Nutritional regulation of postruminal digestive enzymes in ruminants

Nutritional regulation of digestive enzymes has received limited study in ruminants. Comparable studies with nonruminants report adaptations occurring in pancreatic and mucosal enzyme concentrations and synthesis in response to increased consumption of carbohydrate, lipid, or protein. Adaptations in carbohydrases apparently are mediated through changes in blood glucose and insulin, and changes in lipase are mediated through changes in circulating beta-hydroxybutyrate. Increased synthesis and secretion of pancreatic proteases apparently are mediated directly through luminal effects of protein, possibly through a messenger peptide. Available information for ruminants suggests that pancreatic alpha-amylase concentration increases with increased energy intake, but not with increased carbohydrate intake, independent of energy. This result may indicate an adaptation mechanism similar to that of nonruminants, one that is mediated through increased glucose turnover, not intestinal carbohydrate supply alone. Increased intake of energy or increased intestinal supply of carbohydrates results in little or no increase in mucosal carbohydrases in the ruminant. Comparable data for effects of changing intestinal supply of protein are limited to the developing ruminant. Experiments involving lipid feeding are not available, but duodenal lipid infusion decreases pancreatic lipase secretion. Ruminant data are inadequate to predict accurately an adaptive response to feeding any diet.