These facts concerning urea and kreatinin suffice to show how entirely erroneous has been the assumption that the nitrogen of katabolized protein is always distributed in the same proportion among the different waste products. It may therefore be superflous now to go into further details concerning the laws that govern the formation and elimination of the different products. The fact that these laws are widely different for different products, as for urea and kreatinin, demonstrates with a fair degree of certainty that protein metabolism is not all of one kind.
The true minimum katabolism or protein, as obtained in my feeding experiments with starch and fats, is clearly very different from the katabolism of the large quantities of protein demanded by the dietary standards. The former converts not over 60% of the protein nitrogen into urea, and is the source of all the kreatinin eliminated with the urine. The katabolism of that food protein which is not absolutely needed for the maintenance of nitrogen equilibrium, on the other hand, yields probably at least 95% of its nitrogen in the form of urea and yields no kreatinin whatever. The katabolism which yields the kreatinin clearly tends to be constant and independent of the food protein; it can therefore fairly be said to represent the tissue metabolism. The katabolism which yields chiefly urea is the katabolism of the excessive food protein, and its amount depends directly upon the amount of protein contained in the food. This I have called the exogenous metabolism.
Since the exogenous metabolism seems to have nothing to do with the tissue metabolism, and since it increases immediately with every increase of protein furnished with the food and in porportion to such increase, it represents nothing else than the effort of the organism to get rid of nitrogen that it does not need and can not use. The remarkable ability of the human organism to establish nitrogen equilibrium on almost any quantity of protein does therefore not mean, as has been believed, that the organism uses protein by preference instead of fats and carbohydrates. This phenomenon is merely the result of our habitual consumption of more protein than can be used in the tissue metabolism. Being always supplied with an excess, we have always with us the maximum amount of reserve protein that we can advantageously carry, and any further increase in the supply simply leads to an increased elimination.
Such, in brief, are the conclusions which I have drawn from detailed studies of the waste products of protein katabolism.
To recapitulate: We have learned from Voit that protein is not needed to supply energy; and the work of more recent investigators has demonstrated that nothing like 100 grams of protein is needed to maintain nitrogen equilibrium in a man of average size. Further, from detailed analytic studies, we have learned that some waste products, like kreatinin, represent tissue metabolism, and others, like urea, the metabolism of that food protein which is destroyed as rapidly as it is taken in. The two kinds of metabolism are independent. The tissue ' metabolism is for each individual a constant quantity, irrespective of the amount of protein contained in the food. Obviously, therefore, there is a constant minimum protein requirement to prevent loss of tissue material. The amount of protein needed for this purpose is very small, probably not over 25 grams a day. It does, however, not necessarily follow that 25 grams protein should be prescribed instead of 118 grams. The prevailing idea that consumption of more than the minimum amount of protein is detrimental to health may not be true. The minimum may not be the optimum. But what has been considered the minimum, 118 grams, may be beyond the optimum, possibly even above the maximum amount of protein that any normal person should consume.