STEYERMARK, A.C.*; LAM, M.; DIAMOND, J.: Functional design of the gut

Did animals’ bodies evolve such that capacities of body components are matched efficiently to maximum natural loads, warranting the motto “enough but not too much”? Series pathways raise a further question in design optimality: do all steps have similar capacities, or is there a single rate-limiting step? Nutrient processing involves energetically expensive organs arranged in series: absorption in the intestine, metabolism in the liver, circulation by the heart, oxidation by lung gases, and waste excretion by the kidney. Thus, we tested concepts of design economy for the gut vis-a-vis other organs that process nutrients. A practical problem in exploring organ matches of nutrient processing capacities is that nutrients normally reach other organs from the intestine, so that the nutrient loads on those other organs cannot be separately manipulated. We circumvented this problem by nourishing laboratory rats with total parenteral nutrition (TPN)- a clinical procedure for delivering nutrients directly into the circulation of human patients. TPN permits us to administer an identical liquid nutrient mix to rats either as TPN or oral feeding. Thus, under circumstances of nourishing a rat with TPN, with any potential intestinal bottleneck eliminated, can the rat process far more nutrients than its intestine can absorb? Or are processing rates of other gut organs matched to intestinal absorption rates? Or can the intestine absorb nutrients faster than other organs can process them? Which, if any, nutrient-processing organ is rate-limiting?