A recent study found slim women can maintain their weight with an hour a day of moderate exercise, but overweight women have to reduce what they eat as well as exercise to fight obesity.
Out of some 34,000 middle-aged women, those who were not overweight and got the equivalent of at least an hour a day of exercise were the only ones who maintained their weight during the 13-year study conducted by researchers from Harvard Medical School and Brigham and Women's hospital in Boston.
"Their average activity level over the study was 21.5 MET hours per week -- approximately 60 minutes a day of moderate-intensity activity," the researchers wrote in the Journal of the American Medical Association, where the study was published on Tuesday.
One MET, or metabolic equivalent, is the energy expenditure and caloric requirement at rest. Mild exercise such as walking at a leisurely pace increases energy expenditure to around 2.5 METs per hour while vigorous activity such as running can result in six to more than 12 METs per hour.
Twenty-one MET hours per week is the equivalent of about one hour a day of moderate activity.
Women who got less exercise gained considerably more weight than the hour-a-day-of-exercise women, the study showed.
At the beginning of the study and regular intervals throughout, the women reported their physical activity and body weight to the researchers.
They were classified into different groups: less than 7.5 MET hours a week, or around the current US federal health authorities' exercise guidelines of around 20 minutes a day; between 75 and 21.5 METs weekly, or more than 21.5 a week.
While normal-weight women can get away with just exercising to maintain their weight -- even if it means putting in quite a few hours a week on the step machine or jogging path -- heavier women got even heavier with a regimen of exercise alone, the researchers found.
Given the findings, the researchers warned that the current federal guidelines of getting 20 minutes a day of exercise would be "insufficient without caloric restriction" in preventing weight gain in overweight and obese women, even if it would help to lower the risk of developing chronic disease.
The study takes on great weight itself in the light of the dramatic incidence of overweight and obesity in the United States, where one in three adults is obese.
Despite Americans' long-running battle with their bulging midriffs, most research up to now has focused on how to treat overweight and obesity, not on how to prevent weight gain, the authors said.