Want to Stay Trim? Exercise an Hour a Day

March 27, 2010— -- 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.