Statistical issues in life course epidemiology

Direction of associations between added sugar intake in early childhood and body mass index at age 7 years may depend on intake levels

Dietary factors, especially during early childhood, have been discussed as potentially critical for the development of childhood overweight. This study evaluated associations between added sugar intake during early childhood and BMI and body fat at age 7 y. Analysis was based on data from 216 participants of the Dortmund Nutritional and Anthropometric Longitudinally Designed (DONALD) Study. Life-course plots were constructed to evaluate the association between added sugar intake at different ages (0.5, 1, 1.5, and 2 y) and BMI SD score (BMI-SDS) and % body fat (%BF) at age 7 y. Multivariable analyses were performed for the periods identified as critical for later BMI and body fat. Added sugar intake at age 1 y and the change in intake levels during the second year of life emerged as potentially critical. At age 1 y, a higher total added sugar intake was related to a lower BMI-SDS at age 7 y [adjusted β ± SE: -0.116 ± 0.057 BMI-SDS/percent energy (%En) added sugar; P = 0.04]. Conversely, an increase in total added sugar in the second year of life (Δ%En between age 1 and 2 y) tended to be associated with a higher BMI-SDS (adjusted β ± SE: 0.074 ± 0.043 BMI-SDS/Δ%En added sugar; P = 0.09). No associations were observed with %BF. In conclusion, added sugar intake at low intake levels during early childhood does not appear to be critical for BMI and body fat at age 7 y. However, detrimental effects on BMI development may emerge when added sugar intakes are increased to higher levels.

Maternal serum lipids during pregnancy and infant birth weight: the influence of prepregnancy BMI

Maternal obesity may be associated with metabolic factors that affect the intrauterine environment, fetal growth, and the offspring's long-term risk for chronic disease. Among these factors, maternal serum lipids play a particularly important role. Our objective was to estimate the influence of variation in maternal serum lipid levels on variation in infant birth weight (BW) in overweight/obese and normal weight women. In a prospective cohort of 143 gravidas, we measured maternal serum levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglycerides (TG) at 6-10, 10-14, 16-20, 22-26, and 32-36 weeks gestation. Effects of maternal serum lipid levels on infant BW adjusted for gestational age at delivery (aBW) were analyzed using linear regression models. In analyses stratified by maternal prepregnancy BMI categorized as normal (≤25.0 kg/m(2)) and overweight/obese (>25.0 kg/m(2)), we found a significant (P < 0.05) inverse association between aBW and HDL-C at all time points starting at 10 weeks gestation in overweight/obese women. No significant effect was found in normal weight women. In contrast, increased maternal serum TG was significantly associated with increased aBW only for normal weight women at 10-14 and 22-26 weeks gestation. Variation in aBW is not associated with variation in maternal serum TC or LDL-C for either stratum at any time point. We postulate that such differences may be involved in the "physiological programming" that influences later risk of chronic disease in the infants of overweight/obese mothers.

Association of changes in body mass index during earlier adulthood and later adulthood with circulating obesity biomarker concentrations in middle-aged men and women

AIMS/HYPOTHESIS

The objective of our study was to investigate whether changes in BMI during earlier adulthood are more strongly associated with levels of circulating obesity biomarkers in middle age than are BMI changes during later adulthood.

METHODS

The study included 1,612 participants from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam Study. The associations of BMI changes based on recalled BMI for the age ranges 25-40 years (earlier adulthood) and 40-55 years (later adulthood) with later biomarker levels were compared using a linear model, adjusted for BMI at age 25 years and conventional risk factors.

RESULTS

BMI changes during both time periods as well as BMI at age 25 years were significantly associated with circulating levels of adiponectin, γ-glutamyltransferase (GGT), alanine aminotransferase (ALT), high-sensitivity C-reactive protein (hs-CRP) and HDL-cholesterol (HDL-C) in both sexes, and of HbA(1c) in women. However, BMI gain for the age range 25-40 years was significantly more strongly associated with unfavourable levels of adiponectin, hs-CRP, HDL-C and HbA(1c) in men and women, and of GGT and ALT in men (p difference <0.05) than BMI gain for the age range 40-55 years. The percentage change in biomarker levels per unit gain in BMI for the age range 25-40 years ranged from 0.81% (HbA(1c)) to 9.80% (hs-CRP) in men, and from 0.75% (HbA(1c)) to 14.7% (hs-CRP) in women, whereas for the age range 40-55 years, values ranged from -0.15% to 4.82% in men and from 0.25% to 7.06% in women.

CONCLUSIONS/INTERPRETATION

The results support the hypothesis that an increase in BMI in earlier adulthood is more strongly associated with unfavourable circulating levels of obesity biomarkers later in life than is an increase in BMI in later adulthood.

Health differentials in the older population of England: an empirical comparison of the materialist, lifestyle and psychosocial hypotheses

BACKGROUND

In developed countries with old age structures most deaths occur at older ages and older people account for the majority of those in poor health, which suggests a particular need to investigate health inequalities in the older population.

METHODS

We empirically compared the materialist, psychosocial and lifestyle/behavioural theoretical mechanisms of explanation for socio-economic variation in health using data from two waves of the English Longitudinal Study of Ageing (ELSA), a nationally representative multi-purpose sample of the population aged 50 and over living in England. Three dimensions of health were examined: somatic health, depression and well-being.

RESULTS

The materialist and lifestyle/behavioural paths had the most prominent mediating role in the association between socio-economic position and health in the older population, whereas the psychosocial pathway was less influential and exerted most of its influence on depression and well-being, with part of its effect being due to the availability of material resources.

CONCLUSIONS

From a policy perspective there is therefore an indication that population interventions to reduce health differentials and thus improve the overall health of the older population should focus on material circumstances and population based interventions to promote healthy lifestyles.

