Phosphates, nucleoproteins, and phosphatids are all prominent as body constituents.

The insoluble phosphates constitute the chief mineral matter of bone; while soluble phosphates are essential constituents of the blood and protoplasm. It is largely to the presence of the phosphates that the blood and protoplasm owe their ability to remain neutral or faintly alkaline, notwithstanding the constant production of acid in metabolism, as will be seen in connection with the discussion of the maintenance of neutrality below.

The nucleoproteins as constituents of cell nuclei and the phosphatids as prominent constituents of brain and nerve tissue and as less prominent but doubtless essential components of the tissues generally have functions distinct from each other and from the phosphates. On the assumption of a more active metabolism in the cell nuclei or in the brain and nerve tissue than in the bones, there has sometimes been a tendency to regard fluctuations of phosphorus output as indicative of increased or decreased metabolism of nucleoproteins or phosphatids. It is probable, however, that the eliminated phosphorus represents more largely material which has functioned as phosphate. One reason for this is that the bones contain so large a share of the total phosphorus of the body. According to Voit's estimate, a man's skeleton contains about 600 grams of phosphorus; his muscles, about 56 grams; his brain and nerves, about 5 grams. With the bones in possession of such a predominant share of the body phosphorus, it would seem that the metabolism of bone tissue, even though relatively inactive, must exert a considerable influence upon the phosphorus output. Moreover, the soluble phosphates of the blood and protoplasm are constantly tending to be eliminated from the body (through the kidneys or the intestinal walls or both) and perhaps increasingly so in proportion as they become changed into acid phosphates • in the performance of their function of maintaining neutrality by reacting with the acids produced in metabolism. Before taking up the quantitative study of the phosphorus requirement we must consider the nutritive relations of the different types of phosphorus compounds, and whether these are sufficiently interchangeable in nutritive function so that one may properly speak of phosphorus requirement, simply, without discriminating between phosphates, phytates, phosphoproteins, and phosphatids.

Such experimental evidence as is cited here will be given in general in chronological order, to indicate, if possible, how present views have actually developed, and to suggest that they may at any time require modification as a result of further research.

Meischer studied the formation of complex from simpler phosphorus compounds in the adult animal body by observations upon the Rhine salmon, which during the breeding season remain a long time in fresh water, taking no food, but developing large masses of roe and milt at the expense of muscular tissue. This process evidently involves the formation of considerable amounts of nucleoproteins and phosphatids from simpler proteins, fats, and phosphorus compounds of the muscles. Paton * has studied the salmon of Scotland with similar results. Is there then any advantage in feeding phosphorus in organic forms?

Marcuse,† followed by Steinitz,‡ Zadik,§ and Leipziger,|| studied, by metabolism experiments on dogs, the nutritive value of phosphoproteins, when fed to the exclusion of phosphates and when contrasted with equivalent amounts of phosphorus and nitrogen fed in the form of mixtures of inorganic phosphates and simple proteins. Casein and ovovitellin were taken as typical phosphoproteins and compared with either myosin or edestin fed with inorganic phosphates. Rohmann * summarized the results as a whole and found a striking difference in the phosphorus balances in favor of the phosphoproteins as against the mixtures of simple proteins with inorganic phosphates. The storage of nitrogen was also more pronounced in the periods in which the phosphorized proteins were fed. The results appear to justify Rohmann's conclusion that the nutritive values of phosphorized and phosphorus-free proteins are not entirely the same, the former being especially adapted to furnish the material for tissue growth.

* Journal of Physiology, Vol. 22, page 333.

† Archiv fur die gesammte Physiologic (Pfluger), Vol. 67, page 373.

‡ Ibid., Vol. 72, page 75. § Ibid., Vol. 77, page 1.

|| Ibid., Vol. 78, page 402.

In experiments upon men, Ehrstrom† and Gumpert ‡ have found that a smaller amount of phosphorus will maintain phosphorus equilibrium when taken in the form of casein than when taken largely as dicalcium phosphate or as meat, the phosphorus of which is largely in the form of potassium phosphate. On the other hand Keller § in a study of the phosphorus metabolism of young children found evidence that storage of phosphorus was favored by food (like milk) which contained a liberal supply of phosphates in addition to the organic phosphorus compounds; and Von Wendt found that the loss of phosphorus occurring on a diet very poor in ash could be greatly reduced by the addition of dicalcium phosphate to the food.

* Berlin klinische Wochenschrift, Vol. 35, page 789.

†Skandinavisches Archiv fur Physiologie, Vol. 14, page 82.

‡ Medische Klinik, Vol. 1, page 1037.

§ Archiv fur Kinderheilkunde, Vol. 29, page 1.