average MVPA was 47.5 (95% CI 46.1 to 48.9) min at age 12, declining by 1.8 (95% CI 1.5 to 2.1) min/year. There was no strong evidence of differences in physical activity patterns of those with and without asthma or eczema. Obese boys engaged in 11.1 (95% CI 8.7 to 13.6) fewer minutes of MVPA, and obese girls in 5.0 (95% CI 3.3 to 6.8) fewer minutes than their non-obese counterparts. Cross-sectional models showed comparable findings.

CONCLUSIONS

Mean minutes of MVPA per day did not differ between adolescents with asthma or eczema and those without, but obese adolescents engaged in fewer minutes of MVPA. Findings reinforce the need for strategies to help obese adolescents be more active but suggest no need to develop bespoke physical activity strategies for adolescents with mild asthma or eczema.

Biological determinants of physical activity across the life course: a "Determinants of Diet and Physical Activity" (DEDIPAC) umbrella systematic literature review

BACKGROUND

Despite the large number of studies and reviews available, the evidence regarding the biological determinants of physical activity (PA) is inconclusive. In this umbrella review, we summarized the current evidence on the biological determinants of PA across the life course, by pooling the results of the available systematic literature reviews (SLRs) and meta-analyses (MAs).

METHODS

We conducted an online search on MEDLINE, ISI Web of Science, Scopus, and SPORTDiscus databases up to January 2018. SLRs and MAs of observational studies that investigated the association between biological determinants of PA and having PA as outcome were considered eligible. The extracted data were assessed based on the importance of the determinants, the strength of evidence, and the methodological quality.

RESULTS

We identified 19 reviews of which most were of moderate methodological quality. Determinants that were studied most frequently among all ages and demonstrated evidence suggesting a positive association to PA were younger age, being male, higher health status, and higher physical fitness levels. Among adults, normal birth weight was found to be positively associated to PA with convincing strength of evidence, while findings among adolescents were inconsistent and with limited strength of evidence.

CONCLUSIONS

Different social or behavioral factors may contribute to the decrease of PA with age and among females versus males, and creating programmes targeted at diverse ages, female population, and adults with abnormal birth weight is recommended. Future studies should use prospective study designs, standardized definitions of PA, and objective measurement methods of PA assessment.