Climate change and climate variability: personal motivation for adaptation and mitigation

BACKGROUND

Global climate change impacts on human and natural systems are predicted to be severe, far reaching, and to affect the most physically and economically vulnerable disproportionately. Society can respond to these threats through two strategies: mitigation and adaptation. Industry, commerce, and government play indispensable roles in these actions but so do individuals, if they are receptive to behavior change. We explored whether the health frame can be used as a context to motivate behavioral reductions of greenhouse gas emissions and adaptation measures.

METHODS

In 2008, we conducted a cross-sectional survey in the United States using random digit dialing. Personal relevance of climate change from health threats was explored with the Health Belief Model (HBM) as a conceptual frame and analyzed through logistic regressions and path analysis.

RESULTS

Of 771 individuals surveyed, 81% (n = 622) acknowledged that climate change was occurring, and were aware of the associated ecologic and human health risks. Respondents reported reduced energy consumption if they believed climate change could affect their way of life (perceived susceptibility), Odds Ratio (OR) = 2.4 (95% Confidence Interval (CI): 1.4-4.0), endanger their life (perceived severity), OR = 1.9 (95% CI: 1.1-3.1), or saw serious barriers to protecting themselves from climate change, OR = 2.1 (95% CI: 1.2-3.5). Perceived susceptibility had the strongest effect on reduced energy consumption, either directly or indirectly via perceived severity. Those that reported having the necessary information to prepare for climate change impacts were more likely to have an emergency kit OR = 2.1 (95% CI: 1.4-3.1) or plan, OR = 2.2 (95% CI: 1.5-3.2) for their household, but also saw serious barriers to protecting themselves from climate change or climate variability, either by having an emergency kit OR = 1.6 (95% CI: 1.1-2.4) or an emergency plan OR = 1.5 (95%CI: 1.0-2.2).

CONCLUSIONS

Motivation for voluntary mitigation is mostly dependent on perceived susceptibility to threats and severity of climate change or climate variability impacts, whereas adaptation is largely dependent on the availability of information relevant to climate change. Thus, the climate change discourse could be framed from a health perspective to motivate behaviour change.

The ovarian dysgenesis syndrome

New thinking has arisen about the origin of adult onset diseases stemming from a collective body of evidence commonly referred to as the developmental origins of health and disease. This conceptual paradigm posits that certain adult onset diseases arise during critical or sensitive windows of human development or even transgenerationally. The testicular dysgenesis hypothesis (TDS) postulates an in utero origin for adverse male reproductive outcomes, and is an excellent example of the early origins of the paradigm. Despite similarities in the development of the male and female reproductive tracks, noticeably absent is a collective body of evidence focusing on the plausibility of an early origin for gynecologic outcomes and later onset of adult diseases. Using the TDS paradigm, we synthesized the available literature relative to the ovarian dysgenesis syndrome (ODS), which we define as alterations in ovarian structure or function that may manifest as fecundity impairments, gynecologic disorders, gravid diseases or later onset adult diseases. We evaluated environmental exposures, particularly the role of endocrine disrupting chemicals, in relation to these outcomes, and found evidence (although fragmented) consistent with an in utero origin of gynecologic outcomes, which in turn is associated with later onset of adult diseases. The findings are interpreted within the ODS paradigm while delineating methodological challenges and future research opportunities designed to answer critical data gaps regarding the origin of fecundity, gravid health and chronic diseases affecting the female population.

Infant mortality and the health of survivors: Britain, 1910–50

The first half of the twentieth century saw rapid improvements in the health and height of British children. Average height and health can be related to infant mortality through a positive selection effect and a negative scarring effect. Examining town-level panel data on the heights of school children, no evidence is found for the selection effect, but there is some support for the scarring effect. The results suggest that the improvement in the disease environment, as reflected by the decline in infant mortality, increased average height by about half a centimetre per decade in the first half of the twentieth century.

Structural equation modeling in epidemiology

Structural equation modeling (SEM) is an important statistical tool for evaluating complex relations in several research areas. In epidemiology, the use and discussion of SEM have been limited thus far. This article presents basic principles and concepts in SEM, including an application using epidemiological data analysis from a study on the determinants of cognitive development in young children, considering constructs related to organization of the child's home environment, parenting style, and the child's health status. The relations between the constructs and cognitive development were measured. The results showed a positive association between psychosocial stimulus at home and cognitive development in young children. The article presents the contributions by SEM to epidemiology, highlighting the need for an a priori theoretical model for improving the study of epidemiological questions from a new perspective.

The effects of maternal weight gain patterns on term birth weight in African-American women

OBJECTIVE

The goals of our study were (1) to estimate the trends in maternal weight gain patterns and (2) to estimate the influence of variation in maternal weight and rate of weight gain over different time periods in gestation on variation in birth weight in African-American and non-African-American gravidas.

STUDY DESIGN AND SETTING

Data from a prospective cohort study in which pregnant women were monitored at multiple time points during pregnancy were analysed. Maternal weight was measured at three times during pregnancy: preconception (W(0)); 16-20 weeks gestation (W(1)); 30-36 weeks gestation (W(2)), in a cohort of 435 women with full-term singleton pregnancies. The relationship between gestational age-adjusted birth weight (aBW) and measures of maternal weight and rate of weight gain across pregnancy was estimated using a multivariable longitudinal regression analysis stratified on African-American race.

RESULTS

The aBW was significantly associated with maternal weight measured at any visit in both strata. For African-American women, variation in aBW was significantly associated with variation in the rate of maternal weight gain in the first half of pregnancy (W(01)) but not the rate of maternal weight gain in the second half of pregnancy (W(12)); while for non-African-American women, variation in aBW was significantly associated with W(12) but not W(01).

CONCLUSION

Factors influencing the relationship between aBW and maternal weight gain patterns depend on the context of the pregnancy defined by race. Clinical decisions and recommendations about maternal weight and weight gain during pregnancy may need to account for such heterogeneity.

