Effect of shared environmental factors on exercise behavior from age 7 to 12 years

Genetic confounding in the association of early motor development with childhood and adolescent exercise behavior

INTRODUCTION

Early motor development has been found to be a predictor of exercise behavior in children and adolescents, but whether this reflects a causal effect or confounding by genetic or shared environmental factors remains to be established.

METHODS

For 20,911 complete twin pairs from the Netherlands Twin Register a motor development score was obtained from maternal reports on the timing of five motor milestones. During a 12-year follow-up, subsamples of the mothers reported on the twins' ability to perform seven gross motor skills ability (N = 17,189 pairs), and weekly minutes of total metabolic equivalents of task (MET) spent on sports and exercise activities at age 7 (N = 3632 pairs), age 10 (N = 3735 pairs), age 12 (N = 7043 pairs), and age 14 (N = 3990 pairs). Multivariate phenotypic and genetic regression analyses were used to establish the predictive strength of the two motor development traits for future exercise behavior, the contribution of genetic and shared environmental factors to the variance in all traits, and the contribution of familial confounding to the phenotypic prediction.

RESULTS

Significant heritability (h2) and shared environmental (c2) effects were found for early motor development in boys and girls (h2 = 43-65%; c2 = 16-48%). For exercise behavior, genetic influences increased with age (boys: h2age7 = 22% to h2age14 = 51%; girls: h2age7 = 3% to h2age14 = 18%) paired to a parallel decrease in the influence of the shared environment (boys: c2age7 = 68% to c2age14 = 19%; girls: c2age7 = 80% to c2age14 = 48%). Early motor development explained 4.3% (p < 0.001) of the variance in future exercise behavior in boys but only 1.9% (p < 0.001) in girls. If the effect in boys was due to a causal effect of motor development on exercise behavior, all of the factors influencing motor development would, through the causal chain, also influence future exercise behavior. Instead, only the genetic parts of the regression of exercise behavior on motor development were significant. Shared and unique environmental parts of the regression were largely non-significant, which is at odds with the causal hypothesis.

CONCLUSION

No support was found for a direct causal effect in the association between rapid early motor development on future exercise behavior. In boys, early motor development appears to be an expression of the same genetic factors that underlie the heritability of childhood and early adolescent exercise behavior.

Genetic and Environmental Effects on the Early Motor Development as a Function of Parental Educational Attainment

INTRODUCTION

The contribution of genetic and environmental factors to individual differences in early motor development is still largely uncharted. This large-scale twin study establishes the genetic and environmental influences on the timing of motor milestones achievement, and it further tests whether the influences are moderated by parental education.

METHODS

The twins came from families registered in the Netherlands Twin Register (NTR) from 1986 to 2016. In 30,256 complete twin pairs, mother-reported ages at which each twin was able to first-time roll from back to belly, sit unassisted, hands-and-knees crawl, stand up unaided, and walk independently were used to extract an early motor development factor. Parental education was dichotomized ("both parents with low/average education" vs "at least one parent with high education" with university degree as a threshold).

RESULTS

Additive genetics explained 52% of the variance in motor development, the remaining 39% and 9% were explained by shared and nonshared environment separately. Mean age of achieving motor milestones tended to be higher in infants with high educated parents, and a moderation of parental education on the genetic and environmental variance in motor development was seen in female twins with larger heritability in the high educated parents group (64% vs 43%) paired to a lower shared environmental influence (28% vs 48%). Only 7%-8% of the variance was accounted for nonshared environmental factors, including measurement error. The pattern of results did not change when the degree of urbanicity, a correlate of parental education, was additionally considered.

CONCLUSIONS

Genetic factors explain most of the individual differences in the timing of motor milestone achievement, but factors related to the shared home environment also play an important role in early motor development.

Prospective association of family members' sugar-sweetened beverages intake with children's sugar-sweetened beverages consumption in China

PURPOSE

We aimed to investigate whether parental and siblings' sugar-sweetened beverage (SSB) intake had prospective impact on children's SSB consumption, and the potential sex difference in these associations.

