In the battle against obesity, physical activity is a potent weapon, but just how much daily exercise is needed to win the war has been the subject of debate.

Guidelines range from 30 to 60 minutes per day, five days a week, to avoid weight gain. A new study from Brigham and Women’s Hospital leans toward the high end, reporting that women who did at least 60 minutes a day of moderately intense exercise kept extra pounds off, but only if their weight was normal to begin with. I-Min Lee and her colleagues tracked more than 34,000 healthy women who ate a normal diet while enrolled in the national Women’s Health Study. Their average age was 54 when the study began in 1992. Every two to three years they reported their weight and physical activity on questionnaires.

After 13 years, the women gained an average of about 6 pounds, which is typical for middle-age women as they grow older. Only 13 percent of the women gained fewer than 6 pounds. These women were different in two ways. First, they began with a body mass index below 25, which for a 5-foot, 5-inch woman means weighing no more than 150 pounds. Second, they were physically active at least 60 minutes a day.

The findings were published today in the Journal of the American Medical Association. The researchers divided the women into three groups based on how active they were: fewer than 30 minutes a day, 30 to 60 minutes a day, and more than 60 minutes a day.

  Brisk walking and leisurely bicycling were considered moderate exercise, while running, fast bicycling, and swimming laps were called high-intensity activity. Women in each of the less-active groups were equally likely to gain at least 5 pounds, leading the authors to recommend that people follow recommendations from the Institute of Medicine, an organization of leading medical experts who advise the government and public on health issues, for at least 60 minutes of activity a day. The US Department of Health and Human Services’ standard is 30.

But Lee doesn’t want people to give up on exercise, even if they can’t do an hour a day. “It’s the best thing you can do for your health,”’ she said in an interview.

By Elizabeth Cooney
Globe Correspondent
2010 Globe Newspaper Company.