Predictors of early precocious talking: a prospective population study

This study examines potential predictors of 'precocious talking' (expressive language ≥90th percentile) at one and two years of age, and of 'stability' in precocious talking across both time periods, drawing on data from a prospective community cohort comprising over 1,800 children. Logistic regression was used to examine the relationship between precocious talking and the following potential predictors: gender, birth order, birth weight, non-English speaking background, socioeconomic status, maternal age, maternal mental health scores, and vocabulary and educational attainment of parents. The strongest predictors of precocity (being female and having a younger mother) warrant further exploration. Overall, however, it appears that precocity in early vocabulary development is not strongly influenced by the variables examined, which together explained just 2.6% and 1% of the variation at 1;0 and 2;0 respectively.

Risk factors for long-term absence due to psychiatric sickness: a register-based 5-year follow-up from the Oslo health study

OBJECTIVE

To identify individual and work-related predictors of long-term (>8 weeks) sickness absence with psychiatric diagnoses (LSP).

METHODS

Data from the Oslo Health Study (response rate 46%) were linked to public registers. A total of 8333 subjects were followed from 2001 through 2005. Cox regression was used to compute hazard ratios for LSP.

RESULTS

At least one LSP was present in 7.8% of women and 3.9% of men. Poor support from superior had an independent and moderate effect. Path and linear regression analyses indicated that the effect of support from superior was mediated through mental distress and not the other way around. Self-reported mental distress had a strong independent effect.

CONCLUSIONS

Women had a higher risk of LSP than men. Low education and poor support from superior and mental distress were found to be determinants of LSP.

Assessing the impact of body size in childhood and adolescence on blood pressure: an application of partial least squares regression

BACKGROUND

Recent studies have found that postnatal catch-up growth might have a stronger impact than birth size on health in later life. Because growth is a continuing process, the challenge is to tease out the impact of body size at different critical phases. Ordinary least squares regression cannot differentiate the effects of birth size, growth, and current body size simultaneously, because growth is generally defined as the difference between birth size and current size, giving rise to perfect collinearity.

METHODS

This paper aims to describe and justify the use of a novel approach, partial least squares, to estimate life course effects of body size z-scores on later blood pressure, using longitudinal data from a cohort of 960 Filipino boys. Body weight z-scores and changes in z-score weight were measured from birth to age 19 years, and systolic and diastolic blood pressures (BPs) were measured at age 19.

RESULTS

In general, birth size had a very modest association with systolic BP. The early changes in z-scores between birth and the age of 2 years were positively associated with the outcome. Growth after age 8 had a larger effect than early growth, but the confidence intervals are generally large. For diastolic BP, the association was similar for early and later growth. Current body size had the strongest relationships with both outcomes.

CONCLUSION

By creating weighted composites of perfectly collinear variables as components, partial least squares estimates the life course effects of body size on later health according to the correlations between body size and health outcomes.

Life-course analysis of a fat mass and obesity-associated (FTO) gene variant and body mass index in the Northern Finland Birth Cohort 1966 using structural equation modeling

The association between variation in the fat mass and obesity-associated (FTO) gene and adulthood body mass index (BMI; weight (kg)/height (m)²) is well-replicated. More thorough analyses utilizing phenotypic data over the life course may deepen our understanding of the development of BMI and thus help in the prevention of obesity. The authors used a structural equation modeling approach to explore the network of variables associated with BMI from the prenatal period to age 31 years (1965–1997) in 4,435 subjects from the Northern Finland Birth Cohort 1966. The use of structural equation modeling permitted the easy inclusion of variables with missing values in the analyses without separate imputation steps, as well as differentiation between direct and indirect effects. There was an association between the FTO single nucleotide polymorphism rs9939609 and BMI at age 31 years that persisted after controlling for several relevant factors during the life course. The total effect of the FTO variant on adult BMI was mostly composed of the direct effect, but a notable part was also arising indirectly via its effects on earlier BMI development. In addition to well-established genetic determinants, many life-course factors such as physical activity, in spite of not showing mediation or interaction, had a strong independent effect on BMI.

SITAR--a useful instrument for growth curve analysis

Background Growth curve analysis is a statistical issue in life course epidemiology. Height in puberty involves a growth spurt, the timing and intensity of which varies between individuals. Such data can be summarized with individual Preece–Baines (PB) curves, and their five parameters then related to earlier exposures or later outcomes. But it involves fitting many curves.

Methods We present an alternative SuperImposition by Translation And Rotation (SITAR) model, a shape invariant model with a single fitted curve. Curves for individuals are matched to the mean curve by shifting their curve up–down (representing differences in mean size) and left–right (for differences in growth tempo), and the age scale is also shrunk or stretched to indicate how fast time passes in the individual (i.e. velocity). These three parameters per individual are estimated as random effects while fitting the curve. The outcome is a mean curve plus triplets of parameters per individual (size, tempo and velocity) that summarize the individual growth patterns. The data are heights for Christ’s Hospital School (CHS) boys aged 9–19 years (N = 3245, n = 129 508), and girls with Turner syndrome (TS) aged 9–18 years from the UK Turner Study (N = 105, n = 1321).

Results The SITAR model explained 99% of the variance in both datasets [residual standard deviation (RSD) 6–7 mm], matching the fit of individually-fitted PB curves. In CHS, growth tempo was associated with insulin-like growth factor-1 measured 50 years later (P = 0.01, N = 1009). For the girls with TS randomized to receive oxandrolone from 9 years, velocity was substantially increased compared with placebo (P = 10⁻⁸).

Conclusions The SITAR growth curve model is a useful epidemiological instrument for the analysis of height in puberty.

