As explained above, the animal body has two general needs: first, for building material and repairs; second, for fuel. The nitrogenous foods furnish the building material, and the carbonaceous foods the fuels. We have already discussed the carbonaceous foods, this class having been first treated because the foods are somewhat simpler in composition and their use in the body less complicated.

The nitrogenous foods are so called because nitrogen is the most important element in their chemical composition. Besides nitrogen, these foods contain a very large amount of carbon, hydrogen, and oxygen, also small amounts of sulphur and phosphorus. The first foods taken into the animal body are nitrogenous foods, the first need of the animal body being material for growth.

The source of this first food used by animals has already been outlined above. It comes from eggs and milk, milk for the mammals and eggs for all animals lower in rank than the mammals.

A chemical study of these foods prepared for young animals - eggs and milk - shows that in both cases there is an admixture of carbonaceous foods with the nitrogenous. This is indicative of the twofold need of even the youngest animals, whose need for nitrogenous food is simply the predominant need, all the animals, even the youngest, needing also a certain amount of fuel food.

"While the nitrogenous foods possess a large percentage of proteins and possess also a considerable amount of carbonaceous foods in the form of fat, the oil in eggs is hardly sufficient in quantity to be considered as a dietetic factor. The fat in meat, however, is usually sufficient in quantity to require accurate determination, and to be considered as one of the determined sources of fat. On the other hand, in the carbonaceous class all of the vegetables and fruits contain a certain amount of proteins. This protein of the vegetables and fruits is so small in amount as to have little dietetic significance, and may for all practical purposes be ignored. Finally, the carbo-nitrogenous foods contain the carbonaceous and nitrogenous foods in fairly well-balanced quantities, and it is because of this balancing of the two foodstuffs that we are compelled to put them in a class by themselves.

The nitrogenous foods, while required in far smaller quantities than the carbonaceous foods, must be looked upon as absolute and fundamental necessities of life. The amount required by the average individual is between two and four ounces a day, though this is a small amount easily provided from various sources, not only from the meat and eggs, but from milk, cereals, legumes, and nuts. However, this small quantity of nitrogenous foods must be provided' day by day throughout life, or' there is likely to be a disturbance of the nitrogen equilibrium of the body.

In a brief explanation of what is meant by nitrogen equilibrium, it may be said that so long as the body receives enough nitrogen to replace the daily loss of nitrogen the body will maintain a nitrogen equilibrium - that is, it will lose as much nitrogen daily through excretion as it receives through the food. The nitrogen outgo will equal the nitrogen income, thus maintaining the equilibrium.

If the amount of nitrogen received is not sufficient to provide for the repairs - that is, if it is not sufficient to make good the daily loss of nitrogen - there will be a gradual wasting of the active tissues of the body, beginning with the muscles. This wasting of the muscles is possible even while the general body weight is increasing by the deposit of fat. It must be evident that such a process of disturbed equilibrium, or wasting of active tissues, cannot proceed long without serious injury to the body and serious disturbance of its activities.

The first thought that the dietitian should give to a menu is, how much nitrogenous food is needed, and what choice of foods shall be made to provide this need. As stated above, the choice might be confined to the vegetable kingdom, as it would be very easy to procure sufficient protein to maintain nitrogen equilibrium without using any from animal sources, the cereals, legumes, and nuts being all rich in protein. However, human experience seems to indicate that the use of certain animal proteins is essential to the highest physical development. The young mammal, including the human child, is provided by nature with a diet from animal sources alone during the first stage of its development, so that a rational interpretation of nature should not exclude food from animal sources. From the earliest times, man has resorted to the use of eggs as an important addition to his diet. From these sources already named - namely, eggs, milk and vegetable products, fruits, cereals, and nuts - a complete and wholesome diet may readily be devised. Such a diet is not vegetarian but more accurately called ovo-lacto-vegetarian. Four different kinds of diet are therefore recognized by dietitians. First, vegetarian; second, ovo-lacto-vegetarian; third, mixed diet, consisting of eggs, milk, vegetables, fruits, cereals, legumes, nuts, and the flesh of animals; fourth, flesh diet, consisting of the flesh of animals. We find that the last-named diet is the one used by the Eskimos in the Arctic region; the flesh of the seal, the walrus, polar bear, reindeer, and the numerous varieties of fish. While it is true that they sometimes get a little moss and lichen from the stomach of the reindeer that they prepare as a dish which they consider a rare delicacy, and while they occasionally get some milk from the reindeer, still, these substances, not actually from the tissues of animals, are the rare delicacies of their diet and must not be looked upon as a regular and appreciable part.

It must be evident that the reason why the Eskimos are flesh eaters is because they cannot get anything else. They eat what they can get. The more vegetable they can get, the better pleased they are. Subsisting on such a diet, their growth and repair is amply provided for by the lean portion of the animal tissue, while the copious fat deposit of all Arctic animals affords them ample provision for carbonaceous or fuel foods.