Background: Few studies have examined whether the inverse association of age at menarche with adult body mass index (BMI) is due to the tendency of BMI to track between childhood and adult life, with childhood BMI playing a causal role in determining age at menarche.
Objective: The objective was to investigate whether the association of younger age at menarche with a high BMI and increased likelihood of obesity in middle age is due to confounding with early childhood BMI.
Design: In a historical cohort of 3743 Scottish females born between 1950 and 1955, height and weight were measured in early childhood, and age at menarche and height and weight in middle age were obtained by questionnaire.
Results: The age-adjusted change in mean adult BMI per additional year of age at menarche was -0.64 (95% CI: -0.78, -0.50). Adjustment for childhood BMI measured between 4 and 6 y reduced this value to -0.57 (-0.71, -0.43). Adjustment for childhood and adult social class, parity, smoking, and alcohol intake had little effect. The odds ratio for being obese compared with not being obese in adulthood was 0.82 (0.76, 0.86) per 1-y increase in age at menarche and was unchanged by adjustment for childhood BMI and other covariates.
Conclusions: The inverse association of age at menarche with BMI and obesity in middle age is not explained by confounding by early childhood BMI. Instead, age at menarche may simply be a proxy marker for the pace of sexual maturation, which itself leads to differences in adiposity (and BMI) in the peripubertal period that track into adult life.