A new study suggests that the longer infants are breastfed, the lower the likelihood they’ll be overweight as adolescents, a relationship that does not appear to be influenced by sociocultural factors.

The findings, published in the journal Epidemiology, add to the not always consistent body of research on breastfeeding and childhood weight gain. While a number of studies have suggested that breastfed babies are less likely to become overweight than bottle-fed infants, others have found no such benefit or that the weight difference does not last far into childhood.

In the new study, however, Harvard researchers found that even within a single family, children who were breastfed for a longer time were slightly less likely to become overweight than their siblings who were breastfed for a shorter period.

The difference within families was similar to that found in the study population as a whole, where each 4-month increase in breastfeeding was linked to a 6 percent dip in the risk of becoming overweight by adolescence.

Since siblings are raised under much the same circumstances, the findings “lend credence” to the idea that breastfeeding itself confers a weight benefit, Dr. Matthew W. Gillman, the study’s lead author, told Reuters Health.

One of the obstacles in studying the effects of breastfeeding on childhood weight is that both are “socially patterned,” explained Gillman, an associate professor of nutrition at the Harvard School of Public Health in Boston.

For example, mothers with more education or higher incomes are more likely to breastfeed, and their children are also less likely to be overweight. So studies need to control for such influences.

Following families in which siblings had different breastfeeding patterns accomplishes that to a large degree.

For their study, Gillman and his colleagues surveyed 5,614 siblings between the ages of 9 and 14 who were part of a larger study that had previously linked longer breastfeeding duration to a lower risk of obesity later in life.

The fact that the findings within families were close to those in the overall group suggests that breastfeeding itself affects weight later in life, according to the researchers.

The reason is not entirely clear, but one general theory, Gillman said, is that breast milk has lasting metabolic effects that aid in weight control. Another, he added, is that breastfeeding has behavioral effects; with breastfeeding, the length of any one feeding depends mostly on the baby, whereas mothers who bottle-feed may keep feeding their infants until the bottle is empty.

In this way, Gillman explained, breastfeeding may encourage more “self-regulation” of calorie intake later in life.

Whatever the effects of breastfeeding on a child’s weight, he noted, breast milk is considered the best nutrition for infants, so the possibility of weight benefits could be seen as a potential bonus to a healthy practice.

Experts recommend that babies be breastfeed exclusively for at least the first six months of life.

SOURCE: Epidemiology, January 2006.