(USA TODAY) -- Normal-weight women who want to prevent weight gain as they age need to do an hour a day of moderate-intensity physical activity such as brisk walking, a study shows.

Over the years, some experts have suggested it takes that much to control weight, but this is one of the first large-scale studies to examine the question.

Researchers at Brigham and Women's Hospital and Harvard Medical School in Boston tracked the leisure-time physical activity and weight of more than 34,000 healthy women over 13 years. The women, average age 54 at the start of the study, followed a regular diet and didn't try to lose weight. Participants were divided into three groups:

*Women who were the most active and did at least 60 minutes a day of moderate-intensity physical activity, most of it walking.

*Those who were moderately active, at least 150 minutes a week (for example, at least 30 minutes a day five days a week).

*Those who were the least active, at less than 150 minutes a week.

Findings reported in today's issue of the Journal of the American Medical Association:

*Overall, study participants gained an average of 6 pounds in that time.

*Those in the two less-active groups were much more likely to gain 5 pounds or more than the most active women.

*Overweight and obese women tended to be less active than the healthy-weight women, and overall their physical activity was insufficient to help them control their weight.

*13% of women were "successful weight maintainers"-- they started at a healthy weight and gained less than 5 pounds at any checkpoint during the 13 years. They consistently did about 60 minutes a day of moderate-intensity exercise.

"There is no sugarcoating it," says I-Min Lee, lead author of the study and an associate epidemiologist at Brigham and Women's Hospital and associate professor of medicine at Harvard Medical School. This is what it takes to maintain weight given our diets, she says. It's not clear whether these findings apply to men and younger women, she says.

Walking was the most common activity of the women in the study, but if you do more vigorous activity such as running or jogging, 30 minutes is equal to 60 minutes of walking, she says. Other studies show physical activity can be accumulated in short bouts of at least 10 minutes each.

She emphasizes that it's possible to get the health benefits of physical activity, such as lowering the risk of heart disease, some types of cancers and type 2 diabetes, by following the government guidelines and doing at least 150 minutes of moderate-intensity physical activity each week (about 30 minutes a day five days a week or 2 1/2 hours a week) or 75 minutes of a vigorous-intensity activity, such as jogging or swimming laps, or a combination of the two types.

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