METHODS

This study included a total of 904 children and their parents enrolled from 2004 to 2011 China Health and Nutrition Survey (CHNS) cohort study. SSB consumption information was estimated using a short dietary questionnaire and total energy intake was assessed with three-day 24-h dietary assessments at recruitment and follow-up surveys. Multivariate logistic or linear regression analyses were used to assess the association for SSB consumption between parents, siblings and children after adjusting for age, body mass index (BMI) z-score, household income and parental educational level.

RESULTS

In this study, a majority (87.6%) of children consumed SSB. Among them, the median consumption of SSB was 70.3 ml/day per capita and 205.4 ml/day per consumer. Parental SSB consumption was relevant to children's SSB consumption, and this association was more pronounced in boys than in girls. Meanwhile, fathers seemed to have a stronger impact on whether children consume SSB than mothers which was reflected by lower P and higher OR. Additionally, children's SSB intake was prospectively associated with their older siblings' SSB consumption (P _{for trend} < 0.03).

CONCLUSIONS

Parental and older siblings' SSB consumption was relevant to children's SSB intake. Particularly, boys were more susceptible to parental impact than girls, and fathers seemed to have a greater influence on children than mothers.

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.

Better than Better: The New Focus on Well-Being in Child Psychiatry

Training and practice within child psychiatry has focused predominantly on mental illness rather than mental health. A growing body of evidence, however, is demonstrating the importance for clinicians also to be able to focus directly on enhancing positive traits and emotions and increasing well-being and health promotion in their patients. This complementary aspect of mental health care has been called well-being and positive psychiatry, among other terms. Being able to apply these principles to standard practice requires new knowledge, skills, and attitudes that are not part of traditional psychiatric training.

Performing Together: Monozygotic Twin Comedians/Twin research: Mirror-Image Cleft Lip and Palate; Dental Caries; Noninvasive Prenatal testing; Capgras Syndrome with Folie à Deux/In the News: Athletic Twins; Transgendered Twins; Crib-Sharing; Common Careers

Twin studies have shown that our sense of humor has an underlying genetic component, but less investigation of the origins of stand-up comedy has been undertaken. This article briefly reviews twin research findings on humor, then describes the working partnership and social affiliation of a pair of monozygotic male twins who perform together as stand-up comedians. The abilities, personalities and temperaments of these twins suggest future avenues for research in this interesting area. Next, findings from twin studies and case reports of twins with cleft lip and palate, dental caries, noninvasive prenatal testing and Capgras syndrome (with folie à deux) are summarized. In conclusion, recent news about athletic twins, transgendered twins, crib-sharing and career choice are presented.

The effects of parental education on exercise behavior in childhood and youth: a study in Dutch and Finnish twins

Twin studies have estimated the relative contribution of genes and the environment to variance in exercise behavior and it is known that parental education positively affects exercise levels. This study investigates the role of parental education as a potential modifier of variance in exercise behavior from age 7 to 18 years. The study is based on large datasets from the Netherlands Twin Register (NTR: N = 24 874 twins; surveys around the ages of 7, 10, 12, 14, 16 and 18 years) and two Finnish twin cohorts (FinnTwin12: N = 4399; 12, 14 and 17 years; FinnTwin16: N = 4648; 16, 17 and 18 years). Regular participation in moderate-to-vigorous exercise activities during leisure time was assessed by survey. Parental education was dichotomized ("both parents with a low education" vs "at least one parent with a high education"). The mean in exercise behavior tended to be higher and the variance tended to be lower in children of high educated parents. Evidence for gene-by-environment interaction was weak. To develop successful interventions that specifically target children of low educated parents, the mechanisms causing the mean and variance differences between the two groups should be better understood.

Heritability of the affective response to exercise and its correlation to exercise behavior

OBJECTIVES

Individual differences in adolescent exercise behavior are strongly influenced by genetic factors. The affective response to exercise is a potential source of these genetic influences. To test its role in the motivation to exercise, we estimated the heritability of the affective responses during and after exercise and the overlap with the genetic factors influencing regular voluntary exercise behavior.

DESIGN

226 twin pairs and 38 siblings completed two submaximal exercise tests on a cycle ergometer and a treadmill and a maximal exercise test on a cycle ergometer. Affective responses were assessed by the Feeling Scale (FS), Borg's Rating of Perceived Exertion (RPE) and the Activation-Deactivation Adjective Checklist (AD ACL).

