The Science Behind Fat Burning

One of the biggest misunderstandings and “myth-conceptions” in the field of exercise and weight loss has been around the field of fat burning. Aerobic teachers are constantly admonishing their students to work at a slower rate so they can “burn more fat.” Almost all cardio equipment in the gym has a “fat burning” program, and we fitness professionals are constantly bombarded with questions from clients about how to get their heart rate in the target “fat-burning zone.” The misconceptions come from a basic confusion between percentages and absolute amounts. See, at rest, the body is always burning a mix of fuels. All other things being equal, it doesn’t like to burn protein, so that leaves fats and carbohydrates (more technically, fatty acids and glucose). At rest, the “average” person burns about 70% fat and 30% carbs. As one moves from rest to activity, the percentage of fuel coming from fat decreases and the percentage coming from carbs increases. The more intense the exercise, the more carbs and the less fat in the mix, until you reach the point called the “anaerobic threshold,” where you’re going at about your intensity limit. At that point, 99% or more of your fuel is pure carbohydrate and 1 percent or
less is coming from fat. This situation has led many people to assume that in order to “burn fat” they need to exercise at lower intensities. They’re missing the boat. Why? Because while at rest, although a higher percentage of your calories is indeed coming from fat, you are ultimately burning a lower absolute number of calories. At higher intensity exercise, the percentage of calories from fat goes down, true—but it is a percentage of a significantly higher number. To illustrate this critical difference, I often ask audiences to picture multi-millionaire Ross Perot standing next to me. Then I ask them, “Would you rather have 90% of all the money I have in the world, or 3% of all the money Mr. Perot over here has?” When they give the obvious answer, I say, “But why? 90% is so much higher than 3%!” They get the picture. So let’s say you’re exercising at a fairly low intensity that burns, oh, 100 calories in a half-hour. Let’s say that 70% of those calories come from fat. Your neighbor, however, is working out much harder, outside the magical “fat burning” zone: She’s burning up, say 300 calories in that same half hour, but only 50% of those calories are from fat. Now do the math. You’re burning a higher
percentage of fat, but 70% of your 100 calories equals 70 fat calories burned. Your neighbor, on the other hand, is burning a lower percentage of fat, but she has burned up 50% of 300 calories, or 150 fat calories, more than twice what you’ve burned in the same period of time! Get it?