Genetic factors in physical activity levels: a twin study.

BACKGROUND: Substantial interindividual variation is observed in sports participation and physical activity levels in youth. This study aimed to (1) estimate the relative contribution of genes, along with shared and nonshared environmental factors, to variation in sports participation index (SPI) and leisure-time physical activity (LTPA); and (2) test differences in those factors in males and females.

METHODS: The sample was comprised of 411 Portuguese twin pairs of different zygosity aged 12 to 25 years. The SPI and LTPA were assessed with the Baecke questionnaire. Quantitative genetic modeling was used to test alternative models for the presence of additive gene effects (a(2)), common or shared environment within the family (c(2)), and unique environmental factors (e(2)).

RESULTS: The best-fitting models showed sex-specific effects for the two phenotypes. Variance components for SPI in males were a(2)=68.4%, c(2)=20%, and e(2)=11.6%; and in females, a(2)=39.8%, c(2)=28.4%, and e(2)=31.8%. For variation in LTPA, genetic factors in males explained 63%, common environment was not significant, and unique environment explained 37%. In females, contributing factors were a(2)=32%, c(2)=38%, and e(2)=30%.

CONCLUSIONS: Genetic effects explained a considerable amount of variation in SPI and LTPA, which were greater in males than in females. The relevance of shared environmental factors (family and peers) and nonshared environmental factors in SPI and LTPA is particularly evident in females.