The organic foodstuffs are of three distinct types and are classified under three heads, viz.: Proteids or Albuminous foodstuffs, Carbohydrates, and Fats. All animal and vegetable foods, whatever their nature and whatever their origin, are composed simply of representatives of one or more of these three classes of food principles. Proteid substances are characterized by containing about 16 per cent of nitrogen. In addition, they contain on an average 52 per cent of carbon, 7 per cent of hydrogen, 23 per cent of oxygen, and 0.5–2.0 per cent of sulphur. A certain class of proteids, known as nucleoproteids because of their occurrence in the nuclei of cells, contain likewise a small amount of phosphorus in organic combination. Proteid or albuminous substances constitute the chemical basis of all living cells, whether animal or vegetable. This means, expressed in different language, that the organic substance of all organs and tissues, whether of animals or plants, is made up principally of proteid matter. Proteid substances occupy, therefore, a peculiar position in the nutrition of man and of animals in general. They constitute the class of essential foodstuffs without which life is impossible. For tissue-building and for the renewal of tissues and organs, or their component cells, proteid or albuminous foodstuffs are an absolute requirement. The vital part of all tissue is proteid, and only proteid food can serve for its growth or renewal. Hence, no matter how generous the supply of carbohydrates and fats, without some admixture of proteid food the body will weaken and undergo “nitrogen starvation.” It is to be noted, however, that while the element nitrogen (16 per cent) gives character to the proteid or albuminous foodstuffs, so that they are frequently spoken of or classified as the “nitrogenous foodstuffs,” it is not the nitrogen _per se_ that is so essential for the nutrition of the body. Man lives in an atmosphere of oxygen and nitrogen. He can and does absorb and utilize the free oxygen of the air he breathes; indeed, it is absolutely essential for his existence, but the free nitrogen likewise drawn into the lungs at each inspiration is of no avail for the needs of the body. Further, there are many compounds of nitrogen, some of them closely allied to the proteid foodstuffs in chemical composition, which are just as useless as free nitrogen in meeting the wants of the body for nitrogenous foods.