Are adolescents who were breast-fed less likely to be overweight? Analyses of sibling pairs to reduce confounding.

BACKGROUND: Cohort analyses suggesting that breast-feeding protects against being overweight have been criticized for inadequately controlling for confounding associated with the self-selection of feeding practices.
METHODS: Using nationally representative U.S. data from the National Longitudinal Study of Adolescent Health (1994-1996), we performed traditional cohort analyses (n = 11,998) using logistic regression to estimate the relation between breast-feeding and adolescent overweight (body mass index > or =85 percentile, based on year 2000 CDC growth charts), controlling for known potential confounders. Breast-feeding also was assessed in a subsample of 850 sibling pairs to account for unmeasured genetic and environmental factors.
RESULTS: Among girls in the full cohort, the odds of being overweight declined among those who had been breast-fed at least 9 months; odds ratios ranged from 0.90 (95% confidence interval = 0.74-1.09) for <3 months of breast-feeding to 0.78 (0.64-0.96) for > or =9 months. A similar effect was seen in boys, although these trends were less consistent. In contrast, an analysis of sibling pairs provided no evidence of breast-feeding effects on weight within discordant trends.
CONCLUSION: Cohort data indicate that odds of being overweight decrease as breast-feeding duration increases, at least among girls. However, sibling analyses suggest that this relationship may not be causal but rather attributable to unmeasured confounding related to mothers' choice to breast-feed, or to other childhood risk factors for overweight. Our results illustrate the utility of sibling analyses in understanding the true effect of early life exposures (such as breast-feeding) on health outcomes over time, independent of confounding factors that may not be satisfactorily controlled using traditional prospective cohort methods.