STEYERMARK, A.C.*; DIAMOND, J.; University of California, Los Angeles; University of California, Los Angeles: Functional design of the gut: nitrogen processing

Case studies in physiology and anatomy suggest that the design of cells, tissues, and organs is quantitatively adjusted to functional needs, such that the body uses available materials and space efficiently. Series pathways in biological design raise a question in design optimality: do all steps in the series have similar capacities, or is there a single rate-limiting step? Nutrient processing especially involves energetically expensive organs arranged in series. Here we examine the capacity of the intestine and other organs for processing nitrogen. We used total parenteral nutrition (TPN; a method of nourishing individuals through the circulatory system, thereby bypassing the gut) to manipulate amino acid loads on organs other than the intestine that transport and process amino acids and nitrogen. We tested the hypothesis that oral dietary amino acid intake, maximal amino acid uptake capacity by the small intestine, and maximal nitrogen processing capacity by other organs are approximately matched to each other. Urine and serum nitrogen were used as markers that nitrogen processing capacity was being exceeded. We explore the relationships between nitrogen intake, nitrogen uptake by the small intestine, and nitrogen processing by extra-intestinal organs, and then consider safety factors in evolutionary and physiological terms.