Food and You
Dallas E. Boggs
Utilization of Carbohydrates
CARBOHYDRATES are important body fuels; but, unlike fat, they cannot be stored in large quantities. As explained in the chapters on digestion and absorption, all carbohydrates taken into the body are eventually changed into either glucose or glycogen (animal starch). Some excess sugar is stored as glycogen, but the amount that can be put away in this form is quite limited. (If a normal man were forced to get all of his energy from this source, the total storage would be exhausted in half a day.) About two thirds of the total supply is stored as muscle glycogen; a little less than one third exists as liver glycogen; and the rest circulates in the blood as glucose. The total amount in the blood is very small—normally, there is less than 1 gram of glucose in each of the 6 liters of blood in the average man. (All of the glucose in the blood could easily be put in a teaspoon.)
If the carbohydrate storage becomes depleted, it may be replenished by converting amino acids (or fragments of fatty acids) to glucose and glycogen. The ability to convert one type of foodstuff to another enables the body to furnish the carbohydrate fuel required by such a vital organ as the brain. The brain depends, exclusively, on glucose for its fuel. Muscles can burn either sugar or fat—but they perform more efficiently while burning sugar.
Starch and sugars contribute more than one half of the calories in the average diet. It is not possible to store all of the usual day's intake of carbohydrate as glycogen; therefore, what is not burned is converted to fat. If the amount of carbohydrate consumed is not excessive, the fat is burned before the next day's supply arrives; and the body weight remains constant. If, on the other hand, the carbohydrate calories are taken consistently in excess of the daily need, there can be only one outcome—a deposition of fat and a gain in weight. It is misleading, however, to leave the impression that carbohydrate is the only fattening food. Fat may also be deposited as a result of eating either fat or protein. Under normal conditions, transport of carbohydrate to and from the liver works equally well in both directions and represents a very efficient device for maintaining the proper amount of glucose in all tissues.
The Regulation of Blood Sugar
The teaspoonful of glucose dissolved in the blood of a man seems almost insignificant—but it is extremely important that this amount be regulated within narrow limits. If it gets too high, it is spilled out into the urine and lost; if it drops much below normal, the brain is affected with resulting dizziness and a feeling of faintness; and, if the blood glucose drops still lower (as in insulin shock), there may be unconsciousness and convulsions.
Several of the hormones have a part in the regulation of blood glucose, but insulin and epinephrine are probably the most active. Insulin, the substance taken regularly by diabetic patients, is manufactored in the pancreas. When the blood glucose tends to rise for any reason, as it does after a meal, insulin production is increased—to restore the normal level of blood sugar—by stimulating the production of glycogen from the excess glucose in the blood. That effect occurs, to some extent, in the liver; but it is especially marked in the muscles.
When blood glucose is reduced, epinephrine is produced in the adrenal glands. This hormone has the property of accelerating the breakdown of glycogen to glucose, which then makes up the deficit of this substance in the blood. Insulin and epinephrine thus have opposite effects and the fine balance between the two keeps the blood sugar at a fairly constant level. It is important to point out that insulin and epinephrine do not cause the changes in blood sugar; they only speed up the processes that are already in operation. These hormones act like oil on the bearings of a machine, making it run faster and smoother.
An animal that has had its pancreas removed can no longer produce insulin, but it can still form glycogen in his muscles. The process is not as efficient in the diabetic animal as in the normal—the blood sugar rises so high that much of it is lost through the kidney. Despite the absence of insulin and the wastage of sugar in the urine, glycogen is still formed in the muscles. If the epinephrine‑forming portion of the adrenal glands is removed from an animal, he is still able to release glucose from his glycogen stores; but it cannot be done as rapidly as when epinephrine is present. This device for rapid liberation of sugar into the blood is especially useful in meeting emergencies. It probably developed ages ago to serve the primitive animal in preparation either for combat or for a flight from danger—either one of these activities involves violent muscular exercise and hence the burning of large quantities of sugar.
Up to this point our discussion has been limited to the digestion, absorption, transport, storage and release of sugar in the body. The actual burning of sugar occurs in all cells and yields the same end products, carbon dioxide and water, as would be obtained by burning sugar in a furnace. The amount of heat liberated is also the same in both situations. The great difference is that the body burns all of its fuel without flame and at the temperature of the blood. This kind of burning is made possible by a series of enzymes that are able to break down the sugar molecule in small steps. Some of the vitamins (namely, thiamine, riboflavin and nicotinic acid) are important parts of this enzyme machinery. If any or all of these factors are missing from the diet, the oxidation mechanism fails. The result is that partially oxidized products—which are acid in nature—accumulate in the blood, and the person so afflicted becomes very ill.
In summary: It is evident that many factors enter into the utilization of carbohydrate by the body. The mineral element phosphorus is necessary in transforming glucose into glycogen and vice versa. The hormones, insulin and epinephrine, assist in keeping blood sugar at just the right level—and finally the vitamins, thiamine, riboflavin and nicotinic acid help to control the burning process so that the body can extract all of the energy in the sugar without waste and excess heat.