METHODS

Multivariate structural equation modeling was used to estimate heritability of the affective responses during and after submaximal and maximal exercise and the (genetic) correlation with self-reported regular voluntary exercise behavior over the past year.

RESULTS

Genetic factors explained 15% of the individual differences in FS responses during the cycle ergometer test, as well as 29% and 35% of the individual differences in RPE during the cycle ergometer and treadmill tests, respectively. For the AD ACL scales, heritability estimates ranged from 17% to 37% after submaximal exercise and from 12% to 37% after maximal exercise. Without exception, more positive affective responses were associated with higher amounts of regular exercise activity (.15 < r < .21) and this association was accounted for by an overlap in genetic factors influencing affective responding and exercise behavior.

CONCLUSIONS

We demonstrate low to moderate heritability estimates for the affective response during and after exercise and significant (genetic) associations with regular voluntary exercise behavior. These innate individual differences in the affective responses to exercise should be taken into account in interventions aiming to motivate adolescents to adopt and maintain regular exercise.

Active for Life Year 5: a cluster randomised controlled trial of a primary school-based intervention to increase levels of physical activity, decrease sedentary behaviour and improve diet

Background

Previous studies of the effect of school-based interventions to improve healthy behaviours have had important limitations.

Objective

To investigate the effectiveness of a school-based intervention to increase physical activity, reduce sedentary behaviour and increase fruit and vegetable consumption.

Design

Cluster randomised controlled trial.

Setting

Sixty English primary schools.

Participants

Children in year 4 (aged 8–9 years) at recruitment, year 5 (aged 9–10 years) during the intervention and immediate follow-up and year 6 (aged 10–11 years) during 1 year of follow-up.

Intervention

Active for Life Year 5 (AFLY5) included teacher training, lesson plans, materials for 16 lessons, parent-interactive homework and written materials for school newsletters and parents.

Main outcome measures

Primary outcome measures included accelerometer-assessed levels of physical activity and sedentary behaviour, and child-reported consumption of fruit and vegetables. Secondary outcome measures included child-reported screen viewing; consumption of snacks, high-fat food and high-energy drinks; body mass index; and waist circumference.

Results

We recruited 60 schools (2221 children). At the immediate follow-up, no difference was found between children in intervention and control schools for any of the three primary outcomes. The intervention was effective on three of the nine secondary outcomes; children in intervention schools reported spending less time screen viewing at weekends [–21 minutes per day, 95% confidence interval (CI) –37 to –4 minutes per day], eating fewer servings of snacks per day (–0.22, 95% CI –0.38 to –0.05 servings of snacks per day) and drinking fewer servings of high-energy drinks per day (–0.26, 95% CI –0.43 to –0.10 servings of high-energy drinks per day) than the children in control schools. The results remained consistent 1 year later. The intervention increased children’s perception of maternal efforts to limit the time they spent screen viewing and children’s knowledge about healthy physical activity and fruit and vegetable consumption, with these two mediators explaining approximately one-quarter of the effect of the intervention on screen viewing. The intervention did not affect other mediators. The cost of implementing the intervention from a provider perspective was approximately £18 per child. Process evaluation showed that AFLY5 was implemented with a high degree of fidelity. Teachers supported the aims of AFLY5, but their views of the programme itself were mixed.

Limitations

Responses to parental questionnaires for the economic evaluation were low and we struggled to engage all teachers for the process evaluation. Although the participating schools included a range of levels of socioeconomic deprivation, class sizes and rural and urban settings, we cannot assume that results generalise to all primary schools.

Conclusions

AFLY5 is not effective at increasing levels of physical activity, reducing sedentary behaviour and increasing fruit and vegetable consumption in primary school children, but may be effective in reducing time spent screen viewing at weekends and the consumption of snacks and high-energy drinks.

Future work

Our findings suggest that school-based interventions are unlikely to have a major impact on promoting healthy levels of physical activity and healthy diets in primary school children. We would recommend trials of the effect and cost-effectiveness of more intensive family and community interventions.

Trial registration

Current Controlled Trials ISRCTN50133740.

