In cow's milk the greater part of the phosphorus appears to exist as phosphate, but there can be no doubt that the milk phosphorus as a whole is available for the needs of the young of the species, especially in view of the parallelism pointed out by Bunge and Abderhalden between the phosphorus and calcium content of milk and the rate of growth of the young. (See accompanying table.)

It is, however, not without possible significance that the phosphorus of human milk is mainly in organic forms (Soldner) and that, notwithstanding its much lower content of total phosphorus, human milk contains as high a percentage of lecithin as does cow's milk (Stoklasa). An infant fed on diluted cow's milk must therefore receive less lecithin than the breast-fed infant while it may receive more total phosphorus.

In general the more recent investigation favor the view that the body can use inorganic phosphates to meet all its phosphorus requirments.

Hart, McCollum, and Fuller showed in 1909 that with young pigs on a ration too poor in phosphorus to support normal growth the deficit could be made good by feeding phosphates as well as by feeding foods containing organic phosphorus compounds.

The following year (1910) McCollum reported that, other things being satisfactory, all the phosphorus requirements of an animal can be met by feeding inorganic phosphates. In one of these experiments McCollum kept a rat for 104 days on diets of purified food materials in which phosphorus was given only as phosphate. It maintained good condition but suffered some loss of weight as it would not eat enough of the artificial food to meet the energy requirement. In another case in which an amino acid mixture from the hydrolysis of beef muscle was added to the diet the food was eaten more readily and one rat increased in weight from 153 to 176 grams while receiving only inorganic phosphorus.

As young rats eat unpalatable food more readily than do adults, McCollum fed the ration containing phosphate as sole source of phosphorus to young growing rats, one of which ate the ration for 127 days, during which time he doubled in weight. At the end of this experiment the rat was killed and analyzed and found to be of normal composition. There was therefore no reason to doubt that the rat synthesized the nucleoproteins and phosphatids of his growing tissues from the inorganic phosphorus of his food.

Subsequent experiments by McCollum and Davis, as well as those of Osborne and Mendel described in connection with the discussion of proteins (Chapter III (Proteins)), afford many instances of long-continued growth of rats on rations made up of "isolated" foodstuffs in which all or nearly all of the phosphorus was in the form of simple phosphates.

In order to determine whether the synthesis of lecithin in the animal body can be demonstrated experimentally, McCollum, Halpin, and Drescher (1912) fed 3 hens for 10 weeks a ration consisting of 30 per cent skim milk powder and 70 per cent polished rice, both of which were freed from phosphatids. This diet it will be noted contained phosphoprotein as well as phosphate, but very little fat, and it was believed no phosphatid. The hens produced eggs in normal number and of normal composition. The phosphatid in the eggs produced was 27.65 grams per hen, and this was believed to have been synthesized rather than to have come from material previously stored.

Fingerling (1912) kept ducks for 8 months on a diet of potatoes, blood albumin, starch, and lime salts. The ducks laid normally and the phosphatid content of the eggs produced was determined. Since the phosphatid content of the food must have been small and the feces always contained some lecithinlike substances, and since the ducks did not lose weight, Finger-ling concludes that the organic phosphorus compounds in the eggs were synthesized from inorganic phosphorus obtained in the food.

Later he fed the same ducks on food richer in organic phosphorus; and as they produced about the same number of eggs of similar phosphatid content he concluded that the egg-phos-phatids were synthesized as readily from inorganic as from organic phosphorus compounds.

The evidence seems sufficient to warrant the statement that animal organisms are able to synthesize nucleoproteins, phospho-proteins, and phosphatids from inorganic phosphate. It may, however, still be questioned whether the nutritive conditions are as favorable when the body is forced to do this as when a part at least of the phosphorus requirement is met by feeding phosphoproteins and phosphatids.

The above-mentioned experiments of Rohmann and his pupils on dogs and of Ehrstrom and Gumpert on men seemed to demonstrate that the phosphoproteins have a higher food value than a corresponding mixture of simple proteins and simple phosphates; and the recent feeding experiments, while showing the efficiency of phosphates in meeting the phosphorus requirement, do not show conclusively that the phosphates are of fully equal value with the organic phosphorus compounds. Feeding experiments of long duration are well fitted to give convincing evidence on the former point, but are not so well suited for the purposes of exact quantitative comparisons because the very fact of their long duration gives opportunity for other factors to enter, such as differences in vitality among the experimental animals. Masslow, as the result of recent investigation of phosphorus metabolism during growth, holds (1913) that for the best results a considerable part of the phosphorus should preferably be supplied in organic forms.

Some writers have argued that the presence in extracts of intestinal mucosa of enzymes capable of splitting off phosphoric acid from the organic phosphorus compounds of the food may be taken as evidence that phosphorus is absorbed as phosphoric acid or phosphate whatever the form in which it occurs in the food; but in view of the reversibility of enzyme action and the great extent to which it is influenced by conditions, it seems preferable to form our impressions regarding the equivalence or relative values of the different phosphorus compounds from observations or experiments upon animals rather than from tests for enzymes in tissue extracts.

Forbes holds that even though the phosphorus be absorbed as inorganic phosphate there is advantage in having it supplied largely in organic forms since "much larger amounts of phosphorus may be utilized in a normal manner if they are gradually liberated in the usual way by the digestive cleavage of the organic complexes with which they are combined." *

Forbes and Keith (1914) after reviewing most thoroughly the whole literature of phosphorus compounds in animal metabolism, draw, among others, the following conclusion:

"That organic phosphorus is absolutely essential to any animal has not been demonstrated. The proof that inorganic phosphorus can serve all of the purposes for which any animal needs phosphorus is incomplete.† There is much evidence to imply that, with some species at least, some organic phosphorus compounds are more useful than is inorganic phosphorus in the sense of being more readily and economically utilized, and of maintaining a higher state of vitality as revealed by tissue enzyme estimations, the difference probably depending, in part at least, on the fact of the partial absorption and utilization of organic phosphorus compounds as such, without complete digestive cleavage" (Ohio Agricultural Experiment Station, Technical Bulletin No. 5, pages 364-365).

* Ohio Agricultural Experiment Station, Technical Bulletin No. 5, page 357. † (Some of the experiments of Osborne and Mendel and of McCollum and Davis have appeared since the above was written by Forbes and Keith. H. C. S.)

On the other hand Marshall * considers the evidence fully sufficient to warrant the conclusion that organic phosphorus compounds are of no more value as food than are the inorganic phosphates.

In the present state of our knowledge there is at least no quantitative measure of differences in nutritive value as between different forms of phosphorus. If differences in nutritive value between the different groups of phosphorus compounds exist, they are doubtless in favor of the phosphoproteins and phos-phatids and are more significant for the growing than for the full-grown organism. For the reasons explained in Chapters VIII, XIII, and XIV the diet of growing children should always contain a liberal allowance of milk. The milk will provide, in addition to the best form of protein, a high proportion of phosphoprotein and also significant quantities of phosphatids. Hence it seems justifiable to assume that, if the food is properly selected, one may compute its total phosphorus content and compare it with the total phosphorus requirement of the body without separate computation of the different forms of phosphorus.