INTRODUCTION Sugar Metabolism, process by which the body uses sugar for energy. Carbohydrates, one of the three principal constituents of food, form the bulk of the average human diet. The end product of the digestion and assimilation of all forms of carbohydrate is a simple sugar, glucose, commonly called grape sugar when found in food, or blood sugar when found in the human body. The metabolism of fats and of certain protein substances also sometimes leads to the production of glucose. Glucose is the principal fuel that the muscles and other portions of the body consume to produce energy. It is present in every cell and almost every fluid of the body, and its concentration and distribution are among the most important processes in human physiology. A few other sugars are of comparatively minor importance in human physiology, notably lactose, or milk sugar, which is formed in the mammary glands of all lactating animals and is present in their milk. See Metabolism; Sugar.
DIGESTION, ASSIMILATION, AND STORAGE Carbohydrates such as starch, dextrin, glycogen (animal starch), sucrose (cane sugar), maltose (malt sugar), and lactose are broken down in the digestive tract into simple, six-carbon sugars that pass easily through the intestinal wall. Fructose (fruit sugar) and glucose are unchanged in the digestive tract and are absorbed as such. Cellulose, a common constituent of many foods, is an important nutritional element for some animals, notably cattle and termites, but has no value in human nutrition (see Nutrition, Human).
The digestion of carbohydrates is performed by various enzymes (see Enzyme). Amylase, found in saliva and in the intestine, breaks starch, dextrin, and glycogen into maltose, a 12-carbon sugar. Other sugar-converting enzymes in the small intestine break 12-carbon sugars into 6-carbon sugars. Maltase breaks maltose into glucose;
sucrase, or invertase, breaks cane sugar into glucose and fructose; lactase breaks milk sugar into glucose and
The six-carbon sugars, which become the end products of carbohydrate digestion, pass through the wall of the small intestine into minute blood vessels and thence into the portal vein, which carries them to the liver. They are then converted into a single compound, glycogen (see Starch), which is stored there. This glycogen is available at all times and is converted to glucose and released into the bloodstream as required by the body. One of the end products of glucose metabolism in the muscles is lactic acid, which is carried by the bloodstream back to the liver and partly reconverted into glycogen.
ENZYMES AND HORMONES The interconversion between glucose and glycogen is catalyzed by a number of different enzymes. Phosphorylase is responsible for the release of glucose-1-phosphate from glycogen; the reaction is enhanced by the hormones adrenaline and
glucagon. Glucose-1-phosphate is converted to glucose-6-phosphate, which can either be metabolized or converted to free glucose, which enters the bloodstream. The uptake of glucose by cells is stimulated by insulin. Before glucose is used it is converted to glucose-6-phosphate (by
hexokinase), which may be metabolized or (in the liver and in muscle) converted to uridine diphosphate glucose. From the latter compound glucose is transferred to glycogen, in a reaction catalyzed by glycogen synthetase and stimulated by insulin. By as yet unknown mechanisms, cortical and pituitary hormones as well as thyroxin are also involved in the control of carbohydrate metabolism.
GLYCEMIA AND GLYCOSURIA If the body produces too much pituitary hormone or too little insulin, the amount of sugar in the blood rises abnormally, producing a condition known as hyperglycemia. In hyperglycemia the blood may contain as much as four times the normal amount of sugar. Hyperglycemia in itself is not lethal, but it is a symptom of a serious disease, diabetes mellitus. Diabetes is sometimes caused by a tumor or other condition in the pancreas that prevents the formation of insulin. Diabetic patients do not die of hyperglycemia, but if they are not given injections of insulin they may die from such causes as the accumulation of poisons in the body, produced by altered metabolism of fats; the body of the diabetic consumes fats as a substitute for the sugar that it cannot use.
If an excessive amount of insulin is injected into the body, the amount of sugar is reduced to a dangerously low level, a condition known as hypoglycemia or insulin shock. Controlled insulin shock is sometimes used in the treatment of certain types of mental illness.
In a normal individual, if the amount of sugar in the blood rises abnormally, the excess is removed from the blood by the kidneys and excreted in the urine. The presence of sugar in the urine is called
glycosuria, and although it is an important symptom of diabetes, it is not always found in diabetic patients; moreover, glycosuria may appear in normal individuals immediately after a large meal. The critical test for diabetes is neither hyperglycemia nor
glycosuria, but blood-sugar tolerance: after ingesting sugar, both normal and diabetic individuals show an increased percentage of blood sugar; the percentage remains high in the diabetic, whereas in the normal individual the excess glucose is rapidly converted into glycogen.
FERMENTATION The chemical reaction whereby plants such as yeast use sugar is remarkably similar to the metabolism of sugar in the human body. Yeast contains a mixture of 12 enzymes, which are collectively known as
zymase. Most of these enzymes, including hexokinase, are identical to enzymes involved in the human metabolism of glucose. The principal difference occurs at the end of the chain of reactions; a glucose-decomposition product called pyruvic acid is converted in the body into lactic acid, but in plants it is converted by zymase into ethyl alcohol. See Fermentation.
Many problems in the physiology of sugar remain to be solved. Present work in this field has been accelerated since the discovery of tracer elements, especially radioactive carbon. Sugars, synthesized with radioactive carbon, can be followed through the body after ingestion.