Genetic and Environmental Effects on <scp>BMI</scp> Fluctuation Across the Adult Life Course and Its Associations With Baseline <scp>BMI</scp> and <scp>BMI</scp> Change: An Individual‐Based Study of 14 Longitudinal Twin Cohorts

ABSTRACT Introduction Body mass index (BMI) fluctuation has adverse health consequences, yet its determinants remain poorly understood. We examined genetic and environmental influences on BMI fluctuation and its associations with BMI trajectories. Data and Methods Data from 14 longitudinal twin cohorts, including 58 311 individuals (54% women) and 22 714 complete twin pairs (9761 monozygotic), were pooled. BMI trajectories from ages 18 to 99 years were estimated using linear mixed‐effects models. BMI fluctuation was defined as the average squared residual between observed and expected BMI around individual trajectories. Genetic and environmental contributions to BMI fluctuation and its associations with baseline BMI and BMI change were assessed using structural equation modelling. Results Additive genetic effects explained a moderate proportion of variance in BMI fluctuation (a 2 = 0.20 in men, 0.29 in women), with the remainder attributable to unique environmental effects (e 2 = 0.80 and 0.71). No evidence of sex‐specific genetic effects was found. Genetic contributions varied across life stages, peaking in men at ages 31–50 (a 2 = 0.36) and in women at 51–65 (a 2 = 0.36). Higher baseline BMI was associated with greater fluctuation, whereas greater directional BMI change was associated with less fluctuation. Higher BMI at ages 18–30 was associated with greater subsequent BMI fluctuation across later adulthood stages. Previous associations were driven by both genetic and unique environmental factors. Conclusions BMI fluctuation was predominantly explained by unique environmental effects, with moderate heritability that varied across life stages with no evidence of sex‐specific genetic effects. Genetic and environmental factors contribute to how BMI fluctuation is associated with BMI change.