As we continue the analytical study of our food sub-stances, we next come to a group of foods of far greater complexity of make-up than any we have met with so far. It is the group which comprises primarily the flesh products - meat of all kinds, fish, eggs, milk, and cheese. Into this group we can also put two classes of food from the vegetable kingdom, the pulses or legumes (beans and peas) and the nuts. It is found by analysis that all of these substances contain an element entirely lacking in carbohydrates and fats. This is nitrogen. Without nitrogen there can be no life, for it enters into the structure of all living tissue. Although it is around us abundantly in the air we breathe, it is impossible for us to utilize this nitrogen for our body structure and we must obtain all that we need from our food alone. Therefore this group of foods is especially important in the diet. Because of the greater variety of flavor, the group has grown to be the most popular of them all and is used far more plentifully than is wise or necessary. The family that "must have" meat three times a day is putting too heavy a burden on the kidneys as well as on the pocket-book.

This food substance containing nitrogen is known as protein and is extremely complex in its chemical structure. The nitrogen is in close combination with the elements we have already discussed, hydrogen, oxygen, and carbon, and also with several others - sulphur, phosphorus, and iron. There are many kinds of protein substances that vary from one another in structure and behavior, but when used as food they nearly all have in common the power to build up body tissue. A surplus of protein material in the diet over and above what is needed for actual body structure, which in an adult is only enough to repair waste tissue, is split up in the body and partly burned for heat and energy, leaving the nitrogenous waste products to be excreted by the kidneys. Thus protein foods can perform both functions of our definition for food. As it is the most expensive class of foods we have, however, it should be kept exclusively for tissue building, and food from the vegetable kingdom should be relied on for all the energy needed by the body.