Normal-weight women need an hour a day of moderate exercise in order to maintain a healthy weight, a new study found.
Women who exercised fewer than 420 minutes a week gained significantly more weight than those who met the 60 minute a day target, Dr. I-Min Lee of Brigham & Women's Hospital in Boston, and colleagues reported in this week's issue of the Journal of the American Medical Association.
For the heaviest women, however, there was no relationship between exercise and weight gain.
"If you want to prevent your weight gain over time, you need to be physically active at the level of 60 minutes of moderate-intensity physical activity, so the equivalent of one hour a day of brisk walking or 30 minutes a day of jogging or running," Lee said.
For heavier women, Lee said it's a "case of 'too late,' if you will -- for a woman who has a body mass index of 25 or higher, there was no relation between physical activity and weight change."
Federal guidelines have recommended at least 150 minutes a week of moderate-intensity exercise in order to gain substantial health benefits, but it remains unclear whether that amount can also prevent weight gain.
So the researchers assessed data from the 34,079 women in the Women's Health Study from 1992 to 2007. Their average age was 54 and they were all healthy at the start of the study.
Over of 13 years, the women gained an average of 5.7 pounds.
The researchers found that those who exercised the most -- 420 minutes or more a week -- gained significantly less weight than those who exercised less than 420 minutes per week.
They also found that women getting more than 420 minutes of exercise per week were significantly less likely to gain 5 pounds over a three-year period.
In adjusted analyses, those in the group that got the least exercise were 11 percent more likely to gain 5 pounds, and those expending between 150 and 420 minutes per week were 7 percent more likely to do so.
"These two lesser-activity groups of women were significantly more likely to gain weight compared with the most active group of women," Lee said.
The researchers also found that physical activity was inversely related to weight gain, but only among women with a body mass index (BMI) under 25. For those with a BMI greater than that, exercise had no effect on weight control.
"[For] the women who were overweight or obese, physical activity -- with the range done in the study -- was not sufficient to control their weight," Lee said.
Finally, the researchers investigated how much physical activity was done by women who successfully maintained normal weight.
A total of 13.3 percent of the women in the study maintained normal weight by gaining less than 5 pounds over the 13-year study period.
The average activity level among this group was just over 60 minutes a day of moderate-intensity exercise.
The researchers said the results "highlight two important points for weight gain prevention."
First, once patients are overweight, they said, it may be too late for physical activity to have any effects. Second, sustaining high levels of exercise is needed to maintain a normal BMI.
Still, Lee cautioned not to "lose sight of the fact that any amount of physical activity, even 150 minutes a week, is sufficient to lower the risk of developing many chronic diseases."
They noted that the study was limited by self-report of recreational physical activity and body weight.
However, their findings suggest that the "2008 federal recommendation for 150 minutes per week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction."