The Value of Aerobic Exercise in Weight-Loss*

©Copyright 1997 by Robert Chuckrow

Exercise has many benefits and is a vast and varied subject about which many books have been written. Here, we will be mainly concerned with exercise as it pertains to weight loss.

An aerobic activity is one that raises the pulse rate for extended periods of time so that oxygen utilization is trained to become more efficient.1, 2 Some examples of aerobic exercises are running, swimming, cycling, rowing, and tennis. There is no question that aerobic exercise is important for weight loss. However, the reasons usually given for this benefit are worth scrutinizing because some of them are misleading or incorrect. Let us examine each claim in turn:

Claim 1. Vigorous regular exercise can eventually increase muscle mass, and this long-term increase can raise the overall , metabolism rate,,.

This claim is probably true but can only occur over quite a long period of time.

Claim 2. During exercise, the body “burns more calories” than if the exercise were not done.

To see the prevalence of this concept, just go to any fitness center and look at the information listed on the various machines. Many are equipped with a read-out of the number of calories burned, implying that a caloric expenditure during exercise meaningfully corresponds to a weight loss. In fact, the extra energy burned as a result of exercising is quite minor unless the exercise is extremely strenuous and done continuously for many hours. For example, an expenditure of only 73 cal3 is required for a 180-lb person to climb to the top of the Empire State Building. Few are able to sustain exercise sufficiently vigorous to burn a substantial amount of energy, nor is this extreme necessary.

Claim 3. For a sustained period after exercise, the body burns more calories.

It definitely seems as though exercise causes a prolonged increase in metabolism rate because exercising regularly makes it easier to lose weight. However, there is no proof of any increase in metabolism rate for any substantial period after a session of exercise.

The following is what I consider to be the true relationship of exercise to weight loss:

The function of aerobic exercise in a weight-loss program is not so much to burn energy as to change the way the body uses its energy reserves.4

That is, aerobic exercise teaches the body to burn fat rather than sugar. Here is how: Sustained exertion requires the muscles to expend energy, which comes from three sources. One source is stored glycogen that is changed back to sugar by means of epinephrine. Another source is stored fat. The third source is the conversion of muscle protein into glucose, which can be burned (Gluconeogenesis).5

The stores of glycogen are limited and can be depleted with extended exertion. Glycogen depletion causes a drop in blood sugar. Because the brain and vital organs require a certain level of blood sugar, the body takes steps to ensure that the level of blood sugar does not get too low. Where can this sugar come from? One might think that it can come from fat. Certainly, excess sugar is turned into stores of fat. However, whereas sugar can be changed to fat, fat can not easily be changed back to sugar. Protein can be and is easily converted to sugar, but substantial conversion is undesirable because it is at the expense of vital muscle tissue. Thus, frequent, sustained exertion places pressure on the body to burn fat more readily.

Consider running. Unless you run non-stop for very long periods of time, it is unlikely that all of the glycogen stores of your body will be depleted. However, locally accessible stores within the calf and thigh muscles may be substantially depleted during a thirty-minute period of running. Therefore, such extended vigorous muscular activity places a pressure on the body to use as little glycogen as possible. Repetition of such activity trains the body to improve its efficiency in utilizing stores of fat and to use as little glycogen as possible. Also, the body learns to store increased quantities of accessible glycogen to prevent a “glycogen famine.” Except when a seasoned runner is under unusual stress, glycogen levels tend not to be depleted, and the body comfortably utilizes fat for energy. Thus, low blood sugar and its consequent hunger are infrequent. In short, running and other aerobic exercises teach the body to comfortably burn excess fat and go for relatively long periods without hunger.

By combining prolonged exercise with sensible nutrition and supplementation, you can change your metabolism from utilization of sugar to utilization of fat. Then you will become a person who tends not to become fat.

If a fat burner eats a large quantity of carbohydrate in a short period of time, more of this carbohydrate can be stored as glycogen rather than as fat because the glycogen storage capacity is large. Whatever is stored as fat is later burned in a routine fashion. It is only when the stores of fat become so low that they threaten to be insufficient, that the body tries to conserve or increase fat reserves.

Notice that it was not said that a person who runs burns more energy and thus loses weight. Rather, what has been said is that the runner is better at burning fat than someone who does not run. The extra energy that is burned during a fraction of an hour of exercise is not consequential. However, the effect on the manner in which energy is utilized is crucial.

By contrast, the following happens to people who lead a sedentary life: Glycogen storage is small, and the muscles prefer to use glycogen for energy rather than fat. After short amounts of daily activity, glycogen reserves drop and low blood sugar occurs, with a consequent craving for refined carbohydrate. This food is usually eaten in excess of the limited glycogen-storage capacity of the body. The carbohydrate that cannot be stored as glycogen is then stored as fat. However, this fat is never burned. It remains. Thus, a sedentary person will constantly crave food, continue to gain weight easily, and have a difficult time losing weight. If weight is lost, that weight—and more—will most likely be regained.

1See Kenneth H. Cooper, Aerobics, M. Evans and Company, Inc., New York, 1968, pp. 38–39.

2Also see Covert Bailey, The New Fit or Fat, Houghton Mifflen Co., Boston, 1991, pp. 54–59.

3Considering that a pound of fat contains 3,500 cal, 73 cal corresponds to a loss of fat equalling only 1/3 oz!

4The only book I have been able to find that emphatically corroborates this concept is Covert Bailey, Fit or Fat, Houghton Mifflin Co. Boston, 1978.

5See W.D. McArdle, F.I. Katch, and V. L. Katch, Exercise Physiology, Lea & Febiger, Philadelphia, PA 1986, pp. 8–11.

*From Robert Chuckrow, The Intelligent Dieter’s Guide, Rising Mist Publications, Briarcliff Manor, NY, 1997.

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