Since phosphorus compounds are essential to all the tissues of the body, the growth of new tissue requires a storage of phosphorus along with that of protein, but aside from this it is evident that the phosphorus metabolism presents a separate problem from the metabolism of protein.

The phosphorus of the tissues exists largely in the form of nucleoproteins - the characteristic substances of cell nuclei - and, as these are important in metabolism, there was a tendency for a number of years to regard the phosphorus elimination as largely a measure of the metabolism of nucleoproteins somewhat as the nitrogen is taken as a measure of the metabolism of proteins in general. It is probable, however, that such a view of the phosphorus metabolism is of only very limited application, because of the influence of other factors. Voit showed that the material metabolized in fasting comes largely from the bones. Undoubtedly the bones take part in the daily metabolism, and while they may undergo a less active exchange of material than the soft tissues, they possess such a large proportion of the phosphorus in the body that they probably)contribute a considerable part of what is metabolized from day to day. Moreover, recent investigations upon the function of the soluble phosphates of the blood in maintaining neutrality in the body indicate that the neutralization of acid by conversion of di- into mono-phosphates may be followed by an increased excretion of the acid phosphate in the urine. Finally, it is evident that the amount of phosphorus metabolized is very directly influenced by the amount taken in the food.

* Journal of the American Medical Association, Vol. 64, page 573 (1915).

The phosphorus which has been metabolized is excreted from the body almost entirely in the form of inorganic phosphates, the organic phosphorus of the urine constituting as a rule only 1 to 3 per cent of the total.* Carnivorous animals excrete phosphates mainly through the kidneys, but in the herbivora the excretion occurs almost entirely through the intestinal wall, whether the phosphate be taken by the mouth, or injected sub-cutaneously, or be formed by metabolism of organic phosphorus compounds in the body. In man, the elimination of metab-olized phosphorus is partly through the kidneys and partly through the intestinal wall, the relative quantities in urine and feces varying within rather wide limits. As a rule, foods rich in calcium, or which yield an alkaline ash, tend to increase the proportion of phosphorus excreted by way of the intestine.

Attempts have sometimes been made to estimate the phosphorus requirement from the amount excreted in the urine.

* Some investigators have doubted the occurrence of organic phosphorus in urine while others have estimated it as high as 6 per cent of the total urinary phosphorus.

The results thus obtained are always too low (usually very much so), and are largely responsible for the fact that the amount of phosphorus required for the normal nutrition of man is seriously underestimated in many of the standard textbooks.

Since the excretion of metabolized phosphorus through the intestine is in man too large to be neglected and too variable to be allowed for by calculation, we can expect reliable data on phosphorus requirements from those experiments only in which the amounts of phosphorus are actually determined in food, in feces, and in urine. In such experiments it is found (as in the case of nitrogen) that the output obtained upon the experimental days is influenced not only by the food taken at the time, but also by the rate of metabolism to which the body had been accustomed on the preceding days. This is shown by the following results obtained in a 12-day series of experiments upon a healthy man:

Phosphorus Metabolism With Different Amounts Of Phosphorus In The Food

Here the output of phosphorus was greater in the first period with 0.40 gram in the food than in the second when the food furnished 0.77 gram, probably because the first period followed and was influenced by a preceding diet fairly rich in phosphorus, whereas the output in Period II was influenced by the low-phosphorus diet of Period I. For the same reason Period II offered favorable conditions for the establishment of equilibrium on a minimum diet, and the results show that in this case the subject was unable to reach equilibrium on 0.77 gram per day, the output averaging 0.91 gram. When the intake was increased to 1.51 grams, the output rose rapidly and averaged 1.49 grams. In this case the amount which would have been just sufficient for equilibrium evidently lay between 0.91 and 1.49 grams per day. By means of well-planned experiments or series of experiments it is possible to fix for a given individual much narrower limits within which the exact amount required for equilibrium must lie, and when it is known that the intake approximates this required amount, it is justifiable to regard the output as an indication of the normal nutritive requirement. Study of the data of 93 such phosphorus balance experiments upon 27 subjects, 21 men and 6 women, has shown a range of 0.52 to 1.75 grams with an average of 0.96 gram phosphorus 2.20 grams P2O5) per 70 kilograms of body weight per day. This corresponds with the average requirement of 50 grams protein per day per man of 70 kilograms as estimated on page 220. Allowing 50 per cent above the bare minimum would give a phosphorus "standard" of 1.44 grams (3.30 grams P2O5) corresponding to a protein "standard" of 75 grams.