Influence of early life factors on social inequalities in psychiatric outcomes among young adult Norwegian men

BACKGROUND

Social inequalities in health can to a substantial degree be explained by social causation. However, indirect selection by early life factors has been suggested. The main aim of this study was to estimate how much adult social gradients in selected psychiatric outcomes depended on parental and individual characteristics in early life.

METHODS

The population comprised all males born in Norway 1967-71 (n = 170 678). We compiled data on several social and biological variables from birth onwards from different national registers. Health outcomes were collected from the Cause of Death Register (suicide) and the Labour and Welfare Administration (psychiatric disability, psychiatric sickness absence). Indicator of socio-economic position was education level at the age of 28 years. Men were followed up between 4 and 9 years from the age of 29 years.

RESULTS

Crude rates per 100 000 person-years were 21.8 (suicide), 145.7 (disability) and 1164.7 (sickness absence). Social inequalities were strong and consistent for all outcomes. Parental and individual characteristics accounted for a substantial part of the social inequalities in neurosis or personality disorder disability (44.1%) and a moderate role for inequalities in psychiatric sickness absence (25.6%), schizophrenia disability (20.7%) and suicide (17.4%). General ability at the age of 18 years had strongest influence on the social health gradients. Suicide and schizophrenia disability were associated with a combination of high parental and low own education level.

CONCLUSION

This study indicates that indirect selection explains a substantial part of social inequalities in certain psychiatric outcomes and that early life prevention is important to reduce health gradients.

Growth patterns in early childhood and the onset of menarche before age twelve

OBJECTIVE: To examine the relationship between growth patterns in early childhood and the onset of menarche before age 12. METHODS: The study included 2,083 women from a birth cohort study conducted in the city of Pelotas, Southern Brazil, starting in 1982. Anthropometric, behavioral, and pregnancy-related variables were collected through home interviews. Statistical analyses were performed using Pearson's chi-square and chi-square test for linear trends. A multivariable analysis was carried out using Poisson regression based on a hierarchical model. RESULTS: Mean age of menarche was 12.4 years old and the prevalence of menarche before age 12 was 24.3%. Higher weight-for-age, height-for-age, and weight-for-height z-scores at 19.4 and 43.1 months of age were associated with linear tendencies of increased prevalence and relative risks of the onset of menarche before age 12. Girls who experienced rapid growth in weight-for-age z-score from birth to 19.4 months of age and in weight-for-age or height-for-age z-scores from 19.4 to 43.1 months of age also showed higher risk of menarche before age 12. Higher risk was seen when rapid growth in weight-for-age z-score was seen during these age intervals and the highest risk was found among those in the first tertile of Williams' curve at birth. Rapid growth in weight-for-height z-score was not associated with menarche before age 12. CONCLUSIONS: Menarche is affected by nutritional status and growth patterns during early childhood. Preventing overweight and obesity during early childhood and keeping a "normal" growth pattern seem crucial for the prevention of health conditions during adulthood.

Vitamin D status during pregnancy and aspects of offspring health

Low maternal vitamin D levels during pregnancy have been linked to various health outcomes in the offspring, ranging from periconceptional effects to diseases of adult onset. Maternal and infant cord 25(OH)D levels are highly correlated. Here, we review the available evidence for these adverse health effects. Most of the evidence has arisen from observational epidemiological studies, but randomized controlled trials are now underway. The evidence to date supports that women should be monitored and treated for vitamin D deficiency during pregnancy but optimal and upper limit serum 25(OH)D levels during pregnancy are not known.

A life course approach to reproductive health: theory and methods

Taking a life course approach to the study of reproductive health involves the investigation of factors across life and, also across generations, that influence the timing of menarche, fertility, pregnancy outcomes, gynaecological disorders, and age at menopause. It also recognises the important influence of reproductive health on chronic disease risk in later life. Published literature supports the use of an integrated life course approach to study reproductive health, which examines the whole life course, considers the continuity of reproductive health and the interrelationship between the different markers of this. This is in contrast to more traditional approaches that tend to focus only on contemporary risk factors and which consider each marker of reproductive health separately. For instance, we found evidence linking early life factors such as growth, socioeconomic conditions, and parental divorce with ages at menarche and menopause, although the nature of the relationship differs. We discuss the different theoretical models that are used within life course epidemiology and which postulate pathways linking exposures across the life course to health outcomes, using examples of relevance to the study of reproductive health. These highlight the importance of examining timing of exposures, such as during critical periods in early life, and the temporal order of exposures. How life course frameworks of reproductive health can be developed to help identify hypotheses to be tested is also demonstrated. This approach has implications for the development of effective health policy that moves beyond identifying not only the type of intervention but also the most appropriate time across life to intervene.

EDUCATIONAL ATTAINMENT OF NORWEGIAN MEN: INFLUENCE OF PARENTAL AND EARLY INDIVIDUAL CHARACTERISTICS

The life course perspective in social inequalities in health research has resulted in an increased interest in status attainment processes. Adult status is commonly measured as occupational class, income level or educational attainment, and the latter was applied in this study. The study objective was to estimate the relative contribution of parental and early individual characteristics on educational attainment. The study population comprised all males born in Norway in 1967–1971, and alive at age 28 years (n=160,914). Data on social and biological variables were compiled from birth onwards in several national registers. Information on educational attainment at age 28 years was derived from Statistics Norway. Mean years of education was 12.62 years (SD 2.24). Educational attainment was strongly associated with general ability score at age 18 years and parental educational attainment. Parental income had more limited influence; all other early factors had only marginal effect. Path analysis results suggest that the direct effect of general ability was of the same size as the combined direct and indirect effect of parental education and income. The results suggest that status attainment in this young male population is mainly dependent on general ability and parental education level.

