The extent of labile protein stores and their response to protein feeding was studied in undernourished adult subjects. Urinary nitrogen excretion following 3 levels of protein intake (62, 82 and 96 g/day), each protein period alternated with a protein-free diet, was studied in 4 apparently healthy but undernourished young men. Urinary nitrogen with a protein-free diet decreased to a nearly steady value within 2 to 3 days. The initial decrease was small, however, indicating poor labile protein stores in these subjects. With diets containing protein considerable nitrogen was retained. The retained nitrogen, however, was not excreted when the subjects were changed over from protein-containing diets to protein-free diets. Endogenous urinary nitrogen excretion in these subjects was not different from that of normal subjects
