Too busy to spend an hour at the gym? Try 10 minutes at home: Short bouts of exercise can help overweight women shed pounds and keep the weight off just as well as longer workouts, according to a new study.

Researchers at the University of Pittsburgh and Brown University enrolled 148 sedentary, overweight women, ages 25 to 45, in a weight loss program that combined diet and behavior modification with regular exercise.

The findings, published in last week's Journal of the American Medical Association (JAMA), add to the growing evidence of the health benefits of brief periods of physical activity.

"These results are very important," said Kelly Brownell, director of Yale University's Center for Eating and Weight Disorders. "They join those from previous studies showing that exercise does not have to be grueling or take large amounts of time to be beneficial."

Other studies have demonstrated that fitness could be improved by short bouts of regular exercise. But this is the first study to show that significant weight loss can also be achieved with brief bouts of exercise spaced throughout the day.

"It means that people can go for 10-minute walks four times a day and have as great a benefit in losing weight as setting aside 40 minutes to exercise," said Thomas A. Wadden, director of the Weight and Eating Disorders Program at the University of Pennsylvania.

The study randomly assigned participants, all of whom weighed 20 to 75 percent above the government's recommended standard, to one of three exercise groups. In the first group, women were instructed to exercise five times per week beginning with 20 minutes per session. The length of each workout was gradually increased so that by the ninth week of the 18-month program, the women were exercising in one 40-minute session five days per week.

In the second group, women also initially worked out five times a week for 20 minutes daily. By the ninth week, they too had progressed to 40 minutes of exercise. But rather than engage in a long bout of continuous exercise, these participants were instructed to break up their workouts into 10-minute increments to make physical activity easier and more convenient.

The third group of women also engaged in short periods of exercise spaced throughout the day, but were provided with electric treadmills for home use to make physical activity as accessible as possible. Like their counterparts in the study, they gradually increased their workout routine to 40 minutes a day five times a week.

All three groups received identical nutritional instruction on eating a well-balanced diet containing less than 20 percent of total calories as fat. They attended regular group meetings and were advised to limit their total daily intake to 1,200 to 1,500 calories, depending on their weight when they entered the study.

During the initial weeks of the study, the team of researchers, led by psychologists John Jakicic and Rena Wing of Brown University, found that 10-minute increments of exercise spread throughout the day were the most successful in motivating sedentary, overweight women to begin physical activity. Women who exercised in these short bouts wound up engaging in significantly more overall exercise than those who worked out for extended periods of time.

The findings suggest that short bouts of exercise "are the most advantageous" in getting sedentary people to start working out, said Jakicic. "People report that the major barrier to exercise is that they can't find the time. But they can do 10 minutes without being exhausted. It's a great way to start them and you can build from there."

Six months into the study, all three groups were exercising the same amount of time each week and all three groups were equally successful in losing weight. The average weight loss was 26 pounds.

Increasing exercise without cutting calories would have resulted in "five to seven pounds weight lost at best" during the same period, Wadden noted. "Dieting and exercise is necessary if you want to achieve a significant weight loss," he said.

Maintaining weight loss proved to be one of the most difficult aspects of the study. Nearly 80 percent of participants stayed in the study for 18 months. At that point, women in the long-exercise group had regained about six pounds while those who worked out in short bouts throughout the day had regained about nine pounds. These differences were not statistically significant, researchers said.

The group that did best at maintaining their weight loss, however, were those who exercised on home treadmills in short bouts throughout the day. The machines were set up in pleasant but high traffic areas of the home, making them readily accessible. Women in this group exercised more than those in the other two groups and regained on average only four pounds.

The findings suggest that having a treadmill at home "is a wonderful option for helping to build exercise into one's life," Jakicic said. "We had women in the study who were working moms with young kids, and it gave them the flexibility to exercise at times that otherwise would not allow exercise to be fit in."

Obesity continues to plague Americans at unprecedented rates. In a study also published last week in JAMA, researchers from the federal Centers for Disease Control and Prevention (CDC) reported that more than half of all U.S. adults are now overweight or obese. The rising prevalence of obesity-related diseases "emphasizes the need for concerted efforts to prevent and treat obesity," the research team lead by Aviva Must and William Dietz of the CDC concluded.

In a related study, a team of researchers from Columbia University, Johns Hopkins University, Harvard and the University of North Carolina at Chapel Hill concluded that 280,000 people die annually in the United States from obesity-related causes. Drawn from five ongoing studies, the findings note that obesity has become a major cause of death in the United States and a major public health problem.

Gauging a person's ideal body weight is now done by calculating the body mass index, or BMI, a number that takes into account an individual's height and weight. People with a BMI of 25 to 29 are considered overweight. Those with a BMI of 30 and above are considered obese. Under this measurement, a person 5 feet 5 inches tall who weighed 160 pounds would have BMI of 27, while someone the same height who weighed 180 pounds would have a BMI of 30.

Based on the latest findings from the CDC, the share of Americans with a BMI of 30 or more increased from 12 percent in 1991 to 18 percent in 1998. This steady rise in obesity was found in all ages, races and throughout all educational and income levels, according to the CDC. Some of the biggest increases were found among people aged 18 to 29, where obesity rose from 7 percent in 1991 to 12 percent in 1998.

The study found that obesity also increased significantly in individuals who had attended some college, jumping from 11 percent in 1991 to 18 percent in 1998 and among those of Hispanic heritage. In 1991, about 12 percent of Hispanics were obese compared with 21 percent in 1998.

"Without concerted initiatives to prevent and treat overweight in adults, the health care system will increasingly be overwhelmed with individuals who require treatment for obesity-related conditions," the CDC team concluded.