Evidence of the importance of the chemical composition of the protein fed has been had from another aspect which has shown definitely that the organism must receive certain, probably the majority, of its amino acids in the preformed state. Feeding ex periments on young growing animals have been carried out where the protein fed was abnormal in that certain amino acids were completely absent. One of the proteins used was zein (from maize), which lacks the amino acids tryptophan and lysine. When young animals were fed on a well selected diet with zein as the sole protein they rapidly went downhill and died. When to the diet as before the appropriate amount of tryptophan was added to make good the deficiency in this amino acid, the animals re mained alive but did not grow when as a further addition lysine was added the animals not merely lived but grew.
Carbohydrate may be regarded as the food stuff which most readily gives up its energy in the organism. The tendency of modern work is to accept carbohydrate as the fuel. After absorption into the blood stream any excess above immediate requirements would seem to be deposited in the liver and muscles in the form of animal starch or glycogen as a readily available form of reserve. If there is a persistent over ingestion the excess carbohydrate would seem to be converted into fat and thus there is built up a more permanent reserve of energy. Finally if the organism is suddenly flooded with sugar as the result of the ingestion of excessive amounts, much of the sugar is excreted as such in the urine—alimentary glycosuria. Normally the sugar is completely burnt in the organism to form carbon dioxide and water. The complete utilization of the sugar is intimately related to a special secretion of the pancreas to which the name of insulin has been given by its discoverers, Banting, Best, Macleod and Collip. When there is a disturbance of this insulin formation as in the disease known as diabetes, sugar is excreted as such in the urine, the body having lost the power to utilize it. Just how insulin acts is unknown. There is very good reason also, due primarily to the interesting work of Harden and Young on fermentation, to believe, certainly as regards muscle, that before the carbohydrate can be broken down it first enters into combination with phosphorus to form a hexose diphosphate.
Fat is not apparently readily metabolized in the body, but it is not probably so stable as one might deduce from the apparent readiness with which it is deposited. It is undoubtedly, so to speak, invested capital, whereas carbohydrate may be looked upon as capital in current account or at most on deposit receipt. When combustion is carried out normally it is complete, the end products being carbon dioxide and water. For this complete combustion it is essential that a certain amount of carbohydrate be present as is evidenced by the fact that when carbohydrate is withheld from the diet or when it is not available as in diabetes a condition known as acidosis ensues. In acidosis the combustion
of fat ceases before combustion is complete, with the result that there is formed in the body and excreted in the urine 0 -hydroxy butyric acid, aceto-acetic acid and acetone. These substances can be caused to disappear from the urine if a supply of carbohydrate is made available. It would seem that there are two chemical actions involved in the breakdown of the long fatty acid chain, a special process of oxidation known as 0-oxidation, associated with desaturation which renders the chain unstable. Although the evi dence for the conversion of carbohydrate to fat is plain, there is still lacking indubitable evidence of the conversion of fat to car bohydrate, although many workers accept the idea.
Although the mineral salts do not directly contribute to the supply of energy to the body and although generally speak ing, they themselves undergo no metabolic change in the body they play a very important part in the various tissue activities. Unless the salts be present in proper amount and in proper pro portion, metabolism is impossible. The principal mineral elements found in the body are sodium, potassium, calcium, magnesium, iron, phosphorus, sulphur, chlorine and iodine, with in addition traces of many other elements. Of course these various elements do not exist in the tissues in their free state but as compounds which, in the majority of instances, would seem to be inorganic. As all these substances are continually being lost from the body in the various excretions, it is essential that a sufficient supply be always available in the food. These mineral constituents play the main role in the maintenance of the faintly alkaline reaction of the various tissue fluids. As the result of the different metabolic processes which take place in the tissues there is a constant pro duction of acid, chiefly from sulphur and phosphorus, which re quires to be neutralized by basic elements like sodium and potas sium and probably also calcium and magnesium. The kidney for the most part regulates in a very selective fashion the output of these various inorganic constituents. It must not be imagined, however, that the body can completely protect itself from ex cessive salt loss, a loss so great that it may give rise to symptoms of a serious character. Thus it has been shown that men who are, in the course of their work, exposed to high environmental tem peratures with consequently much sweating, are very liable to a form of cramp. Further it has been noted that the cramp is frequently exacerbated where the men drink freely of water to allay their thirst. Recently it has been found that the condition is due to an excessive loss of sodium chloride from the body car ried away in the sweat and that the condition may be cured or prevented by giving the men saline fluid to drink.