Burn fat. Lose fat. Fat free. Fat-related messages bombard us constantly. Yet, much of what is written about fat metabolism is hogwash. There is a lot of misinformation circulating about how our bodies store and lose fat, and when our muscles use fat to produce energy. Two concepts that are often confused are: 1) the role of fat as a fuel during exercise; and 2) how to reduce body fat. Most RT readers shouldn’t be concerned with reducing their already low body fat levels, but distance running performance will benefit from training your muscles to use more fat, thereby sparing your carbohydrate stores. Let’s try to clarify these issues.
Fat’s Role as a Fuel During Running
How much fat your muscles use for fuel during a run depends on the following factors: your training history, the proportion of slow twitch fibers in your muscles, how hard and how far you are running, and how long it has been since you ate a carbohydrate-rich meal. When you run, your muscles use carbohydrates, fats, and a small amount of protein as fuel. Endurance training allows your muscles to use more fat at a given pace. Aerobic training stimulates the following adaptations in the muscles to use more fat: 1) more capillaries to bring oxygen to the individual muscle fibers; 2) more myoglobin to carry oxygen through the muscle fibers to the mitochondria; and 3) more and bigger mitochondria to produce energy aerobically.
Endurance training also increases your muscles’ enzymes for fat metabolism and reduces the enzymes that break down carbohydrates. As a result, you use your glycogen (carbohydrate) stores more slowly, so you can run farther at a given pace before having to slow down due to glycogen depletion. That is a major reason why “the wall” moves closer to the finish line and eventually crumbles if you put in the correct marathon training. Genetics also affects your ability to use fat for energy when you run. The more slow twitch muscle fibers you have, the more fat you can use. That’s because slow twitch muscle fibers have more capillaries, myoglobin, mitochondria, and fat burning enzymes than do fast twitch fibers.
Fat Use vs. Running Speed and Distance
The faster you run, the more carbs you use relative to fat. As your effort increases from sitting to walking to jogging to fast running, you use proportionately less fat and more carbohydrate. During a long run, however, your muscles gradually use fewer carbs and more fat. This is a protective mechanism that helps to prevent glycogen depletion. Your maximal oxygen consumption (VO2 max) is a limiting factor in your running performance. Fats yield 15% less energy than carbs for each liter of oxygen used. Because fats use oxygen less efficiently than carbs, you cannot run as fast burning just fats. That is one of the reasons why it is important to prevent glycogen depletion. If our ancestors ran out of glycogen, they were eaten by sabre-toothed tigers-we just end up jogging to the finish line.
How much fat you burn during running can also be affected by how long it has been since you ate carbohydrates. If you eat carbs during the 90 minutes before you run (particularly foods with a high glycemic index), your body will tend to use less fat and more carbs during the run. This occurs because insulin is released in response to eating carbohydrates, which reduces the mobilization of fatty acids, which reduces your muscles’ ability to use fat for fuel.
The Fat Burning Zone Myth
So far, we have seen how the body uses fat for fuel. Now let’s take a look at the myth of the “fat burning zone.” During the 1980’s, the notion spread that in order to lose fat you must exercise at low intensity. Unfortunately, this misunderstanding of the scientific literature is still widely preached in health clubs. Many aerobic instructors have their clients exercise gently to stay in their “fat burning zone.” Some runners even run slowly in the belief that low intensity exercise is the most effective way to reduce their body fat levels.
The myth developed from the fact we already discussed that when you exercise harder, you use relatively less fat and more carbohydrate. This led to the belief that to lose fat you need to exercise gently. Using this logic, since approximately 80% of the calories you use while sitting are supplied by fat (see table below), then couch potatoes would have very low body fat levels. The missing ingredient is that fat loss is determined by the balance between calories consumed and calories burned over time. The source of the calories burned during a specific workout is not as important as the number of calories burned.
Calories and Fat Burned
used per minute
Calories from Fat
Calories used per minute
Calories used in 30 minutes
exercise is better for losing fat because it uses more calories per
minute. Put in a 30 minute recovery run and you may use 360 calories.
Spend the same amount of time at a faster pace and you may burn 540
calories. If you burn more calories, you will lose more fat than if you
burn fewer calories. Lower intensity exercise, therefore, is only better
for fat loss if you exercise long enough to make up for the lower number
of calories used per minute. The key to losing fat (and if you are reading
this you probably don’t need to), therefore, is to find the balance of
intensity and duration that burns the most calories.