Growth trajectories and intellectual abilities in young adulthood: The Helsinki Birth Cohort study

Slow childhood growth is associated with poorer intellectual ability. The critical periods of growth remain uncertain. Among 2,786 Finnish male military conscripts (1952-1972) born in 1934-1944, the authors tested how specific growth periods from birth to age 20 years predicted verbal, visuospatial, and arithmetic abilities at age 20. Small head circumference at birth predicted poorer verbal, visuospatial, and arithmetic abilities. The latter 2 measures were also associated with lower weight and body mass index (weight (kg)/height (m)(2)) at birth (for a 1-standard-deviation (SD) decrease in test score per SD decrease in body size > or = 0.05, P's < 0.04). Slow linear growth and weight gain between birth and age 6 months, between ages 6 months and 2 years, or both predicted poorer performance on all 3 tests (for a 1-SD decrease in test score per SD decrease in growth > or = 0.05, P's < 0.03). Reduced linear growth between ages 2 and 7 years predicted worse verbal ability, and between age 11 years and conscription it predicted worse performance on all 3 tests. Prenatal brain growth and linear growth up to 2 years after birth form a first critical period for intellectual development. There is a second critical period, specific for verbal development, between ages 2 and 7 years and a third critical period for all 3 tested outcomes during adolescence.

Correcting for multivariate measurement error by regression calibration in meta-analyses of epidemiological studies

Within-person variability in measured values of multiple risk factors can bias their associations with disease. The multivariate regression calibration (RC) approach can correct for such measurement error and has been applied to studies in which true values or independent repeat measurements of the risk factors are observed on a subsample. We extend the multivariate RC techniques to a meta-analysis framework where multiple studies provide independent repeat measurements and information on disease outcome. We consider the cases where some or all studies have repeat measurements, and compare study-specific, averaged and empirical Bayes estimates of RC parameters. Additionally, we allow for binary covariates (e.g. smoking status) and for uncertainty and time trends in the measurement error corrections. Our methods are illustrated using a subset of individual participant data from prospective long-term studies in the Fibrinogen Studies Collaboration to assess the relationship between usual levels of plasma fibrinogen and the risk of coronary heart disease, allowing for measurement error in plasma fibrinogen and several confounders.

Life course path analysis of birth weight, childhood growth, and adult systolic blood pressure

The inverse associations between birth weight and later adverse health outcomes and the positive associations between adult body size and poor health imply that increases in relative body size between birth and adulthood may be undesirable. In this paper, the authors describe life course path analysis, a method that can be used to jointly estimate associations between body sizes at different time points and associations of body sizes throughout life with health outcomes. Additionally, this method makes it possible to assess both the direct effect and the indirect effect mediated through later body size, and thereby the total effect, of size and changes in size on later outcomes. Using data on childhood body size and adult systolic blood pressure from a sample of 1,284 Danish men born between 1936 and 1970, the authors compared results from path analysis with results from 3 standard regression methods. Path analysis produced easily interpretable results, and compared with standard regression methods it produced a noteworthy gain in statistical power. The effect of change in relative body size on adult blood pressure was more pronounced after age 11 years than in earlier childhood. These results suggest that increases in body size prior to age 11 years are less harmful to adult blood pressure than increases occurring after this age.

Associations between birthweight and weight change during infancy and later childhood, and systolic blood pressure at age 15 years: the COMPASS study

Inconsistent findings have been reported on associations between postnatal weight change with later blood pressure. This study analysed the impact of weight change during fetal life, infancy and later childhood on systolic blood pressure (SBP) in adolescence. A longitudinal population-based study was conducted in Stockholm, Sweden. Weight, length at birth and gestational age were extracted from the Medical Birth Register, weight and height in childhood was collected from child health records, and body size and SBP were measured during a health examination at 15 years of age. Complete data on 2438 individuals were analysed using linear regression. In fully adjusted models SBP increased 0.99 mmHg [95% confidence interval (CI), 0.27, 1.71] in girls and 1.02 mmHg [0.31, 1.73] in boys per 1 SD increase in weight during the first year of life. The effect on SBP of weight change from 10 to 15 years among boys was 3.70 mmHg [2.67, 4.73] and among girls 1.30 mmHg [0.38, 2.22] per 1 SD increase in weight. Weight gain in infancy and later childhood was positively associated with SBP at age 15 years, while birthweight showed a weaker inverse association with SBP. No apparent effect of weight gain from 1 to 10 years of age was seen. Degree of weight change in infancy was equally important for SBP in both boys and girls, while growth during puberty showed a stronger association with SBP among boys than among girls.

Decline in physical fitness from childhood to adulthood associated with increased obesity and insulin resistance in adults

OBJECTIVE

To examine how fitness in both childhood and adulthood is associated with adult obesity and insulin resistance.

RESEARCH DESIGN AND METHODS

A prospective cohort study set in Australia in 2004-2006 followed up a cohort of 647 adults who had participated in the Australian Schools Health and Fitness Survey in 1985 and who had undergone anthropometry and cardiorespiratory fitness assessment during the survey. Outcome measures were insulin resistance and obesity, defined as a homeostasis model assessment index above the 75th sex-specific percentile and BMI >or=30 kg/m(2), respectively.

RESULTS

Lower levels of child cardiorespiratory fitness were associated with increased odds of adult obesity (adjusted odds ratio [OR] per unit decrease 3.0 [95% CI 1.6-5.6]) and insulin resistance (1.7 [1.1-2.6]). A decline in fitness level between childhood and adulthood was associated with increased obesity (4.5 [2.6-7.7]) and insulin resistance (2.1 [1.5-2.9]) per unit decline.

CONCLUSIONS

A decline in fitness from childhood to adulthood, and by inference a decline in physical activity, is associated with obesity and insulin resistance in adulthood. Programs aimed at maintaining high childhood physical activity levels into adulthood may have potential for reducing the burden of obesity and type 2 diabetes in adults.

A structural equation model of the developmental origins of blood pressure

BACKGROUND

Birth-size is a problematic proxy for the fetal environment, and regression models testing for associations between birth-size and blood pressure have been criticized.

