Overweight people who lost and kept off 25 to 30 pounds walked briskly for about 50 minutes a day five days a week, according to the longest clinical study ever done on exercise's effect on weight loss and maintenance.

The study was presented here last week at the annual meeting of the Obesity Society, an organization of weight-loss professionals.

To evaluate the effect of exercise on weight loss, John Jakicic and colleagues at the University of Pittsburgh worked with 191 adult women, most of whom were obese — roughly 30 or more pounds over a healthy weight. The dieters were advised to follow a low-fat eating program of 1,200 to 1,500 calories a day.

Participants were assigned different amounts of activity and levels of intensity. About 80% of them chose to walk briskly; some walked more quickly than others. The findings after two years:

• The amount of time people exercised varied from less than 150 minutes a week to more than 200 minutes a week.

• All participants lost weight, an average of 7.2% of their starting body weight.

• Women who exercised the most — more than 309 minutes a week by the end of the first year and more than 270 minutes a week at the end of two years — lost and kept off the most weight, about 13% of their starting weight, or 25 to 30 pounds.

"It appears you need this amount of activity to achieve and sustain weight loss of this magnitude," says Jakicic, chairman of the university's department of health and physical activity.

Most people are going to have to work up to this level of exercise, he says. "It's important to walk with intensity like you're late for an important meeting."

Exercise as a weight-loss tool was much on the mind of obesity researchers at the conference. In another small pilot study, researchers at Fairleigh Dickinson University in Teaneck, N.J., asked 41 women, who weighed an average of 192 pounds, to follow a program that included limiting calories, walking and attending weekly group meetings. The sessions were followed by a group walk.

Half of the women were given portable CD players with headsets and told to listen to the music of their choice every time they walked. The other group did so with no music.

Participants were expected to walk at least three times a week. They followed a prescribed program that gradually increased distance and speed. By the end of the study, participants were doing 2 miles in 32 minutes. They were asked to keep daily records of their exercise and food intake and reduce their calories to 1,200 to 1,500 calories a day.

The findings after six months:

•The music-listening group adhered more closely to the walking program than the non-music group. Its members lost an average of 16 pounds and 4% of their body fat.

• The non-music group dropped an average 8 pounds and 2% body fat.

"The big problem with a lot of exercise programs is people don't adhere to them," says Christopher Capuano, director of the school of psychology. "These data suggest that listening to music while exercising enhances adherence and weight loss."