Can exercise keep you from gaining weight?

Yes, if you do more of it than most of us do, a Harvard Medical School study concludes.

Among normal-weight women, the likelihood of putting on weight decreased as physical activity level increased. Among women who were overweight or obese, there was no relationship between physical activity and weight gain.

Thus, for women who aren’t overweight or obese, exercise can keep off excess pounds, but it must add up to about seven hours per week of moderately intense activity such as brisk walking or casual bicycling — or 3.5 hours per week of vigorous activity such as jogging or aerobic dancing.

For women who are already overweight or obese, increased physical activity alone is not enough to prevent further weight gain. These women also need to reduce their calorie intake. But they should still get at least 30 minutes of moderately intense physical activity most days for the sake of the many health benefits, which include a reduced risk of chronic conditions such as heart disease and stroke.

The researchers analyzed data provided by 34,079 healthy women, average age 54, who were participating in the long-term Women’s Health Study. Between 1992 and 2007, the women reported their body weight and physical activities every three years. They also provided information on matters that could affect the link between physical activity and weight change, such as smoking, postmenopausal hormone use, alcohol intake, and diet.