METHODS

We modelled fetal environment as a latent variable determined by maternal height and arm fat area (AFA) during pregnancy using structural equation modelling. We tested for associations between latent fetal environment (LFE) and systolic blood pressure (SBP) while controlling for birth weight (BW) and current weight (CW). Data are from 1435 male and 1218 female young adult Filipinos (2005; mean age 21 years) enrolled in the Cebu Longitudinal Heath and Nutrition Survey, an ongoing, community-based study of a one-year birth cohort. Using AMOS 6.0, LFE was modelled as a determinant of BW, CW and SBP; CW was modelled as a determinant of SBP.

RESULTS

Overall model fit was excellent (chi2: 32.14, 27 df, P=0.23). The estimated direct relationship between LFE and SBP was inverse for both males ((-0.43) -0.26 (-0.10)) and females ((-0.29) -0.18 (-0.07)).

CONCLUSIONS

These results are consistent with the hypothesis that maternal height and AFA impact fetal development in a manner that is positively associated with fetal growth (as reflected by BW) and inversely associated with SBP in young adulthood.

The association of pediatric low- and high-density lipoprotein cholesterol dyslipidemia classifications and change in dyslipidemia status with carotid intima-media thickness in adulthood evidence from the cardiovascular risk in Young Finns study, the Bogalusa Heart study, and the CDAH (Childhood Determinants of Adult Health) study

OBJECTIVES

This study was designed to determine which of the National Cholesterol Education Program or National Health and Nutrition Examination Survey low- and high-density lipoprotein cholesterol classifications of dyslipidemia status in adolescents is most effective at predicting high common carotid artery intima-media thickness (IMT) in adulthood.

BACKGROUND

Two classifications of pediatric dyslipidemia status have been proposed. No study has assessed which of these is most effective for predicting adolescents who will develop preclinical atherosclerosis in adulthood.

METHODS

Three population-based, prospective cohort studies collected lipoprotein measurements on 1,711 adolescents age 12 to 18 years who were remeasured as young adults age 29 to 39 years. Lipoproteins in adolescence were classified according to National Cholesterol Education Program and National Health and Nutrition Examination Survey cut points, and high IMT in adulthood was defined as those at or above the age-, sex-, race-, and cohort-specific 90th percentile of IMT.

RESULTS

Independent of the classification employed, adolescents with dyslipidemia were at significantly increased risk of having high IMT in adulthood (relative risks from 1.6 to 2.5). Differences in predictive capacity between both classifications were minimal. Overweight or obese adolescents with dyslipidemia had increased carotid IMT (males: 0.11 mm; females: 0.08 mm) in adulthood compared with those who did not have both risk factors. Adolescent dyslipidemia status was more strongly associated with high IMT in adulthood than change in dyslipidemia status.

CONCLUSIONS

Pediatric dyslipidemia classifications perform equally in the prediction of adolescents who are at increased risk of high IMT in young adulthood. Our data suggest that dyslipidemia screening could be limited to overweight or obese adolescents.

A comparison of egg consumption with other modifiable coronary heart disease lifestyle risk factors: a relative risk apportionment study

Guidelines from the American Heart Association (AHA) recommend that healthy adults limit their intake of dietary cholesterol to less than 300 mg per day. Since a large egg contains about 71% of that amount, the AHA recommends restricting egg consumption unless dietary cholesterol intakes from other sources are limited. We applied a risk apportionment approach to estimate the contribution of egg consumption and other modifiable lifestyle risk factors (e.g., smoking, poor diet, minimal exercise, and alcohol intake) to coronary heart disease (CHD) risk at the population level. Specifically, we categorized the U.S. adult population ages 25+ into distinct risk groups based on the prevalence of modifiable lifestyle risk factors and applied an apportionment model, typically used to assess risk contribution at the individual level, to estimate the contribution of egg intake to CHD risk. Our analysis shows that the combination of modifiable lifestyle risk factors accounts for less than 40% of the population CHD mortality. For the majority of U.S. adults age 25+, consuming one egg a day accounts for <1% of CHD risk. Hence, focusing on decreasing egg intake as an approach to modify CHD risk would be expected to yield minimal results relative to changing other behaviors such as smoking and other dietary habits.

The longitudinal effects of physical activity history on metabolic syndrome

PURPOSE

To examine the relationship of physical activity and its changes over a 9-yr follow-up to the prevalence of metabolic syndrome (MetS) in 2060 young adults (24-39 yr) enrolled in the Cardiovascular Risk in Young Finns Study.

METHODS

Leisure-time physical activity (LTPA) was assessed using a self-report questionnaire completed in connection with a medical examination at two consecutive measurements in 1992 and 2001. By summing the LTPA items, a physical activity index (PAI) was formed for both measurement points according to which the participants were divided into tracking groups: persistently active, increasingly active, decreasingly active, and persistently inactive. MetS in 2001 was defined by the guidelines of the European Group for the Study of Insulin Resistance (EGIR), the National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP III), and the International Diabetes Federation (IDF). A continuous metabolic risk score was also calculated by summing the z-scores for the metabolic risk factors.

RESULTS

There was a significant linear relationship between MetS and LTPA at baseline in men and at follow-up in both sexes according to all three definitions. Persistent physical activity during 9 yr was associated with a lower prevalence of MetS than persistent physical inactivity on all definitions (all P < 0.05). Similar results were found for increasingly active women (all P < 0.05). All of these associations remained significant after adjustment for potential confounders. In both men and women, the 9-yr change in LTPA was related to the metabolic risk score after adjustments for baseline LTPA, age, smoking, and education.

CONCLUSIONS

A physically active lifestyle across the lifespan may prevent or delay the onset of metabolic syndrome in young male and female adults.

