CHICAGO (AP) - Two new U.S. studies add to evidence that breast-fed infants may face a lower risk of becoming overweight children, although one suggests factors including the mother's weight are much more important.

In a study by Harvard Medical School researchers, the longer infants had breast-fed, the less likely they were to be overweight in adolescence. In a separate study, U.S. government researchers found that while breast-fed infants were less likely to be overweight at ages 3 to 5 than formula-fed infants, duration of breast-feeding didn't make much difference.

Neither study answers whether breast milk itself, the act of breast-feeding or socio-economic and lifestyle traits in the infants' mothers might explain the results.

Both studies are published in Wednesday's Journal of the American Medical Association.

Mary Hediger, a biologist at the National Institute of Child Health and Human Development and lead author of the government study, said higher income, better educated women are more likely to breast-feed but also more likely to promote healthy eating habits among their children.

In addition, overweight women are more likely to have overweight children and less likely to breast-feed than normal-weight women, Hediger said.

``Obesity tends to run in families,'' she said. ``Whether or not that's modifiable by breast-feeding remains to be seen.''

Her study is based on 1988-1994 health data from a nationally representative government survey. The researchers also interviewed the mothers of 2,685 survey subjects and gave the children physical exams when they were ages 3 to 5.

Children who'd ever been breast-fed as infants were 16 percent less likely to be overweight, and about 37 percent less likely to be at risk.

Children were considered overweight if their body-mass index - a height-to-weight ratio - was in the 95th percentile or higher, meaning higher than at least 95 percent of children their age. They were at risk for being overweight if they were in the 84th to 94th percentile.

Children were three times more likely to be overweight if their mothers were overweight.

The Harvard researchers questioned 15,341 children ages 9 to 14 and their mothers in 1996 and 1997. Youngsters who were breast-fed more than formula-fed were about 20 percent less likely to be overweight than children fed only or mostly formula. The link was weakened slightly when mothers' weight was factored in, said lead author Dr. Matthew Gillman.

Gillman theorized that breast-fed babies learn to ``self-regulate'' food intake better than formula-fed infants because they may have better control over stopping feeding when they're full. By contrast, parents who use formula may see an unfinished bottle and try to induce their infants to drink more, unwittingly encouraging them to ignore their bodies' own hunger cues, which could increase the risk for weight problems later on.

He also noted that breast-fed infants have been found to have lower levels of the hormone insulin, which promotes fat storage, than infants fed formula based on cow's milk.

An editorial published with the studies says their differences likely can be explained by the ages of the study subjects and the methods used.

Regardless, both suggest that breast-feeding may offer at least some protection against obesity, an epidemic in the United States that requires urgent solutions, Dr. William Dietz of the federal Centers for Disease Control and Prevention, wrote in the editorial. Breast milk, ``already acknowledged as the best food for infants,'' may provide a ``low-cost, readily available strategy,'' Dietz said.

Copyright 2001 The Associated Press. All rights reserved.