Blood Glucose

Regardless of the form in which carbohydrate is ingested, it is converted by the digestive enzymes to glucose or similar "simple sugars" and then absorbed into the blood. If there were no provision for rapid storage of glucose in the tissues, its concentration in the blood would rise steeply following a meal, and much of it would be excreted by the kidney and hence lost to the body. Fortunately, much of the glucose absorbed into the blood is removed by the liver and the skeletal muscles and stored as glycogen. The conversion of glucose to glycogen ("glycogenesis") is regulated by the hormone insulin. In the disease diabetes, insulin is deficient or absent and the blood sugar is characteristically high.

The tissues of the body are constantly removing glucose from the blood to use in their metabolic processes and corresponding amounts of glucose must be added to the blood from reserve stores. This is accomplished by the breakdown of glycogen to glucose ("glycogenolysis") under the influence of another hormone, adrenalin. This reaction occurs primarily in the liver and to a lesser extent in the skeletal muscles. The net result is that glucose is stored as glycogen following meals and then reconverted to glucose at a rate sufficient to balance the withdrawal of glucose from the blood by active tissues.

During light exercise, the ordinary rate of delivery of glucose to the blood from the storage depots is adequate to balance the rate of glucose utilization by the muscles, and the blood sugar level is unchanged. As exercise increases in intensity, especially if it is accompanied by emotional excitement, the secretion of adrenalin by the adrenal glands becomes excessive (insofar as blood sugar regulation is concerned) and glucose is added to the blood from the glycogen storage reservoirs at a faster rate than the metabolic activities of the contracting muscles require. The result is a rise in the blood sugar concentration. This effect is more pronounced in intermittent than in continuous exertion. If the exercise is both strenuous and prolonged (as in a marathon race), the blood sugar level often shows a gradual fall, sometimes to half the normal value. This is interpreted as indicating exhaustion of available glycogen stores.

The foregoing discussion has definite implications for the practical management of the diet of athletes. In the first place, the evidence, though conflicting in certain respects, clearly emphasizes the importance of carbohydrate in the energy metabolism of muscle. There is no evidence to support the common belief that candy and other sweets should be restricted during training, unless they diminish the appetite and thus reduce the food intake at the regular meals. On the other hand, since prolonged and exhausting exercise is required to lower the blood sugar level, there would seem to be no practical advantage in the administration of sugar in various forms before or during most types of athletic contests. Any apparent increase in performance which may result is probably psychological in origin. The situation is somewhat different in the case of exhausting exercise which is prolonged over a period of hours. Here the exhaustion of glycogen reserves and the consequent lowering of the blood sugar may be a dominant factor in bringing about complete exhaustion.

It is worth pointing out that the outstanding symptoms of extreme physical exhaustion such as incoordination of movements, collapse and unconsciousness are referable not to the muscles but to the central nervous system. The brain, unlike skeletal muscles, has no available carbohydrate stores and cannot fall back on the metabolism or other substances when its glucose supply is curtailed. It depends, from moment to moment, on the glucose (and lactic acid) brought to it by the blood; when the blood glucose level falls, brain function is depressed and unconsciousness usually occurs when the blood sugar concentration drops below 40 mg. per cent.