Age at dieting onset, body mass index, and dieting practices. A twin study

Objective: Using a twin study design, we sought to determine whether an early age at dieting onset is a risk factor for higher adult body mass index (BMI) or use of risky dieting practices, independent of genetic and familial factors. Method: Female twins ages 18–60 years (N = 950) from the University of Washington Twin Registry completed 2 surveys an average of 3 years apart. Analyses of individual twins and within-twin pairs tested associations of self-reported age at dieting onset with (1) adult BMI at baseline, (2) change in BMI between the two surveys and (3) risky dieting behaviors at baseline. Results: In analyses mimicking studies of unrelated individuals, an earlier age at dieting onset was associated with greater adult BMI (p = 0.003), higher Restraint Scale scores (p < 0.001), greater use of risky dieting behaviors (p = 0.04) and more weight cycling episodes (p < 0.001). In within-pair models that control for genetic and familial factors, the only significant association was between an earlier age at dieting onset and more weight cycling episodes (p = 0.006). Discussion: Underlying genetic and familial factors may influence associations of early dieting with higher adult BMIs and risky dieting practices in women.