A structured approach to modelling the effects of binary exposure variables over the life course

BACKGROUND

There is growing interest in the relationship between time spent in adverse circumstances across life course and increased risk of chronic disease and early mortality. This accumulation hypothesis is usually tested by summing indicators of binary variables across the life span to form an overall score that is then used as the exposure in regression models for health outcomes. This article highlights potential issues in the interpretation of results obtained from such an approach.

METHODS

We propose a model-building framework that can be used to formally compare alternative hypotheses on the effect of multiple binary exposure measurements collected across the life course. The saturated model where the order and value of the binary variable at each time point influence the outcome of interest is compared with nested alternative specifications corresponding to the critical period, cumulative risk or hypotheses about the effect of changes in environment. This framework is illustrated with data on adult body mass index and socioeconomic position measured once in childhood and twice in adulthood from the Medical Research Council National Survey of Health and Development, using a series of liner regression models.

RESULTS

We demonstrate how analyses that only consider the association of a cumulative score with a later outcome may produce misleading results.

CONCLUSION

We recommend comparing a set of nested models-each corresponding to the accumulation, critical period and effect modification hypotheses-to an all-inclusive (saturated) model. This approach can provide a formal and clearer understanding of the relative merits of these alternative hypotheses.

Early childhood aetiology of mental health problems: a longitudinal population-based study

BACKGROUND

Mental health problems comprise an international public health issue affecting up to 20% of children and show considerable stability. We aimed to identify child, parenting, and family predictors from infancy in the development of externalising and internalising behaviour problems by age 3 years.

METHODS

Design Longitudinal, population-based survey completed by primary caregivers when children were 7, 12, 18, 24 and 36 months old. Participants 733 children sequentially recruited at 6-7 months from routine well-child appointments (August-September 2004) across six socio-economically and culturally diverse government areas in Victoria, Australia; 589 (80%) retained at 3 years. Measures 7 months: sociodemographic characteristics, maternal mental health (Depression Anxiety Stress Scale (DASS)), substance misuse, home violence, social isolation, infant temperament; 12 months: partner relationship, parenting (Parent Behavior Checklist (PBC)); 18, 24 and 36 months: child behaviour (Child Behavior Checklist 1(1/2)-5 (CBCL)), PBC, DASS.

RESULTS

Sixty-nine percent of all families attending well-child clinics took part. The consistent and cumulative predictors of externalising behaviours were parent stress and harsh discipline. Predictors of internalising behaviours included small family size, parent distress, and parenting. Twenty-five percent of variation in early externalising behaviour and 17% of variation in early internalising behaviour was explained.

CONCLUSIONS

Effective and cost-efficient population approaches to preventing mental health problems early in childhood are urgently needed. Programmes must support parents in reducing personal stress as well as negative parenting practices.

Dose-response associations between maternal smoking during pregnancy and subsequent childhood obesity: effect modification by maternal race/ethnicity in a low-income US cohort

Studies suggest that children exposed to cigarette smoke in utero are at risk of becoming obese. Few researchers have evaluated the dose-response association between maternal smoking during pregnancy and childhood obesity or whether this association varies by maternal race/ethnicity. The authors obtained retrospective cohort data by linking records from the Pregnancy Nutrition Surveillance System and the Pediatric Nutrition Surveillance System on 155,411 low-income children born during 1995-2001 in 9 US states and 2 tribal nations. The authors examined maternal smoking status, duration of smoking, quantity of smoking, and both duration and quantity combined. Childhood obesity was based on a body mass index greater than or equal to the 95th percentile for sex and age, assessed at age 2-4 years. Maternal race/ethnicity modified the association between smoking during pregnancy and childhood obesity. Among non-Hispanic White mothers, both duration and quantity of smoking were positively associated with childhood obesity in a dose-response manner. Among non-Hispanic Black mothers, only heavy smoking was positively associated with childhood obesity. Among Hispanics, American Indians/Alaska Natives, and Asians/Pacific Islanders, smoking was not associated with childhood obesity. The inconsistent association between smoking during pregnancy and childhood obesity across race/ethnicity categories merits further investigation into potential explanations for this variation, which may include confounding, reporting bias, or unexplored biologic mechanisms.

Adiposity gain during childhood, ACE I/D polymorphisms and metabolic outcomes

We aimed to (i) determine the relative importance of childhood gain in upper body adiposity for insulin resistance (IR) and triglyceridemia (TG); (ii) examine whether the associations between adiposity and metabolic indices were more evident in those with the ACE DD genotype. We examined a birth cohort study of 292 children with measures in the neonatal period (day 4) including subscapular and triceps skinfolds; repeat skinfold measures at age 8, cardiorespiratory (CR) fitness, IR by the homeostasis model assessment (HOMA) equation (HOMA-IR) and serum triglyceride (TG) concentrations and measures of ACE I/D gene variants. A multiple linear regression analysis incorporating a life course approach was undertaken. Childhood gain in upper body adiposity was positively associated with HOMA-IR and TG independently of neonatal skinfolds (P < or = 0.02). The magnitude of these associations was higher among those of the ACE DD genotype. For example, subscapular skinfold gain was not strongly associated with HOMA-IR or TG among those with II or ID genotype (b = 0.03, P = 0.05; b = 0.02, P = 0.18 respectively) but was positively associated among those with the DD genotype (b = 0.11, P = 0.001; b = 0.08, P = 0.003); difference in effect P = 0.05; P = 0.01 respectively. Upper body fat accumulation during childhood was positively associated with HOMA-IR and TG independently of neonatal skinfolds. Further, the stronger associations for those with the ACE DD genotype is consistent with randomised controlled trial findings that ACE inhibition is associated with a reduced risk of developing type 2 diabetes. Further work is required to confirm and extend these findings.

