On account of the firm curd produced in cow's milk by the addition of gastric juice (rennin), and because of the regular occurrence of "curds"in the bowel movements during indigestion, the casein of cow's milk has been, and by many authorities especially in this country still is, considered the chief disturber in feeding cow's milk. This view is no longer held by any of the leading German writers. If the fat in these cases is removed from the milk, and the fat-free milk is given in similar modification, the curds, as well as the indigestion, quite regularly disappear. These curds, we are now taught to believe, are not casein but essentially fat derivatives. They never occur when really fat-free (centrifuged) milk is given and quite regularly reappear in cases with marked fat intolerance, when even a few ounces of whole milk are given in the same food. Under certain abnormal conditions, that are not sharply defined at present, large, hard masses, resembling soaked beans or dried dough, occur as well with fat-free as whole milk. These are probably essentially casein masses. These are rare, occur only when straight or slightly diluted milk is used, and when there is indigestion with increased peristalsis, and are in no way identical with the common "curds" of indigestion. They have seemed rather an accompaniment of indigestion than a cause of it, much as curds will appear in the bowel movements following a cathartic. It is, furthermore, quite probable that the constant motion of the stomach prevents the formation of such massive curds as we find in the stationary test tube. There is, then, very little positive evidence that the protein of cow's milk causes serious trouble in digestion. The view that an excessive amount of protein makes a dangerous demand upon a delicate and inadequate infantile metabolism is equally doubtful. In food intoxications where we have a "metabolic bankruptcy"(Finkelstein), protein alone is well borne, while fat and sugar are toxins. Nature's provision for so small an amount of protein in human milk makes us, however, hesitate to use a larger amount unless necessary.

In the use of too little protein we have, on the other hand, a real danger. It alone contains nitrogen, and no other element can take its place. Anemia, weakness, lack of resistance are early evident in protein deficiency. If the deficiency is too great and prolonged, death results. Such lack, of course, rarely occurs in feeding cow's milk. Protein can, however, easily be given in inadequate amounts in infant foods, when a mistaken fear of "protein indigestion "leads to extreme dilution of the milk.

Nutritional Disturbances Caused By Salts

The salts are essential in growth, digestion, and metabolism. The bearing upon nutrition of the excessive and qualitatively different amount in cow's milk is little understood. The possible connection of an excess of salts with such condition as tetany, eczema, etc., leads one to infer their possible importance. That there is great danger from an inadequate amount when, for example, an excessive carbohydrate diet with little or no milk is given, is evident. The importance of adding common salt to barley water when given alone for only a few days is easily seen.