Funding

This project was funded by the National Institute for Health Research Public Health Research programme and will be published in full inPublic Health Research; Vol. 4, No. 7. See the NIHR Journals Library website for further project information.

Differences in Adolescent Physical Fitness: A Multivariate Approach and Meta-analysis

Physical fitness can be defined as a set of components that determine exercise ability and influence performance in sports. This study investigates the genetic and environmental influences on individual differences in explosive leg strength (vertical jump), handgrip strength, balance, and flexibility (sit-and-reach) in 227 healthy monozygotic and dizygotic twin pairs and 38 of their singleton siblings (mean age 17.2 ± 1.2). Heritability estimates were 49 % (95 % CI 35–60 %) for vertical jump, 59 % (95 % CI 46–69 %) for handgrip strength, 38 % (95 % CI 22–52 %) for balance, and 77 % (95 % CI 69–83 %) for flexibility. In addition, a meta-analysis was performed on all twin studies in children, adolescents and young adults reporting heritability estimates for these phenotypes. Fifteen studies, including results from our own study, were meta-analyzed by computing the weighted average heritability. This showed that genetic factors explained most of the variance in vertical jump (62 %; 95 % CI 47–77 %, N = 874), handgrip strength (63 %; 95 % CI 47–73 %, N = 4516) and flexibility (50 %; 95 % CI 38–61 %, N = 1130) in children and young adults. For balance this was 35 % (95 % CI 19–51 %, N = 978). Finally, multivariate modeling showed that the phenotypic correlations between the phenotypes in current study (0.07 < r < 0.27) were mostly driven by genetic factors. It is concluded that genetic factors contribute significantly to the variance in muscle strength, flexibility and balance; factors that may play a key role in the individual differences in adolescent exercise ability and sports performance.

Genetics of wellbeing and its components satisfaction with life, happiness, and quality of life: a review and meta-analysis of heritability studies

Wellbeing is a major topic of research across several disciplines, reflecting the increasing recognition of its strong value across major domains in life. Previous twin-family studies have revealed that individual differences in wellbeing are accounted for by both genetic as well as environmental factors. A systematic literature search identified 30 twin-family studies on wellbeing or a related measure such as satisfaction with life or happiness. Review of these studies showed considerable variation in heritability estimates (ranging from 0 to 64 %), which makes it difficult to draw firm conclusions regarding the genetic influences on wellbeing. For overall wellbeing twelve heritability estimates, from 10 independent studies, were meta-analyzed by computing a sample size weighted average heritability. Ten heritability estimates, derived from 9 independent samples, were used for the meta-analysis of satisfaction with life. The weighted average heritability of wellbeing, based on a sample size of 55,974 individuals, was 36 % (34–38), while the weighted average heritability for satisfaction with life was 32 % (29–35) (n = 47,750). With this result a more robust estimate of the relative influence of genetic effects on wellbeing is provided.

Tracking of environmental determinants of bone structure and strength development in healthy boys: an eight-year follow up study on the positive interaction between physical activity and protein intake from prepuberty to mid-late adolescence

High protein (> median:Hprot) vs. moderate (< median:MProt) intake was shown to enhance the positive impact of high physical activity (HPA) on proximal femur BMC/aBMD/Area in healthy prepubertal boys. We tested the hypothesis that this synergistic effect would track and influence bone structure and strength until mid-adolescence. BMC/aBMD/Area was measured at femoral neck (FN) and total hip (TotHip) by DXA in 176 boys at 7.4 ± 0.4 and 15.2 ± 0.5 years (± SD). Distal tibia (DistTib) microstructure and strength were also assessed at 15.2 years by high-resolution peripheral computerized tomography (HR-pQCT) and micro-finite element analysis (µFEA). The positive impact of HProt vs. MProt on FN and TotHip BMC/aBMD/Area, recorded at 7.4 years remained unabated at 15.2 years. At this age, at DistTib, HProt-HPA vs. MProt-HPA was associated (p < 0.001) with larger cross-sectional area (CSA, mm(2) ), trabecular number (Tb.N, mm(-1) ) and lower trabecular separation (Tb.Sp, µm). The interaction between physical activity and protein intake was significant for CSA (p = 0.012) and Tb.N (p = 0.043). Under MProt (38.0 ± 6.9 g.d(-1)), a difference in PA from 168 ± 40 to 303 ± 54 kcal.d(-1) was associated with greater stiffness (kN/mm) and failure load (N) of +0.16 and +0.14 Z-score, respectively. In contrast, under HProt (56.2 ± 9.5 g.d(-1) ), a difference in PA of similar magnitude, from 167 ± 33 to 324 ± 80 kcal.d(-1) , was associated with a larger difference in stiffness and failure load of +0.50 and +0.57 Z-score, respectively. In conclusion, the positive influence of relatively HProt on the impact of HPA on proximal femur macrostructure tracks from prepuberty to mid-late puberty. At this stage, the impact of HProt on HPA is also associated with microstructural changes that should confer greater mechanical resistance to weight-bearing bones. These results underscore the importance of protein intake and exercise synergistic interaction in the early prevention of adult osteoporosis.

