Some studies have suggested young children who were breastfed as infants are less likely to be overweight than non-breastfed children, but those results have now been thrown into doubt by a large study of female nurses.
Karin Michels and her colleagues at Harvard Medical School set out to confirm whether breastfeeding reduces the risk of obesity in childhood and adulthood. Their study involved nearly 36,000 nurses born between 1946 and 1965, as well as information from the nurses’ mothers about whether they breastfed or used formula. The nurses were asked about their body shape and weight at age five, 10 and 18 years, and their adult weights were confirmed by medical records.
Just over 40 per cent of the nurses had been breastfed and 59 per cent had not been breastfed or were breastfed for less than one week. Nurses who were breastfed had, on average, a slightly higher birth weight than those who were not breastfed. In addition, their mothers were less likely to smoke, and both parents tended to have higher education.
Taking these factors into account is important, Michels says. While previous studies had shown that children with a history of being breastfed were less likely to be obese than children who were not breastfed, later analyses showed the leaner children also tended to be from families with higher education and better socioeconomic standing.
The nurses’ study showed a slight trend toward breastfed nurses being slightly leaner at age five than those who were not breastfed, but this difference was not considered significant. “From our data set, we found that throughout the life course, there is no association between a history of being breastfed and obesity,” Michels says.