There is no true dietary requirement for carbohydrate since it can be synthesized in the body from either protein or fat. Carbohydrate is utilized chiefly as a source of energy and furnishes 50 to 60 per cent of the calories in the average American diet. It is more efficient than protein or fat in providing fuel for muscular exercise. Carbohydrate has an antiketogenic action in that it inhibits the breakdown of fatty acids in the liver and it has a protein-sparing action by decreasing the rate of deamination of amino acids. Carbohydrate is essential for normal function of the heart and nervous system, the latter using only glucose to supply energy needs. The concentration of glucose in the blood must be maintained above a critical level or serious damage may ensue, particularly to the brain or myocardium. The hemostatic mechanisms responsible for maintaining a relatively con-stand level-of blood glucose include enzymatic, nervous and hormonal components.
Carbohydrate is stored in the liver and muscles in the form of glycogen, the extent of stores varying with the type of diet ingested. Glycogen stores are highest when the diet contains large amounts of carbohydrate, inter-mediate when the diet is high in protein and lowest when the diet is high in fat. Diets rich in carbohydrate protect the liver against injury by ensuring adequate storage of glycogen.
Carbohydrate has a detoxifying action in the liver; acetyl groups derived from carbohydrate are used in acetylation of numerous compounds and glycuronic acid, a carbohydrate derivative, combines with phenolic hydroxal groups. This latter reaction may be important in regulation of the metabolism of steroid hormones and in prevention of excessive accumulation of the sex hormones in the body.
A number of the coenzymes involved in the degradation and synthesis of carbohydrates are formed from vitamins of the B complex. Coenzymes containing thiamine, riboflavin, niacinamide, vitamin B, and pantothenic acid have vital roles in intermediate carbohydrate metabolism. Accordingly, diets rich in carbohydrate require a generous supply of these vitamins and an inadequate quantity of any one of them will prevent metabolic processes from proceeding normally. This information is applied in the diagnosis of deficiency of one of the B vitamins, thiamine, in which pyruvic acid, an intermediate in the breakdown of glucose, accumulates in the blood and tissues.
Much has been learned in recent years about the hormonal regulation of carbohydrate metabolism, of the roles of insulin and of the adrenal and pituitary hormones. This subject will not be discussed since it belongs largely in the field of endocrinology rather than nutrition.
Abnormalities of carbohydrate metabolism are usually hormonal rather than nutritional in origin. However, changes do occur in starvation (19) and in certain nutritional disorders associated with malabsorption. (See p. 123. )
The simplest tests used in evaluating carbohydrate metabolism are determination of glucose excretion in the urine and estimation of glucose concentration in the blood during fasting and at intervals after the administration of a large dose of glucose, the glucose tolerance test. Normal persons excrete less than 0.1 per cent glucose in the urine, an amount not detectable with the usual clinical laboratory procedures. An increase in glucose excretion and an abnormally high concentration of glucose in the blood in the post absorptive state ( greater than 130 mg./ ml/) is suggestive evidence of diabetes mellitus.
The glucose tolerance test assists in establishing the diagnosis; the blood sugar rises to levels above 170-180 mg/100 ml. and remains elevated for three hours or more. The blood sugar response to oral administration of a large dose of glucose is dependent in part on the previous diet. Glucose concentration increases to a greater extent and for a more prolonged period when the diet has been high in fat than when it has been high in protein; the elevation is still less and of shorter duration when a high carbohydrate diet has been consumed. In starvation, the glucose tolerance test is similar to that observed following administration of a diet high in fat; the increase in blood glucose concentration resembles, in degree and duration, that found in diabetes mellitus although the fasting blood sugar level tends to be low.
In sprue, nutritional macrocyptic anemia and some vitamin B-complex deficiency syndromes, a flat glucose tolerance test is observed, i. e., glucose concentration rises very little after administration of a large oral dose. Ad-ministration of folic acid or vitamin Biz improves glucose absorption in sprue and nutritional macrocytic anemia in most instances.
Many factors influence the absorption of glucose from the intestinal tract. These include the food mixture in the intestine, the status of the intestinal mucous membrane, the function of the endocrine glands, particularly the thyroid, anterior pituitary and adrenal cortex and, as noted above, the intake of vitamins of the B complex. Absorption is abnormally rapid in hyperthyroidism and abnormally slow in hypothyroidism, hypopituitarism and adrenal insufficiency. The glucose tolerance test is of diabetic type in hyperthyroidism and in hyperfunction of the pituitary gland and adrenal cortex. A flat glucose tolerance test is found in hypothyroidism, hypopituitarism and hypo-function of the adrenal cortex.
An excessive fall in blood sugar after a meal, especially one high in sugar, is observed not infrequently in persons who are otherwise normal. This functional hypoglycemia may be due to the same mechanism as that which appears to be operative in disease of the pituitary or adrenal gland, namely, failure of the liver to respond with an increased output of glucose until the blood sugar concentration has decreased to levels lower than normal.