After all, that's the widely held belief: Regular workouts turn us into extraordinary fat burners.
Not so, at least not for moderate-intensity exercisers, according to Edward Melanson, PhD, an associate professor of medicine at the University of Colorado, Denver, who presented his research at the annual meeting of the American College of Sports Medicine in Seattle. The study is published in Exercise and Sport Sciences Reviews.
"Moderate duration exercise of an hour or less has little impact on 24-hour fat oxidation," Melanson concludes.
Most studies looking at the fat burning associated with exercise have been short-term studies -- spanning just a couple of hours -- that looked at people who hadn't eaten, he says. Melanson's team evaluated people in a more true-to-life scenario, following them over a 24-hour period during which they exercised and ate or did no exercise and ate.
"It's not that exercise doesn't burn fat," Melanson says. "It's just that we replace the calories."
"Exercise increases the capacity to burn more fat," he says. "But if you replace those calories, that is lost."
The findings shouldn't discourage people from exercise, Melanson says, but rather inspire them to become more realistic about "calories in, calories out" - and to expend more than they take in if they are trying to lose weight and body fat.
Exercise and Fat Burning
Melanson's team evaluated fat burning in 10 lean, endurance-trained participants, 10 lean but untrained people, and eight untrained and obese people during exercise conditions and sedentary conditions.
Participants were fed a diet that was 20 percent fat, 65 percent carbs, and 15 percent protein for three days before each session and on the day they exercised or did not exercise. On the exercise day, participants rode a stationary bike at a moderate intensity for one hour, burning about 400 calories.
When Melanson's team measured calorie expenditures, they were higher in each group when they exercised compared to when they did not, not surprisingly.
But they found that burning of carbohydrate, not fat, seemed to increase in the 24-hour period after exercising.
In the journal report, Melanson reports additional fat-burning studies, including one that compared seven men ages 60-75 with seven other men ages 20-30, with no differences in fat burning between groups for the 24 hours after exercise or no exercise.
Why don't we become long-term fat burners after a good workout? The most likely reason is that we eat. And what we eat affects fat burning.
For instance, eating as little as 240 calories of carbohydrate during the hour before exercise can reduce fat burning during exercise, and the boost in fat burning during exercise can be "blunted" for up to six hours after eating a meal, says Melanson, citing other research.
To maintain their low body fat, endurance-trained exercisers may simply eat less fat than they burn habitually, he says.
Fat Burning: Calories Count
The study findings are "dispelling the myth that you can create a 24-hour fat-burning situation after exercise," says Pete McCall, an exercise physiologist and spokesman for the American Council on Exercise.
But, he tells WebMD, the findings were limited to exercisers who did moderate-intensity exercise, and for an hour or less. "These results might not apply to different forms of exercise or higher-intensity exercise," McCall says.
Still, he says, the research results might be a crucial wake-up call. "The point of this study, I think, is [that] he is trying to get people out of that mind-set: 'I just worked out and I can eat whatever I want.'" At least for people trying to lose weight, McCall says, that's certainly not true
Melanson says that the take-home message from his research depends on whether you are trying to lose weight or just maintain. "If you are using exercise to lose body weight or body fat, you have to consider how many calories you are expending and how many you are taking in," he says. The goal is a negative fat balance.
"If your body mass index is below 25, you shouldn't be concerned about losing more body fat," he says.
By Kathleen Doheny
Reviewed by Louise Chang
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