Exercising Hard Increases Calorie Afterburn

Exercise helps burn extra calories after a workout is over, a new study says, but only if it is so intense that a person is breathing too heavily to carry on a conversation and for a long enough time.

The study examined 10 men, ages 22 to 33, who spent two 24-hours periods inside a metabolic chamber, according to the New York Times. During the first session, the men had to stay perfectly still, sitting in a chair and moving only to eat meals sent in through an air lock and to stretch periodically. The men burned, on average, 2,400 calories on this totally sedentary day.

During the second session inside the chamber, the men also had to stay completely still in a chair -- except for once in the morning, when they rode a stationary bicycle at a high intensity for 45 minutes. They worked out at about 70 percent of their VO2 max, the maximum amount of oxygen a person's body can take in during exercise -- an effort that made them breathe too heavily to carry on a conversation.

The cycling session itself burned about 420 calories, but the men also continued to burn an extra 190 calories over the next 14 hours, increasing the total calories burned by 37 percent.

Compare that to a different study, says the Times, that also used a metabolic chamber and tested the effects of exercise at a moderate level, but found no afterburn.

It's not clear exactly why extra calories continue to burn after an intense exercise session. "Part of the effect may be due to post-exercise energy metabolism: the body starts using more fat and less carbohydrate after a hard exercise session," reports the Times. "Several hormones that are released during exercise remain elevated in the blood afterward, increasing metabolism. And extra calories may be burned when the body replenishes glycogen, the sugar stored in muscles."

"But for the most part, the effect remains a mystery," the Times says.

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