The regulation of intestinal metabolism and its impact on whole animal energetics.

In digesting and absorbing dietary nutrients, the gastrointestinal tract consumes approximately 20% of all incoming energy. A substantial proportion of this consumption is due to the rapid turnover of cellular protein, which permits abrupt changes in gut size to occur, matching capacity with delivery. If it is size of the alimentary tract that constrains nutrient uptake, greater than 20% allocation of ME intake above maintenance to the gut would improve the growth rate of a young animal but the efficiency of ME utilization for growth would deteriorate. Less than 15% allocation in birds seems injurious to both growth rate and efficiency of growth. Nutrient transport capacity of the intestine may be modulated independent of size; in the case of glucose, an up- or down-regulation of the number of brush-border glucose transporters matches absorptive capacity with delivery. The maximum uptake capacity of a small intestine for glucose at any moment in time is a function of its length, the flow rate of digesta, and the distributed-in-space kinetic parameters of transport (e.g., Vmax and Km). An example maximum uptake capacity for glucose in sheep is calculated at 2,112 g/d, assuming continuous digesta flow. Intermittency of flow reduces the uptake capacity to a functional level of 295 g/d, demonstrating a constraining influence of the periodicity of the migrating myoelectric complex. Growth regulation by stimulatory and inhibitory mitotic signals is presented as a candidate for an energy-independent determinant of the upper limit to functional maximum uptake capacity of the small intestine. Both size and functional capacity of the intestine must be considered in assessing the impact this tissue may have on the rest of the animal.