Genetics of Regular Exercise and Sedentary Behaviors

Studies on the determinants of physical activity have traditionally focused on social factors and environmental barriers, but recent research has shown the additional importance of biological factors, including genetic variation. Here we review the major tenets of this research to arrive at three major conclusions: First, individual differences in physical activity traits are significantly influenced by genetic factors, but genetic contribution varies strongly over age, with heritability of leisure time exercise behavior ranging from 27% to 84% and heritability of sedentary behaviors ranging from 9% to 48%. Second, candidate gene approaches based on animal or human QTLs or on biological relevance (e.g., dopaminergic or cannabinoid activity in the brain, or exercise performance influencing muscle physiology) have not yet yielded the necessary evidence to specify the genetic mechanisms underlying the heritability of physical activity traits. Third, there is significant genetic modulation of the beneficial effects of daily physical activity patterns on strength and endurance improvements and on health-related parameters like body mass index. Further increases in our understanding of the genetic determinants of sedentary and exercise behaviors as well as the genetic modulation of their effects on fitness and health will be key to meaningful future intervention on these behaviors.

Effect of intervention aimed at increasing physical activity, reducing sedentary behaviour, and increasing fruit and vegetable consumption in children: active for Life Year 5 (AFLY5) school based cluster randomised controlled trial

OBJECTIVE

To investigate the effectiveness of a school based intervention to increase physical activity, reduce sedentary behaviour, and increase fruit and vegetable consumption in children.

DESIGN

Cluster randomised controlled trial.

SETTING

60 primary schools in the south west of England.

PARTICIPANTS

Primary school children who were in school year 4 (age 8-9 years) at recruitment and baseline assessment, in year 5 during the intervention, and at the end of year 5 (age 9-10) at follow-up assessment.

INTERVENTION

The Active for Life Year 5 (AFLY5) intervention consisted of teacher training, provision of lesson and child-parent interactive homework plans, all materials required for lessons and homework, and written materials for school newsletters and parents. The intervention was delivered when children were in school year 5 (age 9-10 years). Schools allocated to control received standard teaching.

MAIN OUTCOME MEASURES

The pre-specified primary outcomes were accelerometer assessed minutes of moderate to vigorous physical activity per day, accelerometer assessed minutes of sedentary behaviour per day, and reported daily consumption of servings of fruit and vegetables.

RESULTS

60 schools with more than 2221 children were recruited; valid data were available for fruit and vegetable consumption for 2121 children, for accelerometer assessed physical activity and sedentary behaviour for 1252 children, and for secondary outcomes for between 1825 and 2212 children for the main analyses. None of the three primary outcomes differed between children in schools allocated to the AFLY5 intervention and those allocated to the control group. The difference in means comparing the intervention group with the control group was -1.35 (95% confidence interval -5.29 to 2.59) minutes per day for moderate to vigorous physical activity, -0.11 (-9.71 to 9.49) minutes per day for sedentary behaviour, and 0.08 (-0.12 to 0.28) servings per day for fruit and vegetable consumption. The intervention was effective for three out of nine of the secondary outcomes after multiple testing was taken into account: self reported time spent in screen viewing at the weekend (-21 (-37 to -4) minutes per day), self reported servings of snacks per day (-0.22 (-0.38 to -0.05)), and servings of high energy drinks per day (-0.26 (-0.43 to -0.10)) were all reduced. Results from a series of sensitivity analyses testing different assumptions about missing data and from per protocol analyses produced similar results.

