Hoping to keep up that girlish figure into middle age and beyond while still eating as much as you always have? You're going to need a lot of exercise to do it.

Sobering new research finds that women who don't want to cut back on how much they eat are going to need at least an hour of moderate activity every single day to maintain a healthy weight.

That adds up to 420 minutes of exercise a week -- nearly triple the 150 minutes (30 minutes, five days a week) currently recommended by most health authorities.

For those who are already overweight -- and that's the majority of adult women in Canada and the U.S. -- it seems even more exercise is needed to avoid gaining weight.

The researchers at Brigham and Women's Hospital and Harvard Medical School in Boston say they started their study after noticing plenty of research on how to help overweight people lose weight, but not much on how to avoid gaining that weight in the first place.

They say this is the first study to attempt to quantify how much physical activity is needed to prevent long-term weight gain in women.

They tracked for 13 years more than 34,000 U.S. women who took part in the long-term Women's Health Study. The women, who were 54 years old on average at the study start and ate an average American diet, had their weight measured every three years from 1992 to 2007.

Over that time, the women gained an average of almost six pounds.

Only 13 per cent of the women maintained a healthy weight throughout the study. Those were the women who got an hour of "moderate" exercise a day on average or more.

Brisk walking, leisurely cycling, dancing or playing tennis are all examples of moderate exercise. It means working hard enough to raise your heart rate and break a sweat while still being able to carry on a conversation.

"These data suggest that the 2008 federal recommendation for 150 minutes per week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction," the authors state.

The study appears in Wednesday's JAMA, the Journal of the American Medical Association.

Interestingly, though, exercise seemed to benefit only those women who were a healthy weight at the start of the study – those who had a body mass index of less than 25. For the heaviest women, no amount of exercise helped them maintain their weight.

That finding, say the authors, emphasizes "the importance of controlling caloric intake for weight maintenance in this group."

Lead author I-Min Lee, an epidemiologist at Harvard's School of Public Health, says the findings shouldn't discourage women. She notes that even small amounts of exercise bring health benefits.

Exercise can prevent or slow heart disease, reduce falls and offer other health benefits -- even if it won't stave off the weight gain that typically affects women as they age.

Lee notes that as many people age, they tend to become more sedentary but continue to eat as much as they did in their younger years. She says even small changes to everyday living can help to prevent weight gain.

"I think the easiest thing is actually commuting," she said in comments to Reuters, suggesting people walk or bike to work, and if they drive, to park farther away from the office.

She also recommends that women who don't want to take on more physical activity should permanently cut back on calories, to prevent packing on more pounds.