STEYERMARK, A.C*; DIAMOND, J.: Optimal design of the digestive system

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. Here we examine the capacity of the intestine and other organs for processing glucose. We used total parenteral nutrition (TPN; a method of nourishing individuals through the circulatory system, thereby bypassing the gut) to manipulate glucose loads on organs other than the intestine that transport and process glucose. We tested the hypothesis that maximal glucose uptake capacity by the small intestine (as measured by the everted sleeve method) is matched to maximal glucose processing capacity by other organs. We used urine and serum glucose as markers that glucose processing capacity was being exceeded. Mean ad libitum oral glucose intake in the absence of TPN was 42 mmol d^-1, and maximal glucose uptake capacity was 26 mmol d^-1. In general, survivorship was high, presence of glucose in the urine was low, and serum glucose and osmolality were normal, in rats that received less than 94 mmol of glucose d^-1 by TPN. But at infusion rates of 105 mmol d^-1 and higher, urinary glucose, serum glucose, and serum osmolality increased, and survivorship decreased. Thus, rats were able to process approximately 3.7 times more glucose than they could transport through the small intestine, suggesting that glucose-processing organs outside the intestine are over-designed for their function: they can process more glucose than the small intestine can deliver to them. We shall consider safety factors in evolutionary and physiological terms.