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Exercise and Weight Control: Study Results continued...

The two lesser active groups were about equal, however, in the amount of weight gained, she says.

Initially, Lee says, the relationship between physical activity and weight control looked like it applied to everyone. But it did not.

Lee and her team also looked at a subgroup of women -- those who started out at a healthy weight -- that is, with a body mass index or BMI of less than 25 -- and maintained a healthy weight throughout -- that is, gained less than 5 pounds at the three-year interval. Thirteen percent of the women, or 4,540, had a BMI lower than 25 at the study start and maintained a healthy weight throughout. ''We found the relationship between physical activity and less weight gain held only for the women with a BMI of less than 25."

For those whose BMI was 25 or more, ''there was no relationship between physical activity and weight gain, but perhaps because they just weren't very active," Lee says. For these women who are already overweight, she says, it seems exercise must be combined with calorie reduction to control weight.

The women who started at a healthy weight and kept their weight healthy consistently got in an hour of moderate intensity physical activity daily, Lee's team found.

Lee isn't sure whether the findings apply to men as they age. "U.S. men [as a whole] are more physically active than women," she says, and less likely to be obese. Lee has served as a consultant for Virgin HealthMiles, a worksite activity program, and is on its scientific advisory board. The study was funded by the National Institutes of Health.

Exercise and Weight Control: Hard Work

The study results are no surprise to Suzanne Phelan, PhD, assistant professor of kinesiology at California Polytechnic State University in San Luis Obispo and adjunct assistant professor of research at Brown Medical School in Providence, R.I., who has researched the topic.

"To keep a normal body weight is hard work," she says. As you age, she says, "you have to exercise a lot whether you are normal weight or maintaining a weight loss."

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