Researchers looked at more than 43,000 children, aged 7 and 8, with records of how they were fed during infancy.
The study was published online Aug. 12 in the journal JAMA Pediatrics.
The investigators first took into account other factors that could influence the children's weight, such as gender and the amount of time they spent in front of the television or computer, as well as their mothers' education levels, smoking and employment status, according to a journal news release.
Children who were breast-fed at 6 months to 7 months of age were less likely to be overweight than those who were formula-fed, the researchers concluded.
"After adjusting for potential confounders, we demonstrated that breast-feeding is associated with decreased risk of overweight and obesity among school children in Japan, and the protective association is stronger for obesity than overweight," wrote Michiyo Yamakawa, of the Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, and colleagues.
Although the study found an association between being exclusively breast-fed in infancy and a lower risk of obesity or overweight in young children, it did not prove a cause-and-effect relationship