Metabolism is the sum of the chemical changes taking place in the cells of the body in connection with all biological processes, including growth, repair and waste, generation and maintenance of heat, all manifestations of functional activity, storage of surplus nutriment, etc. It comprises two phases, anabolism and katabolism.

Anabolism embraces the changes involved in the upbuilding of the cells, and corresponds in a general way to nutrition. Assimilation is the selective act of the cells in appropriating the special form of nutriment in the circulating blood which is suited for their needs. The special phases of anabolism comprise the assimilation of nitrogen for growth and repair of protoplasm, the storage of a certain amount of fat and carbohydrate, the assimilation of oxygen for maintenance of heat and functional activity, and the retention of inorganic salts in the bones and other tissues.

Katabolism comprises the changes involved in the continuous molecular waste of the cells; in the constant oxidation going on in the tissues; in the maintenance of animal heat; in the exercise of muscular, nervous and secretory activity; and perhaps in the breaking up of protein into oxidizable carbohydrates and nitrogenous waste products. We speak less of the acts of katabolism than of the products of katabolism, which are a measure of those acts. The chief of these products are: (1) urea, the chief form of excretion of nitrogen, and (2) carbon dioxide and water, which represent the products of energy-yielding oxidations.

Elimination is the escape or expulsion of the products of katabolism from the tissues into the blood, and corresponds to disassimilation or denutrition. Excretion is the final expulsion of the end products of katabolism from the excretory organs and includes the expulsion of unabsorbed aliment by the bowel.

In the case of certain products of metabolism, it is hardly possible to state positively whether they represent anabolism or katabolism. Here belong especially the substances formed in connection with secretion, as the digestive enzymes, the hydrochloric acid of the gastric juice, the iodothyrin of the thyroid gland and the adrenalin of the suprarenal gland. In all likelihood both factors participate, for certain substances must be assimilated from the food to form these bodies which in turn are set free in the blood. It is also true that some of these substances act by their mere presence, without apparent waste (catalytic action).

Metabolism Of Protein

A relatively small proportion of protein is normally disposed of by oxidation for energy-formation, when carbohydrates and fats enter fully into the diet. But in their absence, protein, by virtue of the fact that it contains the elements of carbohydrates and fats (carbon, hydrogen and oxygen) in its molecule, can furnish all the heat and force required. Certain savages subsist wholly on meat and remain in good health, and in the so-called Salisbury treatment of obesity, etc., it can be demonstrated at will that a patient may subsist for weeks on raw beef without apparent detriment, provided plenty of water is drunk, the excess of water aiding in the elimination of unutilized protein.

When absorbed protein reaches the liver, a large part of the nitrogen is split off and excreted as urea. The remaining portion goes to make good the small daily waste of body protein. The portion of the protein molecule left after the nitrogen is split off is available as fuel. When carbohydrates and fats are liberally supplied, so that protein is not required for fuel, the daily amount needed is comparatively small. Even when the body is losing nitrogen rapidly, as in certain fevers, it has been found that this is partly because the body protein is being used as fuel, and the loss can be largely prevented by furnishing more energy in the form of carbohydrate or fat. Much confusion has arisen because the excess of nitrogen in the diet is to some extent identical in form with the waste products of cellular activity. Besides urea, some of the chief nitrogenous products of protein metabolism are ammonium salts, uric acid, creatin and creatinin.

The non-nitrogenous portion of protein may be burned or possibly stored, but the nitrogen excess must be eliminated. Because of the fact that excess of nitrogen throws work on the kidneys, it is assumed that an excessive protein diet is injurious to the integrity of these organs. This is evident in diseased kidneys, but cannot be proved for healthy organs. However, it seems unnecessary to consume a great excess of protein and perhaps unwise, owing to the great frequency of fatal kidney diseases. At any rate, except where protein is specially advantageous for ease of digestion, it is unnecessary to make it a large part of the diet.

Metabolism Of Fat

Fat, which is passed into the blood from the thoracic duct, is carried over the body and deposited in many organs and tissues, unless needed for immediate use as a source of energy, in which case it is oxidized to carbon dioxide and water. It has been shown that diet-fat may be deposited in the body in the same form as eaten, but the readiest means of forming body fat is usually to give an excess of carbohydrate. The ease with which carbohydrates form fat is shown in the case of cows which produce large quantities of butter fat on an herbivorous diet. It is also possible to form fat from protein, but it is difficult, as a considerable part of the energy of the protein molecule goes to get rid of the nitrogen, and protein increases the rate of metabolism. We know very little of the details of fat metabolism. Some investigators hold that it must be converted into dextrose before it can be oxidized. As already stated, an excess of carbohydrate may be stored in the form of fat.

Metabolism Of Carbohydrates

The blood maintains a constant sugar content of about 1. 5 per cent. As fast as this sugar is oxidized, it is replaced by more from the store of glycogen in the liver and muscles, or from newly-formed sugar of recently digested food. The final products of carbohydrate oxidation are carbon dioxide and water, but several intermediate products (as lactic acid) are first formed, probably by the action of enzymes.

When the body is unable to store any more carbohydrate as glycogen, all further storage of excess is in the form of fat.

When sugar is lacking in the blood, as in starvation or failure to utilize carbohydrates, the sugar content is maintained at the expense of protein. This explains why the diabetic who is eating no carbohydrate, may still excrete sugar in the urine. There is some experimental evidence that sugar may be formed also from fat. At any rate, fat is oxidized in absence of carbohydrate, and whether it is first changed to sugar, or oxidized directly, it is a compact means of storing energy, since one gram of fat will yield 2¼ times as much as an equal amount of protein or carbohydrate.