CONCLUSION

The findings suggest that the AFLY5 school based intervention is not effective at increasing levels of physical activity, decreasing sedentary behaviour, and increasing fruit and vegetable consumption in primary school children. Change in these activities may require more intensive behavioural interventions with children or upstream interventions at the family and societal level, as well as at the school environment level. These findings have relevance for researchers, policy makers, public health practitioners, and doctors who are involved in health promotion, policy making, and commissioning services. Trial registration Current Controlled Trials ISRCTN50133740.

Genetic and environmental influences on the allocation of adolescent leisure time activities

There is a growing recognition of the importance of the out-of-school activities in which adolescents choose to participate. Youth activities vary widely in terms of specific activities and in time devoted to them but can generally be grouped by the type and total duration spent per type. We collected leisure time information using a 17-item leisure time questionnaire in a large sample of same- and opposite-sex adolescent twin pairs (N = 2847). Using both univariate and multivariate genetic models, we sought to determine the type and magnitude of genetic and environmental influences on the allocation of time toward different leisure times. Results indicated that both genetic and shared and nonshared environmental influences were important contributors to individual differences in physical, social, intellectual, family, and passive activities such as watching television. The magnitude of these influences differed between males and females. Environmental influences were the primary factors contributing to the covariation of different leisure time activities. Our results suggest the importance of heritable influences on the allocation of leisure time activity by adolescents and highlight the importance of environmental experiences in these choices.

Factors behind leisure-time physical activity behavior based on Finnish twin studies: the role of genetic and environmental influences and the role of motives

Different approaches are being taken to clarify the role of various factors in the development of physical activity behaviors. Genetic studies are a new area of physical activity research and also the motives for physical activity have been widely studied. The purpose of this paper is to review the findings emerging from the longitudinal genetic studies on leisure-time physical activity and to evaluate the associations between motivational factors and leisure-time physical activity. The focus is to review recent findings of longitudinal Finnish twin studies. The results of the latest longitudinal Finnish twin studies point to the existence of age-specific genetic and environmental influences on leisure-time physical activity. Variations in environmental factors seem to explain the observed deterioration in leisure-time physical activity levels. A decline in genetic influences is seen first from adolescence to young adulthood and again from the age of thirty to the mid-thirties. In the Finnish twin participants, mastery, physical fitness, and psychological state were the major motivation factors associated with consistent leisure-time physical activity behavior. The results also indicate that intrinsic motivation factors may be important for engagement in leisure-time physical activity.

The nature of behavioural correlates of healthy ageing: a twin study of lifestyle in mid to late life

BACKGROUND

With the greying of the industrialized world has come increased interest in identifying the modifiable lifestyle factors that promote healthy and successful ageing. Whereas many of the behavioural correlates of late-life morbidity and mortality have been identified, relatively little is known about the origins of individual differences in these factors.

METHODS

A sample of 12,714 twins, including both members of 3806 pairs of known zygosity, ascertained through the Danish Twin Registry and aged 40 to 80 years, completed a self-report assessment of six lifestyle factors associated with ageing: smoking, drinking, diet and physical, social and intellectual activities. Standard biometric methods were used to analyse the twin data and determine the extent to which individual differences in each of the lifestyle factors are heritable.

RESULTS

For each of the six lifestyle factors, the estimate of heritability ranged from 32% (95% CI: 19-42%) for the diet scale to 69% (62-72%) for the smoking measure. Biometric estimates of the contribution of the twins' common rearing environment were uniformly small (≤6%). There was little evidence that standardized biometric estimates varied by gender or age.

CONCLUSIONS

Individuals likely construct lifestyles in part to complement and reinforce underlying genetically influenced dispositions and talents. The heritable nature of lifestyle factors implies that the behavioural and genetic contributors to ageing processes are not necessarily conceptually distinct but rather reflect the complexity of gene-environment interplay in ageing.

