THE TRUE RELATIONSHIP OF EXERCISE TO FAT-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.

With aerobic exercise we train the body to burn fat rather than sugar. 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 an hormone epinephrine. The other source is stored fat. The third source is the conversion of muscle protein into glucose, which can be burned.

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.
Extended vigorous muscular activity places a pressure on the body to use as little glycogen as possible. Repetition of any 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.” 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.
It is 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.