Although much uncertainty and obscurity still surround the subject of vitamins, the condition is somewhat clearer than when the first edition of this volume was published. Much speculation and nonsense still exist in the literature of the subject. It is still not possible to satisfactorily define them and they are frequently referred to as "vital chemicals." It is still said: "we do not know exactly how the vitamins act in the body," although it is now generally agreed, as I suggested in the first edition of this work, that they are enzymes.

Although certain of the vitamins are of a protein nature, some of them being somewhat like the amino acids, vitamins do not constitute a group of chemically related compounds, as do the proteins or carbohydrates. Chemically, about the only thing they have in common is that they are organic compounds. Functionally, rather than structurally, they are of a group. It is said to be merely accidental that they are classed together as "vitamins" (Berg preferred the term "complettins" which, perhaps, more correctly expresses their roles in nutrition.).

Vitamins are regulating substances. They are appropriately described as part of the chemical regulators of the activities of living organisms. They share this work with the hormones of the internal secretions and the various enzymes of the body. I have thought that they may be essential to the formation of hormones and the various enzymes. They are very complex substances and are derived from a wide variety of sources. Not all vitamins are known and of those now known only certain ones are thought to be essential to human life. They are not foods in the regular sense of the term, but they enable the body to utilize and assimilate the proteins, carbohydrates, fats and minerals. As enzymes they lose much of their mysteriousness.

Some vitamins are found in almost every living cell indicating that their role in nutrition is a very fundamental one. Indeed, it is probable that they constitute an integral part of the grand admixture of many ingredients that we know as protoplasm. They are required in very small amounts; certain of them being required in unbelievably small amounts, yet they are indispensable to the life and well-being of the higher animals. Not all the known vitamins are required by all animals. Although about twenty-two vitamins have been announced (only about twelve of these have been isolated in pure form), only about seven or eight have been definitely shown to be needed by man. It is thought that there may be many vitamins that have not yet been discovered. Perhaps not more than one to three of the unknown vitamins will prove, when discovered, to be essential to human nutrition. I know of no reason to doubt that the lower forms of animal life also require vitamins for their life and well-being.

Vitamins are one link in a chain of essential nutritive substances requisite for the harmonious regulation of the chemical and organic processes of the body. Although the several vitamins are closely related and inter-related, at least, functionally, it is believed that each one plays a specific role in nutrition. Summarizing from McCarrison's Studies in Deficiency Diseases, vitamins are constant constituents of living tissues, being present in small amounts and, although, they do not contribute to the energy-supply of the body, they do make it possible for the body to utilize proteins, carbohydrates, fats and salts and are essential to growth, regeneration and to maintenance of health. There exists a distinct relation between the amount of vitamins required and the other food elements, so that efficiency of the vitamins is dependent upon the composition of the food mixture. There is also a distinct relation between the amount of vitamins required and the rate of metabolism.

The capacity of any given cell for work is impaired in proportion to the degree of vitamin starvation. The result of vitamin deficiency is destruction--the greater the deprivation, the more rapid the development of deficiency states; the lesser the deprivation, the slower their development.