It has the makings of a nutritional Catch-22: Physical activity helps burn calories, which is important for reaching a healthy weight. But what if exercise fuels appetite and leads to consumption of more calories?
Scientists have tried to answer that question for years. Numerous studies suggest that moderate- to high-intensity aerobic exercise -- such as brisk walking, jogging, rowing or cycling -- has no impact on appetite in men. But a recent report by University of Ottawa researchers paints a different picture for women.
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The study compared the effects of easy workouts and high-intensity exercise on appetite in 13 fit, lean women in their twenties. After each morning exercise session, the women had lunch at a buffet where they could eat as much as they wanted. They also had access to unlimited snacks and dined at the buffet that evening.
Participants ate the most after high-intensity workouts, equivalent to jogging at too fast a pace to carry on a conversation. After these sessions, participants consumed an average of 878 calories for lunch. Compare that with the 819 calories they ate after an easier workout, equivalent to a brisk walk.
The study found that women ate about 35 percent of the calories they had burned in low- and moderate-intensity workouts. But after high-intensity workouts, they consumed "in excess of 90 percent of the calories burned," said Eric Doucet, assistant professor at the University of Ottawa's School of Human Kinetics and a co-author of the study, which was published in the American Journal of Clinical Nutrition. Even accounting for a slight boost in metabolism from exercise, the participants still overcompensated by eating more food after high-intensity exercise compared with easier workouts. Most of the added calories came from fat.
"Women just need to be aware of the fact that after high-intensity exercise, they might be tempted to overconsume food," said Doucet.
The results also help explain, he said, why "in weight-loss programs, men seem to respond better to exercise treatment than women do." In the Midwest Exercise Trial, a 16-month study of 131 overweight young adults who simply added physical activity to their daily regimen but didn't change what they ate, researchers found that men lost an average of 11 pounds while women maintained their weight.
Here's what experts say to keep in mind about exercise:
Physical activity always beats sedentary living. At the University of Ottawa, researchers also included a control session of no activity, after which the participants ate 751 calories for lunch. This was more than the net calories consumed after both exercise sessions, which were 565 calories for the high-intensity workouts and 530 for low-intensity workouts.
Skip the snack after workouts. You may feel that you deserve an energy bar or a sports drink for your efforts, but if you're trying to trim pounds, adding calories in the two hours after workouts can be counterproductive, Doucet said. Doing so short-circuits fat burning and can undercut "what you have done during exercise," he said. So quench your thirst with water. And if you must eat soon after a workout, avoid food that raises blood sugar and results in a burst of insulin. Smart choices: A piece of fruit or a glass of vegetable juice, Doucet suggests.
Expect only modest calorie burn from exercise. A 140-pound person who walks 1.5 miles in 30 minutes burns 100 calories -- about the amount found in a medium chocolate chip cookie. Most people are so sedentary that almost any activity feels strenuous, leading them to "overestimate the calories burned," said R. James Stubbs, who studies exercise and appetite at the Rowett Research Institute in Aberdeen, Scotland. It may feel like you burned a ton of calories in that step aerobics class, but you probably didn't.
Stay the course. A year-long study at the University of Pittsburgh found that adding regular physical activity produced weight loss in 200 overweight, sedentary women who were also dieting. Results ranged from a 5 percent reduction (about a seven-pound loss for a 150-pound person) for those who worked out about 20 minutes daily to a 13 percent loss (equal to about 20 pounds) for women who spent at least 30 minutes daily exercising. Plus, experts say exercise delivers benefits that go far beyond trimming pounds or maintaining weight: increased energy, better cardiovascular health, reduced stress, improved sleep and better mood. "Being active sharpens you," Stubbs said. "It closes down blood flow to the gut and opens it to the muscles, which is the exact opposite of sitting down in front of the television and eating food."
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