Overweight women who exercised one to two hours a week lost several pounds in six months without dieting, a study shows. But those who exercised the most — about three hours a week — didn't lose as much as they should have, possibly because they increased the calories they consumed.
"There is a great lesson here: People generally overestimate the calories they are burning with exercise, and they may reward themselves by eating more," says lead researcher Tim Church, director of preventive-medicine research at the Pennington Biomedical Research Center in Baton Rouge.
If a person runs for an hour then goes out to have a large high-calorie coffee drink and a large muffin, she probably consumed more calories than she burned, he says.
For the latest study, Church and colleagues recruited 411 overweight or obese sedentary women, average age 57. Some of the women were told not to change their level of physical activity. The others were divided into three groups and instructed to do different amounts each week. They rotated walking briskly on a treadmill at a fitness center with riding a stationary bike.
Their workouts were monitored closely by the researchers, and the women were asked not to change their eating habits during the study.
At the end of six months, one group of exercisers was doing an average of 72 minutes of moderate physical activity a week; another group was doing about 136 minutes (a little more than two hours); and the third was doing 194 minutes (slightly more than three hours).
The findings reported in PLoSONE, a Public Library of Science online journal:
• Women who did 72 minutes of physical activity a week lost 2 to 3 pounds in six months, which was what was expected from the amount of exercise they were doing.
• Those doing more than two hours of activity a week lost slightly more than 4 pounds in that time, which also was what was expected.
• Those who did three hours of activity a week lost only about 2 to 3 pounds, but they should have lost almost 6 pounds from the increase in physical activity.
• Everyone who exercised lost dangerous belly fat. Those who didn't lose weight dropped about an inch around the middle. Those who did lose weight trimmed an average of 2 inches around the waist.
This confirms other research that physical activity alone can contribute to some weight loss. Another recent study showed that women who increased their activity level by an additional 3,500 steps a day lost 5 pounds during the year. Men who added that many steps lost 8½ pounds in a year.
Another study showed that when obese women improved their eating habits and walked briskly for an additional 50 minutes to an hour a day, they lost about 10% of their body weight in six months.
Church says a growing body of evidence shows that physical activity decreases dangerous abdominal fat.
"Exercise without weight loss has a benefit to your waist, but exercise with weight loss has even more benefit to your waist," he says. This study did not include men, but he says other research he has done shows the same phenomenon in men.
The loss of belly fat through exercise is important because excess abdominal fat is considered a risk factor for many chronic diseases, including heart disease, diabetes, stroke and some types of cancer, Church says.
Experts believe the fat cells deep in the abdomen are harmful because they secrete chemicals that play a role in a number of diseases. The cells produce about three times more bad chemicals than subcutaneous fat, the stuff you can pinch right under your skin, Church says. "Plus, the plumbing of visceral fat drains directly to the liver, allowing the bad chemicals to directly interfere with the liver's ability to metabolize blood sugar and cholesterol."
In addition to lowering the risk of many serious medical conditions, regular physical activity also improves quality of life by reducing stress, depression and anxiety, and by improving bone and joint health, sex drive, sleep and memory, Church says.
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