By LINDSEY TANNER
AP Medical Writer

updated 4:33 p.m. PT, Tues., March. 23, 2010
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-aged 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-aged 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 who exercises for an hour twice a week.

Thin as a younger adult, Katzin said the pounds started creeping up after she had her two children in the 1980s, despite exercising and watching what she eats. "It's just extremely frustrating and discouraging."

The study appears 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 research "reinforces in a nice, clear way the idea of how difficult it is to maintain a healthy weight in our society," said Foreyt, of the Baylor College of Medicine in Texas.