The Fat Burning Zone: Myth or Magic?

The most popular myth about exercise may be that you have to exercise at a low intensity in a specific range of heart rates to burn fat, referred to as the “fat burning zone.” I hear personal trainers all the time tell their clients to stay in that zone so they can burn fat. Wrong!

The two main fuels you use during exercise are fat and carbohydrates, with these two fuels providing that energy on a sliding scale—at low intensity, you use more fat and, as intensity increases, you use less fat and more carbohydrates. While more fat is used at low intensities, what matters is how many calories you burn during your workout, not whether those calories come from fat or carbohydrates. Despite what people think, you don’t have to use fat during exercise to lose fat from your waistline. Carbohydrates are actually muscles’ preferred fuel during exercise. For fat and weight loss, what matters most is the difference between the number of calories you expend and the number of calories you consume. To burn lots of calories, you have to go long or you have to go hard.

With high intensity exercise, metabolic rate also remains elevated for hours afterward, which means you’ll burn more calories even when you’re not exercising, as your body recovers from the tough workout. Research has shown that the more intense the workout, the more and longer your post-workout metabolic rate is elevated and the more calories you burn. Since you can perform a greater intensity of work by breaking up the work with periods of rest, interval training is a great way to perform high-intensity exercise and help you lose fat.

To maximize fat loss, try these workouts:

Go Hard

Interval training burns lots of calories in a short amount of time and keeps your metabolic rate elevated for hours after your workout. Do one or two of these workouts each week:

8 x 2 minutes at 95-100% max heart rate with 2 minutes active recovery
8 to 12 x 30 seconds fast with 1 minute active recovery
4 x 4 minutes at 95-100% max heart rate with 3 minutes active recovery

Go Very Long

Long aerobic workouts (≥ 1.5 to 2 hours) burn lots of calories and make you a better fat burning machine by increasing the number of mitochondria in your muscles. Mitochondria are your aerobic furnaces, where fat is burned. With more mitochondria, metabolism is steered to a greater reliance on fat at the same exercise intensity. Long workouts also deplete your stored carbohydrates, which threatens your muscles’ survival since carbohydrates are muscles’ preferred fuel. In response to this threat, muscles “learn” how to use fat more effectively.

So, if you want to burn fat and lose weight, just remember that high-intensity workouts and very long workouts burn more calories both during and after your workouts. And don’t worry about staying in your fat burning zone. Because there’s no such thing.

Dr. Jason Karp is a nationally-recognized speaker, writer, exercise physiologist, and owner of RunCoachJason.com, a state-of-the-science running coaching and personal training company in San Diego. The 2011 IDEA Personal Trainer of the Year, he holds a Ph.D. in exercise physiology and is founder and coach of REVO2LT Running Team and Dr. Karp’s Run-Fit Boot Camp. He has written more than 200 articles for international running, coaching, and fitness magazines, is the author of four books, including 101 Developmental Concepts & Workouts for Cross Country Runners, Winning Racing Strategies, and Running for Women, and is a frequent presenter at national fitness and coaching conferences.

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