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Is the “Fat-Burning Zone” a Myth?

A look at the misconceptions and truths about fat-burning workouts—and how to make the most of your sweat sessions.

Until about five or so years ago, most exercisers and fitness trainers embraced the idea of the fat-burning zone—a moderate exercise intensity range associated with burning more fat. Today, you’ll find countless articles about “The Fat-Burning Zone Myth". These articles say you need to exercise at a high intensity to burn the most fat. Myth or no myth, the important question is how this all affects your personal weight loss strategy.

MORE: How to Lose Weight—and Keep It Off

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Is the Fat-Burning Zone a Myth?

The idea of a fat-burning zone has its roots in solid science. When you exercise—and even when you’re at rest—your body uses the carbohydrates, proteins and fats it obtains from the foods you eat as fuel. Under normal conditions, the amount of protein your body uses for energy is so small (under 2 percent at rest and for exercise sessions lasting less than an hour) that we don’t consider it in the fuel-burning equation. The battle is between carbohydrates and fats. 

When your body burns food to create energy, it uses oxygen and produces carbon dioxide.  You remember the elementary school diagrams: While plants “breathe in” carbon dioxide and give off oxygen, people do the reverse.  By analyzing the air you breathe out, scientists can measure the amounts of these gases produced and consumed.  Then, they can determine how much fat and carbohydrate you are burning.

The relative levels of carbon dioxide produced to oxygen used are expressed in a ratio called the respiratory quotient, or RQ. This number reveals the relative use of carbohydrates and fats in the energy equation. A quotient of 1.0 would point to complete use of carbohydrates for energy, while a quotient of 0.7 would indicate pure use of fats. Most people have a number that falls somewhere in between.

Your body uses varying percentages of fats and carbohydrates throughout the day, largely determined by the level of activity you are engaged in. The less intensely you are working out, the higher the ratio of fat burning to carbohydrate burning. As you increase the intensity, a greater percentage of your calories are burned from carbohydrates rather than fat. In this respect, the low- to moderate-intensity fat-burning zone exists.

MORE: The Sexy Benefit of Working Out

Now for the myth part. You might infer that you burn more total fat per session when exercising in your “fat-burning” zone than at a higher intensity during which carbohydrates are used more. Minute for minute, this isn’t true. Here’s why: Because you burn more total calories when exercising at higher intensities, the overall total calories burned from fat is still greater at those intensities, even though the percentage of calories coming from fat is slightly lower.

Here is a hypothetical example: If Sally exercises at 65 percent of her maximum heart rate, she burns 150 calories in 30 minutes. Of those, 50 percent (or 75 calories) will come from fat. If she increases her intensity to 85 percent of her maximum heart rate, she burns 210 calories. Only 40.5 percent of those come from fat, but that totals 85 fat calories—10 more than she would burn at the lower intensity. So even though your body uses a higher percentage of calories from fat at the lower intensity, it still uses more overall fat at higher intensities because the total number of calories you burn is higher.

So is the fat-burning zone a myth? No. Is it a myth that you should exercise in that zone to burn the most fat or lose the most weight? Sometimes. It all depends on your personal preference, how much time you have to exercise and your physical condition.

Personal preference:  If you do not enjoy working at high or moderately high intensities, you will likely not do it very often or very long. And if you don’t do it often or only do it for a few minutes, you’re not going to burn many calories from any fuel source. From a fat-burning/weight loss perspective, you would be better off working out for 45 minutes at 65 percent of your maximum heart rate than for 20 minutes at 85 percent. If you’re into high-intensity workouts, shoot for a longer duration of 30 to 60 minutes.

Time: If you want to lose the most weight and burn the most fat as efficiently as possible, meaning with the least amount of time spent, ramp up the intensity.

MORE: How Much Exercise Do You Really Need?

Physical condition: If you have circumstances that make exercising at high intensities unsafe or uncomfortable (such as excess weight, cardiac issues, arthritis, body alignment challenges or multiple sclerosis), you are better off going lighter (light to moderate exercises) and for longer. In a nutshell:

Good: Short duration (20 minutes total) at higher intensities.

Better: Long duration (45 minutes total) at moderate intensities.

Best: Long duration (45 minutes total) at higher intensities.

The next time you workout, also keep these points in mind: Calories are cumulative. They don’t have to be burned in one workout session. Four 5-minute sessions are as good as a 20-minute bout—and maybe better, because you can likely work at higher intensities in the 5-minute sessions and burn more total calories. Also, all calories are created equal. It doesn’t matter if you’re burning calories from formal exercise, recreation, romance or work.  Calories are calories, and burning them from any physical activity will reap benefits.  Last but not least, when it comes to exercise and weight loss, more is more. The more you workout, the more you calories you’ll burn.

QUIZ: How Physically Active Are You?

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