Breastfeeding and body composition in children: will there ever be conclusive empirical evidence for a protective effect against overweight?

An increased prevalence of childhood overweight has been observed worldwide over the past decades, which indicates the need for strategies to prevent obesity. There is some evidence that risk of obesity is primed by exposures early in life. Among other factors, breastfeeding has been hypothesized as a potential priming factor against overweight. Although the properties of human milk suggest possible mechanisms for a protective effect of breastfeeding compared with formula feeding with respect to later overweight, empirical evidence is more difficult to establish. This article reviews the available epidemiologic literature on this topic. Several observational studies have shown evidence for a small protective effect with respect to overweight in childhood. Three meta-analyses reported significant protective effects of breastfeeding against overweight in later life, whereas another meta-analysis showed no effect of breastfeeding on mean body mass index (BMI) after adjustment for confounding factors. These seemingly inconsistent results might potentially be explained by different effects of breastfeeding in normal-weight compared with overweight children. Evidence from interventional studies is limited. A randomized trial failed to confirm an effect of a breastfeeding promotion on children's BMI, but this trial lacked statistical power because rates of breastfeeding were relatively similar in the intervention and control groups. In conclusion, protective priming effects of breastfeeding on later overweight appear to be possible but are difficult to prove. Although observational studies have to deal with confounding issues, interventional studies on breastfeeding promotion may lack power.