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Preventing Childhood Obesity: Health in the Balance
included overweight of the mother, maternal smoking during pregnancy, and low social status. In research on the weight status of 12,587 children in the United States at 4 years of age, Grummer-Strawn and Mei (2004) found that greater duration of breastfeeding showed a protective effect on the risk of overweight among non-Hispanic whites, but not among non-Hispanic blacks or Hispanics. The reasons for differences among ethnic groups are not clear; the study did not examine supplementation by formula or foods or varying dietary or physical activity patterns. A study by Bogen and colleagues (2004) also found no association between breastfeeding and obesity among 20,518 low-income black children (the study sample did not include Hispanics).
Breastfeeding is thought to promote the infant’s ability to regulate energy intake, allowing him or her to eat in response to internal hunger and satiety cues—that is, to assume greater control in determining meal size (Fisher et al., 2000). In contrast, a caregiver who is formula feeding an infant may use visual information about how much remains in the bottle to “encourage” the infant to finish the bottle, potentially fostering overfeeding. Even if the caregiver makes no such effort, the uniform composition of formula, both during a single feeding and over the duration of infancy, may not provide the infant with the same metabolic/hormonal cues that are supplied with breast milk. Because the composition of breast milk changes during each feed and from one feeding to the next over the course of lactation, the full effects of this variation are not experienced when breastfeeding is nonexclusive or of short duration (Lederman et al., 2004).
Factors in breast milk may elicit metabolic programming effects that contribute to the protective association between breastfeeding and childhood obesity. There is the possibility that other parental lifestyle factors and behaviors, not yet identified, may undermine or overwhelm that protection (Dewey, 2003). Lifestyle and cultural factors may also explain the discrepant findings among different ethnic groups. It is worth emphasizing that a protective effect of breastfeeding was found in the majority of studies reviewed although not in all. But in none of the 11 studies reviewed by Dewey (2003) or those published since that review has breastfeeding been associated with increased risk for childhood obesity; breastfeeding was found to be either protective or neutral. None of the studies have found formula feeding to be protective against childhood obesity.
Research indicates that many flavors from the mother’s diet are transmitted to her breast milk (Mennella and Beauchamp, 1991; Mennella, 1995). By the time complementary foods are introduced, therefore, the breastfed infant has already had experience with a variety of flavors from the adult diet, which may promote acceptance of foods during weaning (Sullivan and Birch, 1994; Mennella et al., 2001; Lederman et al., 2004). Experience with numerous flavors in breast milk (as opposed to the lack of