No. What counts for losing weight is “the total number of calories burned,” said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at Tufts University’s Friedman School of Nutrition Science and Policy. It’s “bogus,” she added, to think that some type or intensity of exercise will burn fat while others will not.
Dr. George Blackburn, associate director of the Division of Nutrition at Harvard Medical School, emphatically agreed: “There is no such thing as fat-burning exercise as opposed to some other kind.”
What is true is that the percentages of fuel burned change during a workout. If you start out with an empty stomach, for the first 5 minutes of exercise, said Blackburn, you burn carbohydrate that is stored as glycogen in your muscles. Then the mitochondria, the energy-producing miniorgans inside cells, “start switching over to fat” as fuel. Eventually, you’ll be burning about half and half, glycogen and fat. In other words, the ratio of fuels changes.
But that has nothing to do with your heart rate, or how hard someone exercises.
“If you go slowly, the percent of fat that you burn will be more, but the overall calories you expend will be less,” said Nelson. It’s not the percent of carbohydrate or fat in the fuel that matters, but the total amount of calories burned. And the faster the exercise, the more calories burned.
As Blackburn put it, if you walk slowly, you’ll burn about 5 calories per minute; if you run, you’ll burn 10. In other words, it takes an hour of walking (at 5 calories burned per minute) to work off a 300-calorie hamburger; it takes 30 minutes of running.