Before I get started, let me ask everyone reading this one question. “Have you ever seen a fat sprinter?” Think about this question while you read this post and I will explain why I asked it at the end.

Myth #1: After 20 minutes of aerobic exercise, your body is predominantly using fat as the fuel for exercise.

The belief that after 20 minutes of aerobic exercise, your body is predominantly using fat as the fuel for exercise, has pervaded the exercise world for a very long time. Unfortunately, it is incorrect. First, the body always uses some combination of fat and carbohydrate during exercise. Second, it takes much longer than 20 minutes before fat becomes the primary source of energy during exercise.
The results of a study of trained cyclists will help to explain why this myth is incorrect. Basically, a group of cyclists rode at a moderate intensity while the amount of each substrate (energy source) being used was measured. At the one hour period, they were still deriving over 60% of the calories burned from carbohydrate. Of the approx. 40% fat being burned, only 27% came from fat cells while the other 13% came from intramuscular fat. This merely serves to point out that it can take upwards of an hour for fat to be a major energy source during exercise. We’ll see later on that it may not matter with respect to fat loss.

Myth #2: Well trained people will shift over to fat burning faster than untrained people. Also, low intensity exercise will burn more fat than high intensity exercise.

A study was done at Georgia State U. on very well trained aerobics teachers. They stepped for approx 40 minutes (about the length of a typical step class) while the source of energy was measured as above. Mind you, these were extremely well trained teachers and the intensity of their exercise was approx 60% of their max, which is fairly low. Of the total energy expended, over 90% came directly from carbohydrate with only a small amount coming from fat. This draws some serious doubt on both the above myths. We’ll also see later that this doesn’t matter for fat loss.
The myth of low intensity exercise being better for fat loss is erroneous for this reason. Yes, it is true that you burn a greater percentage of fat during low intensity exercise (actually you burn the greatest percentage of fat during sleep which is the ultimate in low intensity activity). However, the total number of calories burned is less so the absolute number of fat calories burned is less. During high intensity exercise, you burn a lower percentage of fat calories but a higher absolute number of fat calories due to the higher number of total calories burned. You also get done faster.
At this point, I’m guessing that you’re saying “Well, if I can only burn fat if I exercise an hour or more, why should I bother?” This brings us to the final myth.

Myth #3: You must burn fat during exercise to lose fat.

Two very similar studies have found evidence that this is simply not the case. One was done at UCLA while the other was performed at Georgia State U. Both had two groups exercising at either a very high intensity or a very low intensity. The exercise was standardized so that both groups burned the same number of calories.
One group exercised approx. 50 minutes at a very low intensity (~50%VO2max) while the other exercised approx. 25 minutes at a very high intensity (~90% VO2 max). Well, based on the pervading myths, only the low intensity group should have lost fat. But, at the end of 18 week, bodyfat loss was identical. Now you’re saying “But if the high intensity group burned only carbs, how did they lose fat?” They don’t really know. However, what seems to be important is the caloric deficit rather than the manner of burning the calories.
Even if you do tons, of low intensity exercise, if you still eat too much, you will not lose fat. High intensity, which burns more calories will produce a greater caloric deficit making overall fat loss greater than low intensity.
So, how about our sprinter. Well, sprinters, who tend to be phenomenally lean break all of the supposed rules for fat loss. They do lots of short duration, high intensity work, burning only carbs during exercise. However, their bodyfat levels are extremely low.
Now, am I recommending that everyone go out and start running sprints to lose fat? Well, no. First and foremost, high intensity exercise greatly increases the chance for injury, especially if you are just starting out. Second, high intensity exercise is very uncomfortable. If you are just starting, you will not continue exercising if you put yourself through lots of unenjoyable and painful exercise. However, assuming you have been working out a sufficient amount of time to be physically able to do high intensity work, you will definitely see greater fat loss by exercising at a higher intensity because you will be burning more calories for a given time period. This is especially important for those people who have a given amount of time that they can exercise. In order to burn more calories they should increase the intensity since they cannot increase the time.

Fat Burning Truth

If you have ever worked out at a gym you may have noticed treadmill ergometer notations of “fat burning zone” in the area of 65% of maximum heart rate. And if your workout is generally at a higher intensity, you may have wondered whether you may be running too fast to burn fat. Nothing could be more ridiculous.
The idea that low intensity aerobic activities are better for fat burning than high intensity exercise was given credence when several research studies indicated that low intensity activities burned a greater percentage of fat calories than high intensity activities.
These studies validated that the body prefers to use fat as its fuel source during low-intensity exercise. This equates roughly to 60% of the total calories burned, as opposed to about 40% from high intensity exercise.
While this fuel preference is true, it is misguided to believe that the selective use of fat for fuel will translate into burning more total fat calories. High-intensity exercise burns more fat calories, as well as carbohydrate calories, on an absolute basis than lower intensity activities. Because an important aspect of training intensity is the total amount of fat calories burned–not the percentage from fat–higher intensity exercise has the decided edge. Further, reduced body fat results from total calorie deficits–that is, burning more calories than you take in from your foods.
When you consider the time-related efficiency of training, low-intensity exercise provides a very poor cost/benefit dividend. You burn about the same number of calories walking a mile as running a mile–it just takes a bit longer walking them off. If fat burning is one of your aims, performing cardiovascular exercise at a higher intensity gives you more bang for your workout buck.