The effect of breastfeeding on mean body mass index throughout life: a quantitative review of published and unpublished observational evidence.

BACKGROUND Evidence from observational studies has suggested that breastfeeding may reduce the prevalence of obesity in later life. OBJECTIVE The objective was to examine whether initial breastfeeding is related to lower mean body mass index (BMI; in kg/m(2)) throughout life. DESIGN The study was a systematic review of published studies investigating the association between infant feeding and a measure of obesity or adiposity in later life, which was supplemented with data from unpublished sources. Analyses were based on the mean differences in BMI between those subjects who were initially breastfed and those who were formula-fed (expressed as breastfed minus bottle-fed), which were pooled by using fixed-effects models throughout. RESULTS From 70 eligible studies, 36 mean differences in BMI (from 355 301 subjects) between those breastfed and those formula-fed (reported as exclusive feeding in 20 studies) were obtained. Breastfeeding was associated with a slightly lower mean BMI than was formula feeding (-0.04; 95% CI: -0.05, -0.02). The mean difference in BMIs appeared larger in 15 small studies of <1000 subjects (-0.19; 95% CI: -0.31, -0.08) and smaller in larger studies of >or=1000 subjects (-0.03; 95% CI: -0.05, -0.02). An Egger test was statistically significant (P = 0.002). Adjustment for socioeconomic status, maternal smoking in pregnancy, and maternal BMI in 11 studies abolished the effect (-0.10; 95% CI: -0.14, -0.06 before adjustment; -0.01; 95% CI: -0.05, 0.03 after adjustment). CONCLUSIONS Mean BMI is lower among breastfed subjects. However, the difference is small and is likely to be strongly influenced by publication bias and confounding factors. Promotion of breastfeeding, although important for other reasons, is not likely to reduce mean BMI.