The dopaminergic reward system and leisure time exercise behavior: a candidate allele study

PURPOSE

Twin studies provide evidence that genetic influences contribute strongly to individual differences in exercise behavior. We hypothesize that part of this heritability is explained by genetic variation in the dopaminergic reward system. Eight single nucleotide polymorphisms (SNPs in DRD1: rs265981, DRD2: rs6275, rs1800497, DRD3: rs6280, DRD4: rs1800955, DBH: rs1611115, rs2519152, and in COMT: rs4680) and three variable number of tandem repeats (VNTRs in DRD4, upstream of DRD5, and in DAT1) were investigated for an association with regular leisure time exercise behavior.

MATERIALS AND METHODS

Data on exercise activities and at least one SNP/VNTR were available for 8,768 individuals aged 7 to 50 years old that were part of the Netherlands Twin Register. Exercise behavior was quantified as weekly metabolic equivalents of task (MET) spent on exercise activities. Mixed models were fitted in SPSS with genetic relatedness as a random effect.

RESULTS

None of the genetic variants were associated with exercise behavior (P>.02), despite sufficient power to detect small effects.

DISCUSSION AND CONCLUSIONS

We did not confirm that allelic variants involved in dopaminergic function play a role in creating individual differences in exercise behavior. A plea is made for large genome-wide association studies to unravel the genetic pathways that affect this health-enhancing behavior.

A twin-sibling study on the relationship between exercise attitudes and exercise behavior

Social cognitive models of health behavior propose that individual differences in leisure time exercise behavior are influenced by the attitudes towards exercise. At the same time, large scale twin-family studies show a significant influence of genetic factors on regular exercise behavior. This twin-sibling study aimed to unite these findings by demonstrating that exercise attitudes can be heritable themselves. Secondly, the genetic and environmental cross-trait correlations and the monozygotic (MZ) twin intrapair differences model were used to test whether the association between exercise attitudes and exercise behavior can be causal. Survey data were obtained from 5,095 twins and siblings (18-50 years). A genetic contribution was found for exercise behavior (50 % in males, 43 % in females) and for the six exercise attitude components derived from principal component analysis: perceived benefits (21, 27 %), lack of skills, support and/or resources (45, 48 %), time constraints (25, 30 %), lack of energy (34, 44 %), lack of enjoyment (47, 44 %), and embarrassment (42, 49 %). These components were predictive of leisure time exercise behavior (R(2) = 28 %). Bivariate modeling further showed that all the genetic (0.36 < |rA| < 0.80) and all but two unique environmental (0.00 < |rE| < 0.27) correlations between exercise attitudes and exercise behavior were significantly different from zero, which is a necessary condition for the existence of a causal effect driving the association. The correlations between the MZ twins' difference scores were in line with this finding. It is concluded that exercise attitudes and exercise behavior are heritable, that attitudes and behavior are partly correlated through pleiotropic genetic effects, but that the data are compatible with a causal association between exercise attitudes and behavior.

The Young Netherlands Twin Register (YNTR): longitudinal twin and family studies in over 70,000 children

The Netherlands Twin Register (NTR) began in 1987 with data collection in twins and their families, including families with newborn twins and triplets. Twenty-five years later, the NTR has collected at least one survey for 70,784 children, born after 1985. For the majority of twins, longitudinal data collection has been done by age-specific surveys. Shortly after giving birth, mothers receive a first survey with items on pregnancy and birth. At age 2, a survey on growth and achievement of milestones is sent. At ages 3, 7, 9/10, and 12 parents and teachers receive a series of surveys that are targeted at the development of emotional and behavior problems. From age 14 years onward, adolescent twins and their siblings report on their behavior problems, health, and lifestyle. When the twins are 18 years and older, parents are also invited to take part in survey studies. In sub-groups of different ages, in-depth phenotyping was done for IQ, electroencephalography , MRI, growth, hormones, neuropsychological assessments, and cardiovascular measures. DNA and biological samples have also been collected and large numbers of twin pairs and parents have been genotyped for zygosity by either micro-satellites or sets of short nucleotide polymorphisms and repeat polymorphisms in candidate genes. Subject recruitment and data collection is still ongoing and the longitudinal database is growing. Data collection by record linkage in the Netherlands is beginning and we expect these combined longitudinal data to provide increased insights into the genetic etiology of development of mental and physical health in children and adolescents.