Habitual physical activity in children: the role of genes and the environment

Background: Understanding the factors that contribute to physical inactivity in children is important because sedentary behavior strongly relates to metabolic disorders such as obesity and diabetes. Objective: We aimed to quantify the genetic and environmental influences on physical activity energy expenditure (PAEE) in 100 sex-concordant dizygotic (n = 38) and monozygotic (n = 62) twin pairs aged 4-10 y. Design: Resting metabolic rate (RMR) was assessed by using respiratory gas exchange, total energy expenditure (TEE) by using doubly labeled water, and body composition by using dual-energy X-ray absorptiometry. Structural equation modeling was used to partition the phenotypic variance into additive genetic (a(2)) and common (c(2)) and unshared (e(2)) environmental components. Results: Because PAEE [TEE - (RMR + 0.1 x TEE)] depends on body weight, which is highly heritable, we tested several models: 1) after adjustment for age, sex, ethnicity, study date, season, and weight, a(2) explained none of the phenotypic variance in PAEE (95% CI: 0%, 38%), whereas c(2) and e(2) accounted for 69% (33 %, 77%; P = 0.001) and 31% (23%, 39%; P < 0.001) of the variance, respectively; 2) after adjustment for the cofactors in model 1, a(2) explained 19% of the phenotypic variance in TEE (0%, 60%; P = 0.13), whereas c(2) and e(2) accounted for 59% (16%, 79%; P = 0.007) and 23% (17%, 31%; P < 0.0001) of the variance, respectively; 3) in models adjusted as above (excluding weight), a(2) explained no variance in physical activity level (TEE/RMR) (0%, 32%; P = 0.50), whereas c(2) and e(2) explained 65% (34%, 60%; P = 0.001) and 35% (28%, 45%; P < 0.0001) of the variance, respectively. Conclusions: Our data suggest that the familial resemblance in physical activity in these children is explained predominantly by shared environmental factors and not by genetic variability.