It is to supply material with which to carry on the building up of tissue and replace that which is broken down; in other words, to supply material for growth and repair, that we eat. At least this is one of the purposes served by food.

Other processes besides those of growth and repair are continually going on in the body. For example, there is the work of preparing food for use by the body. This work is known as digestion and is accomplished largely by the action of certain juices or secretions which act upon the food chemically. These juices have to be manufactured by the body for its own use. Food furnishes the materials necessary for the production of these and the many other secretions of the body.

The broken down products of the cells are acid in character and are highly irritating and poisonous. If permitted to remain in the body unchanged they would soon destroy life. Therefore, they are not only eliminated, but are changed chemically by being combined with certain alkaline mineral elements, thus rendering them less irritating and harmful and also preparing them for elimination. The mineral elements with which this detoxifying change is made are supplied by our food.

Foods are burned in the body to supply heat and energy. At least this is the present theory of scientists. There are those who deny this and who insist that both the heat and energy of the body are independent of its food supply, that food serves solely as replacement material in building up new and repairing old tissues and in forming the body's secretions. The claim has been made that heat is derived from the assimilation of food rather than from its oxidation. It is also claimed that the body's heat is due to friction.

We eat carbon, take in oxygen and give off carbon dioxide. It is quite evident that the carbon is oxidized in the body. It would certainly give off heat in this process. The body may have other sources of heat, but this seems to be certainly one of them. The a mount of heat produced by the body seems to parallel the amount of carbon dioxide it gives off.

The chemical energies of the body are directed by something which is not itself a chemical energy, but which is intimately associated with the organic synthesis which the chemical energy serves to maintain. At least, I cannot see how we can escape this position. I have no doubt that chemical as well as mechanical energies are utilized in the body, although, they are subordinate to a controlling and unifying non-chemical force. However, this is still a much mooted question and will be solved only in the future. I do not think that all the energies of the living body are derived from foods.

The normal specific gravity and normal alkalinity of the blood are maintained by food. As will be shown later, these two functions are performed chiefly by the minerals of the diet.

These uses of food may be summed up in a few words by saying: food is any substance which, when taken into the body, can be used by it for the replenishment of tissue (growth and repair) and for the performance of organic function. This definition can be made to include water and the oxygen of the air; however, water and oxygen are not usually classed as foods. Such substances, if they are to be classed as true foods, must be with deleterious effects. Many things that are eaten by man have deleterious effects, although, they do possess food value. Obviously, such foods should be abstained from so long as other foods are to be had.

The human body is a wonderfully complex and ingenuous mechanism made up of thousands of different parts and containing hundreds of different chemical compositions. Yet all of these must be nourished by a single blood stream, a stream which itself is of remarkably uniform composition so far as any chemical analysis can determine.

If the blood derived its substance from a single source of supply, as does the blood of a nursing baby, for example, life would seem marvelous enough. But, when one considers that the blood is often nourished by hundreds of different food substances, particularly in the case of modern civilized man, it seems almost inconceivably complex. We find it difficult to comprehend how life can exist at all.

The body must secure all the necessary food elements from all the great mass of diverse foods, in order to avoid the deficiencies or "starvations" and, at the same time, it must avoid all excess of certain materials which we almost always consume in excess. Food substances which are not needed and cannot be used, injure and do not help the body.

As the study of nutrition continues, the essentials of man's diet multiply. The older books gave man's nutritive requirements as proteins, carbohydrates and fats. Today we say he needs proteins, fats, carbohydrates, minerals, vitamins and cellulose, or roughage. The normal dietary should include all of these factors.

Since food serves so many and such vital functions in the body, it is highly important that we supply our bodies with all of the needed food elements. It is essential that the diet adequately nourish the whole body and not merely some part or parts of it. The dietary ensemble must meet all of the needs of the ensemble of nutrition. The whole of the diet and not one article of food or one element of nutrition, determines the nutritive result. The adequacy of a given dietary to feed the whole body and not its theoretical adequacy to meet the needs of one organ, will determine its fitness in any given case.

The human body has never been fully analyzed nor has there ever been made a full and complete analysis of all foodstuffs. This, however, is not a matter of great importance. Neither man nor foods can be analyzed without thoroughly destroying him or them. The products of the destructive processes are not the same as those that exist in the cells and tissues of the body or of the food. It is only possible to analyze a dead body and this throws but little light on the chemistry of a live one. An analysis of a dead body and an analysis of a handful of soil will show them to both be composed of the same elements, but no one can mistake the flesh of a man for a handful of soil. An apple, too, is made up of the same elements as the soil, but we easily recognize the vast difference between this product of vital synthesis and the soil in our garden. Fortunately it is not necessary to know the exact chemistry of the body nor the exact chemistry of foods in order to properly feed ourselves, our families and our patients. If we feed our bodies natural foods, so that we may be sure they contain all the nutritive essentials, we can trust the orderly and very ancient processes of life to take care of the rest of the matter for us.