My own studies of protein metabolism in man, though pursued in a different way and for a different purpose, have a direct bearing on the problem of the necessary minimum protein consumption. The specific waste products formed from the destruction of protein within the human organism are eliminated in soluble form with the urine. They are therefore easily available for detailed chemical investigations, and as the result of innumerable studies much exact knowledge has been gained concerning the normal katabolism products of protein. My investigations lie within this field.
The views that have till recently prevailed concerning the chemistry of urine, in so far as it relates to the problem of protein metabolism, may be concisely stated as follows: All the nitrogen of the protein destroyed in the body is eliminated with the urine, and almost 90% of it appears in the form of urea. From 95% to 98% of the nitrogen is eliminated as urea, kreatinin, ammonia, and uric acid. The absolute amount of each of these nitrogenous products depends upon the amount of protein katabolized, but changes in the amount of protein destroyed affect them all equally, thus leaving them always in about the same proportion to each other and to the total nitrogen. This fact, if correct, is very important, because it clearly indicates a certain unity in the chemical processes concerned with the use and destruction of protein within the body. It indicates that the protein of the food and the protein of the living tissues are essentially alike and in the same condition with reference to the organism at the time of their final destruction. The two rival theories concerning this subject accordingly agree in assuming the essential unity of the chemical processes involved in protein katabolism, and the only point of difference between the two is that one, the theory of Voit, holds that the protein must be in solution and dead before being oxidized and destroyed, while the other, that of Pfliiger, assumes that it is only actually living protoplasm that is destroyed.
It would be useless in this connection to go into a detailed discussion of these two theories, because I think it can be shown that the fundamental premise of both, namely, the supposed constancy in the relative distribution of the urinary nitrogenous products, is no longer tenable.
The fact that the relative proportions of the various nitrogenous constituents of normal human urine have so long been supposed to be approximately constant is in a measure directly the result of Voit's teachings concerning the minimum protein requirement. The destruction of 100 grams protein or more within the organism, as demanded by the dietary standards, rendered it well nigh impossible to discover the laws that govern the formation and elimination of each product. About a year and a half ago I discovered accidentally that the urine corresponding to a very low protein katabolism has a chemical composition which is very different from that of urine derived from the standard diets. This led to numerous attempts to reduce the daily protein destruction in normal persons to the lowest possible level.
Nearly all preceding attempts to reduce the protein katabolism have also been attempts to maintain at the same time nitrogen equilibrium. In mine the question of nitrogen equilibrium, or loss of protein, was not considered, and I have as a matter of fact used a diet containing almost no protein. I have kept several normal persons for a week or more, two or three persons for two weeks, and one for 17 days on a diet consisting exclusively of pure arrow root starch and 300 ec'of cream. In this way the daily protein katabolism has repeatedly been reduced to about 20 grams a day.
Detailed chemical studies of the urines corresponding to such greatly reduced protein katabolism have shown that the relative proportion which the nitrogenous products bear to each other and to the total nitrogen does change and very greatly. For example, the kreatinin elimination is entirely independent of the total amount of protein katabolized. It is just as great on a diet containing no protein as on one containing 118 grams of protein. In the one case it represents from 17% to 20% of the total nitrogen, in the other from 3% to 4%. The urea, on the other hand, is peculiarly a product of excessive protein katabolism. When the urinary nitrogen represents a katabolism of 100 grams of protein, 90% of that nitrogen is present as urea, while when the protein katabolism has been reduced to 20 grams, only from 50% to 60% of its nitrogen appears in the form of urea.