Use of multiple imputation in the epidemiologic literature

The authors attempted to catalog the use of procedures to impute missing data in the epidemiologic literature and to determine the degree to which imputed results differed in practice from unimputed results. The full text of articles published in 2005 and 2006 in four leading epidemiologic journals was searched for the text imput. Sixteen articles utilizing multiple imputation, inverse probability weighting, or the expectation-maximization algorithm to impute missing data were found. The small number of relevant manuscripts and diversity of detail provided precluded systematic analysis of the use of imputation procedures. To form a bridge between current and future practice, the authors suggest details that should be included in articles that utilize these procedures.

Prospective study of breast-feeding in relation to wheeze, atopy, and bronchial hyperresponsiveness in the Avon Longitudinal Study of Parents and Children (ALSPAC)

BACKGROUND

Breast-feeding clearly protects against early wheezing, but recent data suggest that it might increase later risk of atopic disease and asthma.

OBJECTIVE

We sought to examine the relationship between breast-feeding and later asthma and allergy outcomes by using data from the Avon Longitudinal Study of Parents and Children, a large birth cohort in the United Kingdom.

METHODS

We used adjusted logistic regression models to evaluate the association between breast-feeding and atopy at age 7 years, bronchial responsiveness to methacholine at age 8 years, and wheeze at ages 3 and 7 1/2 years. Bayesian methods were used to assess the possibility of bias caused by an influence of early wheezing on the duration of breast-feeding, as well as selection bias.

RESULTS

Breast-feeding was protective for wheeze in the first 3 years of life (odds ratio [OR] of 0.80 [95% CI, 0.70-0.90] for > or = 6 months relative to never) but not wheeze (OR, 0.98; 95% CI, 0.79-1.22), atopy (OR, 1.12; 95% CI, 0.92-1.35), or bronchial hyperresponsiveness (OR, 1.07; 95% CI, 0.82-1.40) at ages 7 to 8 years. Bayesian models adjusting for the longer duration of breast-feeding among children with wheezing in early infancy produced virtually identical results.

CONCLUSIONS

We did not find consistent evidence for either a deleterious effect or a protective effect of breast-feeding on later risk of allergic disease in a large prospective birth cohort of children with objective outcome measures and extensive data on potential confounders and effect modifiers. Neither reverse causation nor loss to follow-up appears to have materially biased our results.

Perspective: risk apportionment and disease intervention strategies

The ultimate public health objective is the ability to predict and prevent disease, and not necessarily to identify an exhaustive list of potential disease risk factors. For any important public health outcome with multiple and potentially interrelated risk factors, an improved understanding of the contribution of individual and combinations of modifiable risk factors to the disease burden is essential for formulating an appropriate public health strategy. Partitioning techniques that divide the combined impact of multiple risk factors into exposure-specific components while taking into account the potential interrelations among those components, have been described in the epidemiological literature. In this article, we review and compare the available methods and options for such apportionment and apply them in a more general public health context as a method of selecting and prioritizing coronary heart disease (CHD) prevention strategies.

Simpson's Paradox, Lord's Paradox, and Suppression Effects are the same phenomenon--the reversal paradox

This article discusses three statistical paradoxes that pervade epidemiological research: Simpson's paradox, Lord's paradox, and suppression. These paradoxes have important implications for the interpretation of evidence from observational studies. This article uses hypothetical scenarios to illustrate how the three paradoxes are different manifestations of one phenomenon – the reversal paradox – depending on whether the outcome and explanatory variables are categorical, continuous or a combination of both; this renders the issues and remedies for any one to be similar for all three. Although the three statistical paradoxes occur in different types of variables, they share the same characteristic: the association between two variables can be reversed, diminished, or enhanced when another variable is statistically controlled for. Understanding the concepts and theory behind these paradoxes provides insights into some controversial or contradictory research findings. These paradoxes show that prior knowledge and underlying causal theory play an important role in the statistical modelling of epidemiological data, where incorrect use of statistical models might produce consistent, replicable, yet erroneous results.

Ethnic heterogeneity in the longitudinal effects of placental vascular blood flow on birthweight

OBJECTIVE

Our goals were: (1) to estimate the longitudinal trends in uterine artery (UtArt) and umbilical artery (UmArt) resistance indices (RIs) in different ethnic strata; (2) to estimate time-dependent changes across gestation in the influence of variation in UtArt and UmArt RI on variation in birthweight in different ethnic strata; and (3) to determine the optimum set of UtArt and UmArt RIs for predicting birthweight in different ethnic strata.

STUDY DESIGN

Analyses were carried out on data collected in a prospective study of 535 multiethnic gravidas recruited from the Cedars-Sinai Medical Center (Los Angeles, CA). Baseline maternal characteristics were recorded at time of entry into the study. UtArt and UmArt RIs were measured on 3 occasions during pregnancy (visit 1, 16-20 weeks' gestation; visit 2, 21-29 weeks' gestation; and visit 3, 30-36 weeks' gestation). The outcome for this study was gestational age-adjusted birthweight (aBW).

RESULTS

The average UtArt and UmArt RI decreased steadily across gestation for all ethnicities. The average UtArt RI at each visit and the average rate of change between visits were not significantly different among the ethnicities. However, the UmArt RI measured at visit 3 and its rate of change in the last trimester were significantly different among the ethnic groups (P < .02). After adjustment for traditional risk factors for fetal growth restriction, the magnitude and rate of change of UtArt RI significantly predicted aBW only in Hispanic women, whereas the magnitude and rate of change of UmArt RI predicted aBW only in African American women. The most parsimonious combination of UtArt and UmArt RI measurements at visits 1, 2, and 3 that predicted statistically significant variation in aBW differed by ethnicity.

CONCLUSION

The relationships between aBW and longitudinally collected measures of UtArt and UmArt RI depend on the context defined by ethnicity and time of measurement after adjusting for a parsimoniously selected subset of traditional risk factors.