“A colleague once told me, ‘I’m only walking at two miles an hour because I’m burning more fat that way,’” recalls John Porcari, professor of exercise and sport science at the University of Wisconsin−La Crosse.
Why? She wanted to be in the “fat-burning zone.”
It’s true that when you’re strolling or sitting on the sofa, you do burn more calories from fat than from carbs.
“Your body doesn’t burn fat very efficiently,” notes Porcari. “But it doesn’t need to be efficient when you’re not working hard.”
“But as we progress to a higher heart rate—that is, higher-intensity exercise—our ability to rely on fat for energy decreases and carbohydrate becomes our preferred fuel,” explains Jenna Gillen, assistant professor of kinesiology and physical education at the University of Toronto.
That's no reason to slow down
“You may be burning a greater percentage of fat during low-intensity exercise,” says Gillen, “but that doesn’t mean you’re burning a larger amount of fat. Unless you exercise for a long time, you’re not burning many calories.”
In one study, 27 women with obesity who were on a low-calorie diet were randomly assigned to cycle at either high or low intensity three times a week. The “high” group cycled for 25 minutes; the “low” group cycled for 50 minutes.
“They burned the same calories, and after eight weeks, both groups had lost the same amount of weight and body fat,” says Porcari.
The bottom line
Ignore the fat-burning zone. Gillen’s advice: “Focus on burning more calories rather than the ratio of fat-to-carbohydrate burn.”