The Association Between Cognitive Ability and Body Mass Index: A Sibling-Comparison Analysis in Four Longitudinal Studies

Abstract

AbstractBackgroundBody mass index (BMI) and obesity rates have increased sharply since the 1980s. While multiple epidemiologic studies have found higher adolescent cognitive ability is associated with lower adult BMI, residual and unobserved confounding due to family background may explain these associations. We used a sibling design to test this association accounting for confounding factors shared within households.MethodsWe used data from four cohort studies: the National Longitudinal Study of Youth 1979 (NLSY-79), the NLSY-79 Children and Young Adult, the NLSY 1997 (NLSY-97) and the Wisconsin Longitudinal Study (WLS); a total of 12,250 siblings from 5,602 households. We used random effects within-between (REWB) and residualized quantile regression (RQR) models to compare between- and within-family estimates of the association between adolescent cognitive ability and adult BMI (20-64 years).ResultsIn REWB models, moving from 0th to 100th percentile of adolescent cognitive ability was associated with −1.89 kg/m2 (95% CI = −2.41, −1.37) lower BMI between families. Adjusting for family socioeconomic position reduced the association to −1.23 (−1.79, −0.66) points. However, within families the association was just −0.13 (−0.70, 0.45) points. This pattern of results was found across multiple specifications, including analyses conducted in separate cohorts, models examining age-differences in association, and in RQR models examining the association across the distribution of BMI.ConclusionThe association between high adolescent cognitive ability and low adult BMI was substantially smaller in within-family compared with between-family analysis. The well-replicated associations between cognitive ability and subsequent BMI may largely reflect confounding by family background factors.