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CHICAGO -- Rev up the treadmill: Sobering new research spells out just how much exercise women need to keep the flab off as they age -- and it's a lot.
At least an hour of moderate activity a day is needed for older women at a healthy weight who aren't dieting. For those who are already overweight -- and that's most American women -- even more exercise is called for to avoid gaining weight without eating less, the study results suggest.
"We all have to work at it. If it were easy to be skinny, we would all be skinny," said John Foreyt, a behavioral medicine expert who reviewed the study but wasn't involved in the research.
Brisk walking, leisurely bicycling and golfing are all examples of moderate exercise. But don't throw in the towel if you can't do those things for at least an hour a day.
Even a little exercise is good for your health even if it won't make you thin, the researchers said.
Their findings are based on 34,079 non-dieting middle-age women followed for about 13 years. The women gained an average of almost 6 pounds during the study.
Those who started out at a healthy weight, with a body mass index less than 25, and who gained little or no weight during the study consistently got the equivalent of about an hour of moderate activity daily. Few women -- only 13 percent -- were in this category.
Few already overweight women got that amount of exercise, and the results suggest it wasn't enough to stop them from gaining weight.
The results echo what gymfuls of middle-age American women see every time they step off the treadmill and onto the scale.
"Talk to any group of women and they all say the same thing," said Janet Katzin, 61, a "slightly overweight" marketing director from Long Island, N.Y., who exercises for an hour twice a week.
Katzin said the pounds started creeping up after she had her two children in the 1980s, despite exercising and watching what she eats.
The study appeared in Wednesday's Journal of the American Medical Association. Only women were studied, so the researchers from Harvard's Brigham and Women Hospital said it's uncertain whether the results would apply to men.
The results bolster a 2002 Institute of Medicine report that emphasized the importance of balancing diet and exercise and recommended at least 60 minutes daily of moderate activity for adults and children. But the study also indicates that the 2008 U.S. guidelines urging about a half-hour of exercise five days a week won't stop weight gain while getting older without cutting calories, said Dr. I-Min Lee, the study's lead author.
The study underscores some inevitabilities about aging. Men and women often put on weight, partly because their metabolism slows down.
Hormonal changes in menopause also can make women prone to weight gain, especially around the belly.
Still, Lee emphasized that the benefits of exercise extend beyond what you see in the mirror, helping keep the heart healthy and protecting against chronic disease even if you don't get enough activity to lose weight.
The researchers analyzed data on women who took part in a long-running federal study. Participants were 54 on average at the start and periodically reported how much they exercised and weighed. They also reported eating habits at the start, but not throughout, a limitation the authors acknowledged.
Dr. Howard Eisenson, who heads Duke University's diet and fitness center, said it's likely some women underestimated what they ate and overestimated how much they exercised, which could have skewed the results.
Still, Eisenson said he doesn't encourage anybody to try to lose weight by exercise alone. To combat age-related weight gain, "you're fighting in many cases a losing battle" if you don't also cut calories, he said.
That doesn't mean you have to starve yourself, but it does mean watching what you eat and avoiding frequent indulgences.
"You can eat a candy bar in two minutes. Most are at least 200 calories," and to burn that off requires walking for about an hour, Lee said. Knowing that equation can help people make wise decisions about activity and food choices, she said.