Nutritional supply of proteins and absorption of their hydrolysis products: consequences on metabolism.

Except in nibblers such as the rat the food intake of single-stomached animals, particularly omnivores, is discontinuous and varies in amount and composition. These quantitative and qualitative variations in food intake pose a number of questions. Do they affect the digestive processes which are related to food transit and enzyme hydrolysis, especially the extent of nutrient absorption and its kinetics? What effect do they have on the storage of nutrients in the liver and the temporal distribution in the tissues? Do they lead to a modification of the nutritive value? These questions cannot be answered merely by measuring digestibility. Digestibility is used to evaluate the disappearance of ingested nutrients either in the whole digestive tract (rectal digestibility) or in its proximal part (ileal digestibility), where the nutrients are not metabolized by the flora and are, thus, absorbed in their initial form of diet components. Whichever technique is used, these digestibility measures are global and static and do not account for the different absorption rates which vary for different nutrients. Since body reserves of free amino acids are small, the relative timing of the input of amino acids and of carbohydrates can markedly affect protein synthesis and the efficiency of feed utilization (Geiger, 1950). For example, if a pig is supplied with a limiting amino acid (AA) at a time other than when the rest of the diet, deficient in that one AA, is given the growth rate and feed efficiency will be lower than when the diet and supplement are given simultaneously (RCrat & Bourdon, 1975). Study of kinetics associated with differences in the relative absorption rate of the various nutrients is necessary for a fuller understanding of variations in the nutritive value of feeds. Also, the chemical substances which appear in the body during digestion must be identified so that we may evaluate the changes in feeds which result mainly from microbial metabolism, as they pass through the digestive tract and during their transport by the enterocyte. The importance of these transformations is that they can lie at the origin of changes in their nutritive value. The use of a technique which allows direct measurement of intestinal absorption and hepatic metabolism of nutrients has helped to resolve some of these problems. The technique has allowed assessment of the amounts of nutrients appearing in blood, their composition and changes with time, as well as the kinetics of their uptake and release by the liver. Furthermore, it has provided us with an appreciation of the endocrine reactions of the enterohepatic axis involved in the regulation of absorption and metabolism. This paper reports some findings relating to proteins and gives experimental results obtained in our laboratory on the kinetics of intestinal absorption of nitrogenous nutrients and their consequences for hepatic and peripheral metabolism.