Phosphorus compounds are as widely distributed in the body and as strictly essential to every living cell as are proteins.

Phosphates are constantly excreted from the body even after long fasting. During a fast the rate of excretion of phosphates does not fall off rapidly like that of chlorides, but tends to run more nearly parallel with the nitrogen excretion, as would be expected in view of the fact that the phosphates of the urine represent not only an excretion of preexistent salts, but also the result of the metabolism of body tissue.

Some of the relations of the phosphorus compounds to nutritional functions are outlined by Forbes and Keith as follows:

"Among the several inorganic elements involved in animal life phosphorus is of especial interest. No other one enters into such a diversity of compounds and plays an important part in so many functions. Structurally, it is important as a constituent of every cell nucleus and so of all cellular structures; it is also prominent in the skeleton, in sexual elements, glandular tissue, and the nervous system. Functionally, it is involved in all cell multiplication, in the activation and control of enzyme actions, in the maintenance of neutrality in the organism, in the conduct of nerve stimuli, and through its rela-tion to osmotic pressure, surface tension, and imbibation of water by colloids it has to do with the movement of liquids, with the maintenance of proper liquid contents of the tissues, with cell movements, and with absorption and secretion." (Ohio Agricultural Experiment Station, Technical Bulletin No. 5, page 11).

While the phosphorus compounds of the body and of the food are very numerous and might be classified differently according to the standpoint from which they are being considered, it will be convenient for our present purposes to divide them into four main groups:

1. Inorganic phosphates, of which potassium phosphate is probably the most abundant in food and in the fluids and soft tissues of the body, while calcium phosphate is the chief inorganic constituent of bones.

2. Phosphorus-containing proteins, including the nucleo-proteins of cell nuclei, the lecitho-proteins, and the true phos-phoproteiris such as casein or caseinogen of milk and ovovitellin of egg yolk.

3. Phosphatids, phospholipins or phosphorized fats - including lecithins, lecithans, kephalins, etc. which occur in large quantity in brain and nerve tissue and in smaller concentra-tion (but probably as essential components) in all the cells and tissues of the body, not only of man, but of plants and animals generally. The phosphatids are therefore widely distributed in food materials, but are found in extremely varying proportions in foods of different types. Egg yolks are conspicuously rich in phosphatids, about two thirds of the phosphorus of the egg being present in this form.

4. Phosphoric acid esters of carbohydrates and related sub-stances such as inositol ("inosite") and the natural salts of. such esters. The calcium, magnesium, and potassium salts of "phytic acid," * collectively known as phytates, phytins, or phytin, have for some years been regarded as the most abundant phosphorus compounds of the wheat kernel and probably of the grains and legumes generally, if not of all vegetable foods. Recent investigations indicate, however, that not all the phosphorus compounds which were supposed to be phytins are really salts of phytic acid. As has been explained in Chapter I (Carbohydrates), the recent work of Northrup and Nelson indicates that starch contains phosphorus as an essential constituent, and there are other indications of phosphorus-containing carbohydrates or carbohydrate-phosphoric acid esters in food materials and also of the formation of hexose-phosphoric acid esters in the body in the course of the carbohydrate metabolism.

Thus we may think of the phosphorus with which we have to deal in food and nutrition as being partly in the form of inorganic phosphates and partly in combination with (or present as a constituent of) each of the three groups of organic foodstuffs - proteins, fats, and carbohydrates, or closely related substances.

In the course of digestion and metabolism the phosphoric acid radicles are split off from the organic radicles and ulti-mately nearly all of the phosphorus leaves the body as inorganic phosphate. To what extent the cleavage of the organic phosphorus compounds occurs in the digestive tract under ordinary conditions and to what extent, if at all, the phosphorus of phos-phoproteins or phosphatids, for example, is absorbed in organic form is still a subject of investigation.

* Phytic acid is probably inositol-hexa-orthophosphoric acid, C6H24O24P6 (Robinson and Mueller).