As we have seen, the conclusion reached by many extended and severe investigations is that the compounds of foods have certain functions in common. For instance, the proteins, carbohydrates, and fats are all oxidized wholly or in part to supply the necessary energy for muscular activity. The proteins then serve both constructive and fuel purposes. Carbohydrates and fats are alike in being sources of energy through oxidation, and in being utilized for the deposition of animal fat. In view of these facts, the question arises whether the physical welfare of the human subject requires the mixture of nutrients that commonly exist in the average human diet and that is enforced in the dietary standards that are recommended by students of human nutrition. It is certain that some species of animals may exist wholly on a flesh diet which is practically devoid of carbohydrates. Even man himself, in the wild state or when confined to game as a source of food, is able to subsist for a considerable period of time on animal food alone.

174. Carbohydrates Physiologically Economical

Why do food standards call for a large proportion of non-nitrogenous material, particularly carbohydrates? The necessity of protein in the diet is abundantly demonstrated. Many investigations have shown that when the food contains no protein, the waste of nitrogen continues, no matter how abundant is the supply of carbohydrates and fats. In other words, a continuous protein cleavage is demanded by the animal organism, and no other nutrients can serve as a substitute for protein in meeting this demand. If the food contains no protein, body tissue will be depleted. It cannot be said that either carbohydrates or fats are an essential part of the diet in the sense protein is, because it is possible as energy producers to substitute one for the other and protein for both.

In spite of these facts, it is safe to assert that the welfare of the human organism is best promoted by a food carrying a mixture of the three classes of nutrients. The larger part of man's food is used for the production of energy, and it is physiologically economical that this energy be supplied by the non-nitrogenous nutrients, particularly carbohydrates. If the proteins are broken down to supply energy, there is always a definite proportion of urea and uric acid residue that must be eliminated through the kidneys. An exclusive protein diet would burden these organs beyond their accustomed habit, and flooding the system with these nitrogenous wastes, in the opinion of medical experts, increases the tendency to gout and other forms of rheumatism. On the other hand, the carbohydrates, when not stored as fat, are completely oxidized to the simplest compounds, carbon dioxid and water, which are eliminated through the lungs and skin, possibly part of the water so formed acting as a solvent of the urinary compounds. Investigation seems to prove conclusively that the animal body has a physiological preference for carbohydrates over the fats or other nutrients as a source of energy. After the free ingestion of sugar the respiratory quotient in certain experiments has become 1.00 when just previously it was much less than 1.00. This demonstrates that while fat was being oxidized before the sugar was taken, the oxidation immediately changed wholly to the sugar. This indicates the physiological adaptability of starches and sugars for maintaining muscular activity. 176. Protein sparers. - The carbohydrates and fats are sometimes classed as "protein sparers." This means that, with an adequate supply of these bodies in the food, protein destruction may be reduced to the lowest possible limit. To illustrate, if a man doing moderate work were maintaining an energy balance when eating of digestible nutrients 218 grams of protein, 400 grams of carbohydrates, and 56 grams of fat, and 100 grams of digestible carbohydrates were added to the daily food, approximately 100 grams of digestible protein could undoubtedly be withdrawn from the daily food without causing any drain upon body protein to meet the demands of the organism. As stated, however, such a substitution cannot be carried beyond certain limits without depressing the protein supply below the body needs for maintenance. Fats are not as efficient protein sparers as are carbohydrates. To be more explicit, fats and carbohydrates do not replace protein in proportion to their energy equivalents, carbohydrates being the more efficient. In brief, then, experience and science both indicate that carbohydrates are the most healthful, and physiologically the most economical, source of a large proportion of the food energy used by the human subject. There is every justification for the relative abundance of starch foods in man's diet.