[Concept of labile protein and the adaptation of nitrogen metabolism in protein deficiency in man].

Investigations involving 30 healthy males, who for 45 days consumed standard semisynthetic diets differing in the caseine content (from 0 to 15.1 g of nitrogen per day) and serving as a sole source of protein, furnished data on the nitrogen excretion kinetics and nitrogen balance. Mathematical analysis of these data allowed it to find the existence of general regularity in the nitrogen excretion and to deduce an integral equation of its losses at different levels of nitrogen consumption. Pertinent calculations have ascertained that by the labile protein one should understand not the specific proportion of the total protein in the organism, but the actual rate of its metabolism, manifesting itself in the routine lag-phase during adaptation of the nitrogen metabolism to a new level of the protein allowances in the organism. The data thus made available permitted it, at the same time, to propose a criterion for estimating the optimal requirements of man in proteins.