American Journal of Epidemiology Advance Access originally published online on March 22, 2006
American Journal of Epidemiology 2006 163(9):870-872; doi:10.1093/aje/kwj134
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Letter to the Editor |
RE: "DURATION OF BREASTFEEDING AND RISK OF OVERWEIGHT: A META-ANALYSIS"
National Perinatal Epidemiology Unit, Oxford University, Headington, Oxford OX3 7LF, United Kingdom
(e-mail: maria.quigley{at}npeu.ox.ac.uk)
In a recent paper, Harder et al. (1
) presented results from a meta-analysis of the association between duration of breastfeeding and risk of overweight. In a meta-analysis of linear trends, 1 month of breastfeeding was associated with a 4 percent decrease in risk of overweight (odds ratio (OR) = 0.96, 95 percent confidence interval (CI): 0.94, 0.98). Significant heterogeneity was evident. Below, I propose several possible causes of this heterogeneity and suggest that a meta-analysis might have been inappropriate.
The age at which the outcome was measured varied between the 17 studies (2–17), ranging from infants aged 
1 year (4, 13, 17) to adults aged 33 years (10). The remaining 13 studies presented data on children aged 2–14 years. While there was no difference in the effect of breastfeeding between persons under and over age 6 years (1), there is likely to have been heterogeneity due to pooling of data with a wide age range. For example, in the studies of infants, current breastfeeding may have had a different effect on risk of overweight than past breastfeeding, which may have occurred up to 33 years previously.
Studies included in the meta-analysis varied according to their definition of overweight. It is plausible that the effect of breastfeeding is different for the outcomes obesity, overweight, and risk of becoming overweight. The definition of breastfeeding varied between studies: Four used exclusive breastfeeding, eight used "any" (exclusive or partial) breastfeeding, and five did not specify the definition. Infants who were never breastfed formed the baseline category for breastfeeding duration. These baseline categories might not be comparable between studies because of variation in the composition of infant formula between countries and by year of birth (1940s–1990s). In addition, breastfeeding duration was grouped differently in different studies. Different choices of the score assigned to each group, particularly the last group (e.g., 
6 months), give rise to different trends. Finally, no meta-analysis was conducted on adjusted odds ratios, although 11 studies had adjusted for possible confounding factors. Being overweight and having been formula-fed share many of the same risk factors, most notably low socioeconomic status and maternal overweight. Hence, an observed effect of breastfeeding on risk of overweight could be partly explained by confounding.
I repeated the linear-trends meta-analysis (pool-first method (18)) in children aged 2–14 years, using adjusted effects and analyzing body mass index cutoffs separately. Graphical examination of the data revealed heterogeneity of effects (figures 1 and 2). Some decreasing trends were weaker after adjustment for confounders. Six studies used body mass index greater than the 95th or 97th percentile, and of these studies, five (5, 6, 9, 15, 16) presented results that were adjusted for confounders (figure 1). In these five studies, the pooled crude odds ratio for risk of overweight per month of breastfeeding was 0.95 (95 percent CI: 0.92, 0.98). There was significant heterogeneity (Q = 15.8, 4 df, p = 0.003). Adjustment for confounders weakened the trend (pooled adjusted OR = 0.97, 95 percent CI: 0.94, 0.99) and removed much of the heterogeneity (Q = 7.3, 4 df, p = 0.121). Of the six studies that used body mass index greater than the 90th percentile, four (8, 11, 15, 16) presented results that were adjusted for confounders (figure 2). In these four studies, the pooled crude odds ratio for risk of overweight per month of breastfeeding was 0.94 (95 percent CI: 0.89, 1.01; Q = 35.9, 3 df, p < 0.001). Adjustment for confounders weakened the trend (pooled adjusted OR = 0.97, 95 percent CI: 0.93, 1.02), but there was still heterogeneity (Q = 17.9, 3 df, p < 0.001).
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Adjustment for confounders reduces the heterogeneity in these findings, but residual confounding cannot be excluded; for example, only three studies (5
, 6, 11) adjusted for maternal body mass index. While breastfeeding appears to be associated with a reduced risk of overweight, it is unclear whether this is due to confounding. Similarly, consistency of effect across age groups, settings, and types of breastfeeding is not assured. The number of studies analyzed by Harder et al. (1), particularly those with adjustment for confounders, was too small relative to the possible sources of heterogeneity to conduct a meaningful meta-analysis. ACKNOWLEDGMENTS
Conflict of interest: none declared.
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