Burning Fat: Myths and Facts

A popular myth is that there is a specific range of heart rates in which you must exercise to burn fat. Even many cardio machines display a “fat-burning zone” on their panels, encouraging people to exercise in a specific heart rate range. Have you ever wondered if you really have to exercise in a specific heart rate zone to lose fat? And what happens if you venture out of that zone? Jason R. Karp, PhD, writer and exercise physiologist who coaches recreational runners to Olympic hopefuls through his company, RunCoachJason.com, sheds light on this issue.

Fuel Use During Exercise
You use both fat and carbohydrates for energy during exercise, with these two fuels providing that energy on a sliding scale. During exercise at a very low intensity (e.g., walking), fat accounts for most of the energy expenditure. As exercise intensity increases up to the lactate threshold (level 8 or 9 out of a scale of 1 to 10), the contribution from fat decreases while the contribution from carbohydrates increases. When exercising just below the lactate threshold, you are using mostly carbohydrates. At the highest intensity of exercise, you are only burning carbohydrates.

If you exercise long enough (1.5–2 hours), your muscle carbohydrate (glycogen) content and blood glucose concentration become low. This metabolic state presents a threat to the muscles’ survival, since carbohydrates are muscles’ preferred fuel. When carbohydrates are not available, the muscles are forced to rely on fat as fuel.

Since more fat is used at low exercise intensities, people often assume that low-intensity exercise is best for burning fat. However at higher intensity, the rate of caloric expenditure and the total number of calories expended are much greater than they are when exercising at a lower intensity: so the total amount of fat used is also greater (IDEA Health and Fitness Association).

to be continued

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