Gene × environment interaction model adapted from reference 40. VE, vigorous exercise; BMI1 and BMI2, BMI in twins 1 and 2 within a twin pair, respectively; A1 and A2, additive genetic effects for twins 1 and 2 within a twin pair, respectively; C1 and C2, shared environmental effects for twins 1 and 2 within a twin pair, respectively; E1 and E2, nonshared environmental effects for twins 1 and 2 within a twin pair, respectively; M, moderator 1 (VE); N, moderator 2 (age); T, VE-moderated component of A; U, VE-moderated component of C; V, VE-moderated component of E; F, age-moderated component of A; G, age-moderated component of C; H, age-moderated component of E; I, effect of VE on mean BMI; J, effect of age on mean BMI; K, effect of VE × age interaction on mean BMI.