The weekly total of 420 minutes is nearly triple the 150 minutes of moderate daily exercise currently recommended by U.S. health officials and illustrates the challenge American women face in maintaining a healthy weight.

Winning that war will require individuals to make changes in their daily routines -- like walking or biking to work -- but it may also take a shift in policy to make it easier for people in fit exercise into their lives, researchers said.

Two-thirds of American adults are overweight or obese, increasing the risk of heart disease, diabetes and other chronic illnesses, and adding about $150 billion a year to U.S. healthcare costs.

From a public health perspective, it would be better to prevent the weight gain in the first place, said I-Min Lee of Brigham and Women's Hospital and Harvard Medical School in Boston, whose study appears in the Journal of the American Medical Association.

Lee said there is ample research on people who are already overweight, but little on how to prevent weight gain as people grow older.

Current U.S. exercise recommendations differ, with 2008 guidelines recommending 150 minutes of moderate exercise -- brisk walking, gardening, ballroom dancing -- as a way to improve overall health, and a 2002 report by the Institute of Medicine, one of the National Academies of Sciences, recommending an hour a day.

If people exercise vigorously, by running or cycling hard, for instance, less time is needed to get the same benefits.

GAINING WITH THE YEARS

Lee's team studied the guidelines in more than 34,000 healthy U.S. women with an average age of 54 who ate a typical American diet.

The women reported their weight and weekly exercise totals in the first year, and then at 3-year intervals from 1992 to 2007.

Over the course of the study, the women gained an average of 5.7 pounds (2.6 kg) overall.

Only 13 percent of women in the study maintained a healthy weight throughout the study -- and those who got an hour of exercise a day on average or more were by far the most likely to be in that group.

Only in this group of women did we find physical activity was associated with less weight gain, Lee said. For the heaviest women, no amount of exercise helped, they found. They needed to diet, also.

Lee said the results suggest that the current recommendations of two and a half hours per week are not enough to keep middle-aged women from gaining weight as they age.

Lee said women should not let the findings discourage them from exercising at all, but they may want to make small changes now to prevent later weight gain.

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

If seven hours a week are just too hard to fit in, Lee said people might want to consider vigorous exercise such as jogging, which can cut the weekly time requirement in half.

And she said policymakers need to consider changes that make it easier for people to exercise, such as building sidewalks or bike lanes that make it easier and safer for people to exercise.

Medicine