If your No. 1 excuse for not exercising is lack of time, we just fixed that.

Women who bike just five minutes a day gain about 1.5 fewer pounds as they enter middle age than their sedentary friends, according to researchers from the Harvard School of Public Health in Boston. Health.com reports that biking--either on a bicycle or stationary bike--appears to prevent weight gain in women as they age, especially if they are overweight to begin with.

The study: The Harvard team followed more than 18,000 premenopausal women between the ages of 25 and 42 for 16 years, during which time the women gained, on average, 20.5 pounds. The study participants were all nurses enrolled in a larger, national study on health and lifestyle that began in 1989. At the start of the study, about half reported walking slowly, 39 percent walked briskly and 48 percent biked, which included working out on a stationary bike. Sixteen years later in 2005, the average physical activity had increased a bit, but still remained low overall, with participants walking briskly just one hour a week and biking just 18 minutes a week.

The results: The women who biked for just five minutes a day gained about 1.5 fewer pounds over the course of the study than similar women who didn't take up biking. And it gets better. Women who spent a half-hour a day biking kept even more weight off, gaining about 3.5 fewer pounds than those who only biked five minutes a day.

Is biking better than walking? "Bicycling is an answer to weight control," lead study author Dr. Anne Lusk told Health.com. "Walking is not necessarily an answer, unless the person is walking briskly." She said that walking only helps women prevent weight gain if they walk briskly at a pace of at least three miles per hour. Women who walk at a slower pace are not able to fight off the weight gain.

The women who began the study either overweight or obese experienced even better results than normal-weight women, especially when they increased their daily physical activity to 30 minutes of biking. Over the study period, they gained about seven pounds less than those who didn't bike as long.

This is not only the first study to focus specifically on the benefits of biking, but also the first to compare biking with walking. "A lot of information on physical activity provided to women is very general, encouraging daily activity, but not specifically what kind," Keri Gans, a spokesperson for the American Dietetic Association, told Health.com. "This study encourages an activity that is not expensive and that almost all women can easily engage in. And if a woman is presently a walker, it's good to know that she must pick up her pace."

The study findings were published in the Archives of Internal Medicine.

--From the Editors at Netscape