24-Hour Fat Burning From Exercise?

Women on exercise bikes.

CBS

Many of us get through a tough workout by thinking about the fat burning that will occur during theexercise and for the next 24 hours or so.

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, anassociate professor of medicine at the University of Colorado, Denver, who presented his research atthe annual meeting of the American College of Sports Medicine in Seattle. The study is published inExercise and Sport Sciences Reviews.

"Moderate duration exercise of an hour or less has little impact on 24-hour fat oxidation," Melansonconcludes.

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 teamevaluated people in a more true-to-life scenario, following them over a 24-hour period during whichthey 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 islost."

The findings shouldn't discourage people from exercise, Melanson says, but rather inspire them tobecome more realistic about "calories in, calories out" - and to expend more than they take in if theyare 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 butuntrained people, and eight untrained and obese people during exercise conditions and sedentaryconditions.

Participants were fed a diet that was 20 percent fat, 65 percent carbs, and 15 percent protein forthree days before each session and on the day they exercised or did not exercise. On the exerciseday, participants rode a stationary bike at a moderate intensity for one hour, burning about 400calories.

When Melanson's team measured calorie expenditures, they were higher in each group when theyexercised 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 afterexercising.

In the journal report, Melanson reports additional fat-burning studies, including one that comparedseven men ages 60-75 with seven other men ages 20-30, with no differences in fat burning betweengroups 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 weeat. And what we eat affects fat burning.

For instance, eating as little as 240 calories of carbohydrate during the hour before exercise canreduce fat burning during exercise, and the boost in fat burning during exercise can be "blunted" forup 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 burnhabitually, he says.

Fat Burning: Calories Count

The study findings are "dispelling the myth that you can create a 24-hour fat-burning situation afterexercise," says Pete McCall, an exercise physiologist and spokesman for the American Council onExercise.

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 crucialwake-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.'" Atleast for people trying to lose weight, McCall says,that's certainly not true

Melanson says that the take-home message from hisresearch depends on whether you are trying to loseweight 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 howmany 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," hesays.

By Kathleen Doheny

Reviewed by Louise Chang

©2005-2008 WebMD, LLC. All rights reserved

© 2009 WebMD, LLC.. All Rights Reserved.

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