Depending on whom you ask, the answer to this question is either one of the great myths of exercise or one of the great unappreciated truths: Is there an afterburn effect from a workout? Whether the metabolism speeds up for hours after exercise an old question, first studied a century ago, and over the years, study after study has been carried out, with decidedly mixed results. Some investigators found no post-exercise effect. Others reported effects so small they were almost unnoticeable — one found male triathletes burned just 12 to 30 extra calories after a workout. Others found as many as 700 additional calories were burned after a long and exhausting exercise session.
The latest sally comes in a recent paper in the journal Medicine & Science in Sports & Exercise. Its lead author, Amy A. Knab of Appalachian State University, says it trumps studies that preceded it because of its careful design. And its results are good news — sort of.
Dr. Knab and her colleagues recruited 10 men, ages 22 to 33, who agreed to spend two periods of 24 hours each in a metabolic chamber, a small room that measures the calories people burn while they are inside. The men were not all athletes, but they did have to be able to ride a bike vigorously.
On the first visit to the chamber, the subjects had to stay perfectly still, sitting in a chair and moving only to eat meals, which were sent in through an air lock. In the afternoon, they were permitted a two-minute stretch every hour. Bedtime was 10:30 p.m. At 6:30 the next morning, the subjects were awakened and allowed to leave. They burned, on average, 2,400 calories on this totally sedentary day.
The second visit to the chamber came two days later. Everything was the same, with one exception. At 11 a.m., the subjects rode a stationary bicycle at a high intensity for 45 minutes.
The exercise itself burned about 420 calories, Dr. Knab and her colleagues reported. But what was most interesting was the calories burned afterward. Over the next 14 hours, the men burned an extra 190 calories, increasing the total calories burned by 37 percent.
“We were surprised,” Dr. Knab said. She thought there might be extra calories burned, but she did not expect so many, nor did she expect the effect to last so long.
She suspects one reason she saw such a pronounced effect was that the exercise was so intense. The subjects had had to cycle at 70 percent of their so-called 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. And they had to keep it up for 45 minutes.
A different study, also using a metabolic chamber, tested the effects of moderate exercise and found no afterburn. Those subjects exercised at 50 percent of their VO2 max, a level that still allows conversation.
Claude Bouchard, a scientist at the Pennington Biomedical Research Center in Baton Rouge, La., along with other researchers, investigated the exercise effect with conventional methods using a mouthpiece and nose clip or a ventilated hood to determine oxygen used and carbon dioxide exhaled. From those measures, researchers can calculate calories burned.
They find, he says, that when studies are done properly (many are not), extra calories are burned in the hours after exercise — but only if subjects exercise at least as hard and long as Dr. Knab’s subjects. And if they exercise even harder, they burn even more calories afterward.
A recent book that Dr. Bouchard and a colleague edited notes two studies that found this effect. The researchers found that if subjects ran at 70 percent of their VO2 max or cycled at 75 percent of it, they could burn 300 to 700 extra calories after the exercise was over, though 700 calories was unusual.
It is not clear why extra calories should be burned after a bout of intense exercise, Dr. Bouchard says. 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. 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.
Whatever the cause, researchers say, the extra calories burned after exercise can help people lose weight. Unfortunately, those who may have the most to lose may have the hardest time doing the sort of exercise that gives them a calorie-burning bonus.
The usual guideline for general health is 30 minutes of moderate exercise most days of the week. That’s doable for most people and should increase heart health, even if it provides no calorie bonus.
But that sort of moderate exercise, Dr. Bouchard said, “is what we recommend — that’s the target.”
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