Pancreatic Adaptation to Diet — Regulation of Specific Enzyme Synthesis
The exocrine pancreas provides the intestine with a mixture of enzymes capable of digesting the three kinds of nutritional substrates generally found in a normal diet: protein, carbohydrate, and lipid. Several mechanisms regulating the rate of pancreatic secretion and its enzyme composition have developed during evolution. These mechanisms enable the organism to adjust quantitatively and qualitatively the enzyme output to changes in the pattern of food intake and to changes in food composition. Quantitative adjustments (control of enzyme output) are exerted by hormones and neurotransmitters released during feeding. These messengers are also involved in the regulation of pancreatic growth (see Chapter 1). Qualitative adjustments of enzyme levels in secretion have been described following changes in the proportions of dietary constituents. This phenomenon has been called pancreatic adaptation to diet, since the changes in digestive enzyme proportions were directed towards an optimization of digestion. The purpose of this chapter is to review our present knowledge on pancreatic adaptation. Contributions which led to the understanding that adaptation is not a matter of independent secretion but rather a gradual phenomenon involving changes in the relative rates of enzyme synthesis will be summarized in the first part. Recent evidence that this is actually due to differential regulation of pancreatic gene expression will be reported in the second part. The third part will deal with the important question of the messengers transmitting information on food composition from the intestinal lumen to the pancreas, an area in which our knowledge is still limited. Pancreatic response to nonphysiologic alterations of diet, such as protein deprivation, malnutrition, or the ingestion of toxic substances, will not be reviewed.