While breast-feeding has many benefits, it won't prevent a child from becoming fat as an adult, says a new study that challenges dogma from U.S. health officials.

The research is the largest study to date on breast-feeding and its effect on adult obesity.

"I'm the first to say breast-feeding is good. But I don't think it's the solution to reducing childhood or adult obesity," said the study's lead author, Karin Michels of Harvard Medical School.

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The U.S. Centers for Disease Control and Prevention promotes breast-feeding as a way to reduce children's excess weight, and the guidelines for federal chronic disease prevention grants to states call for breast-feeding promotion. Some health officials say 15 to 20 percent of obesity could be prevented through breast-feeding.

A CDC official said he couldn't comment on the new research because he hadn't fully reviewed it. But many previous studies have linked breast-feeding and lower rates of childhood obesity, he noted.

Perhaps the obesity-preventing benefits of breast-feeding are strong for children but wane by adulthood, said the official, Larry Grummer-Strawn.

"It would be remarkable to find a behavior that you engage in for one year of life and see detectable effects from it 40 years later," said Grummer-Strawn, chief of the CDC's maternal and child nutrition branch.

Good or bad eating and exercise habits, developed later in life, may sustain or erase initial weight-related benefits from breast-feeding, he and other experts said. Of course, that doesn't take away the other benefits of breast-feeding, such as building a child's immunity to disease.

The Harvard study, published online this week in the International Journal of Obesity, involved nearly 14,500 women who were breast-fed as infants and more than 21,000 who were not.

In 1989, the women were asked their height and weight and what those measurements were when they were children and at age 18. Then every two years, through 2001, they were asked to update their weight information. The surveyed women were all between 25 and 42 at the time of the 1989 questionnaires, Michels said.

In 2001, the mothers of these women were sent a questionnaire asking if their daughters had been breast-fed and for how long.

When possible, researchers checked medical records to confirm what the mothers and daughters recalled, but breast-feeding is not routinely documented. Still, the researchers believe the women's recollections of breast-feeding are reliable.

"A mother knows whether she breast-fed her child," said Michels, an associate professor of epidemiology.

Women who were breast-fed for at least a week had a risk of being overweight or obese that was nearly identical to that of women who were bottle-fed, the study found. And duration of breast-feeding didn't seem to make a difference. The women who had been breast-fed for more than nine months had a risk of becoming overweight or obese similar to that of women breast-fed less than one week.

The study involved only women, but the researchers believe the results are equally true for men, Michels said.

Michels believes that one reason previous studies might have been misleading about breast-feeding's effects on weight gain is that many of those studies failed to properly account for socioeconomic factors that also may have had an influence.

An Emory University pediatrics expert said he is not surprised by the study's findings about adults. But he said breast-feeding is beneficial for children, and the government's health message should not change.

"The first step is preventing childhood obesity. We know obese children have to work harder to not become obese adults," said Dr. Robert Geller, an associate professor who also is chief of pediatrics for Atlanta's Grady Health System.

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