Fat Burning Exercise
Chester J. Zelasko, Ph.D. | January 16, 2001

Whenever the new year begins, the health magazines are full of new ways to burn off body fat faster than ever before. It’s tough for the layperson to know what’s fact and what’s fiction. This should clarify at least some of the claims for you.

When you exercise, your body would rather burn sugar (glucose) than anything else. Why? Because it can maximize the energy it gets from that source. Fat and, to a limited extent, protein can also provide energy but not as efficiently. When you’re sitting comfortably in a chair, you’re using a little more fat than carbohydrate—unless you ate something within the past hour. When you rise and start walking, you shift to burning a higher percentage of carbohydrates. Moving from a walk to a jog to a run to an all-out sprint will continue to shift the carbohydrate percentage higher until you are using carbohydrates exclusively. The theory behind fat-burning exercise is to workout at a low level to burn more fat. While it works, the problem is that you have to invest more time—something we hold as a premium. Here’s an example of what that means.

Let’s say a 180-pound woman walks on a treadmill for 45 minutes at 3 mph. For her, that’s low-intensity exercise and represents about 50% of her aerobic capacity. Based on calculations from established exercise tables, she would burn 216 total calories using 50% carbohydrate and 50% fat as fuel. That’s 108 calories from carbohydrates and 108 calories from fat.

Now let’s say that same woman walks at 4 mph for 45 minutes. For her, this represents moderate-intensity exercise which is 60% of her aerobic capacity. Based on the tables, she would burn 288 calories with 61% from carbohydrate and 39% from fat. She’s still walking for 45 minutes, but she expends 288 kcal in that amount of time. Calculating the percentages means she used 176 calories from carbohydrate and 112 calories from fat.

She invested no more time and just a little more effort, and she burned 70 more total calories. Interestingly, she burned about the same number of calories from fat. She could have cut the time she walked at a faster pace to 30 minutes and would still have burned about the same total calories as she would have walking more slowly but longer.

So what should you think about fat-burning exercise? While it’s absolutely true that you use a higher percentage of fat exercising less strenuously, the additional investment of time doesn’t seem worth it. To date, there are no studies which demonstrate that burning fat while you exercise helps you lose any more body fat than exercising harder and faster. The bottom line is that what you use as a fuel during exercise doesn’t matter. As long as you don’t overeat, you will burn body fat sometime during the day, so don’t be concerned about it. Work at a level that’s comfortable but challenging and leave the rest to your body to sort out.


  1. McCardle, W., Katch, F., and Katch, V. Exercise Physiology. (1996) Publisher: Williams and Wilkins. Baltimore:Maryland.
  2. Zelasko, C. J. Exercise for weight loss: What are the facts? (1995). Journal of the American Dietetic Association. 95:1414-1417.
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