Exercise guidelines for adults differ among health organizations, depending on whether they emphasize overall health, weight loss or weight maintenance.
American College of Sports Medicine: Moderate activity 150 to 250 minutes a week to prevent weight gain. Institute of Medicine: 60 minutes a day to prevent becoming overweight.
Department of Health and Human Services: At least 150 minutes a week for health benefits, The Los Angeles Times reports.
The study is being published in the March 24/31 edition of the Journal of the American Medical Association.
"I don't want people to throw up their hands and say 'I can't do it,' " said I. Min Lee, the study's lead researcher and a doctor and associate professor at Harvard Medical School and Brigham and Women's Hospital in Boston.
Women at a normal weight who consume a normal diet can beat middle-age weight gain by working out intensely for 30 minutes a day, whether by running, cycling, swimming laps or working out at a gym.
Weight gain can also be prevented with 60 minutes of moderate activity, such as walking, a leisurely bike ride or playing catch, The Wall Street Journal reported.