Availability of amino acids in monogastric animals. Variations of digestive origin.

The supply of amino acids to the organism, required for maximum protein synthesis, results from several interdependent digestive processes: the sequential degradation of dietary proteins during their passage in the gastro-intestinal tract, and the absorption of their hydrolysis products. In this paper we survey the possible variations in these phenomena which are liable to affect amino acid availability. Concerns the transit of amino acids, we first examine gastric emptying which determines the length of stay of digestion products in the stomach and their passage to the small intestine. Thereafter the transit of digesta at the ileo-caeco-colic junction where the quantitative, qualitative and kinetic aspects of digestion in the small intestine may be investigated. The influence of the nature of proteins, and of their technological treatments is studied as well as the possible occurrence of a differential transit of free amino acids as compared to that of proteins and of proteins as compared to that of the carbohydrate fraction. With respect to enzyme hydrolysis, two kinds of problems may be considered: the accessibility of proteins to enzymes (due to the structure of proteins or of the other dietary components, or due to antinutritional factors), and the mode of action of some of the proteases (in particular trypsin, in relation with the involvement of technological treatments). The mechanisms thought to operate for normal absorption of free amino acids and oligopeptides are discussed, followed by some data concerning the absorption of amino acid derivatives, appearing during technological processing, and their influence on the absorption of normal forms of amino acids. The fate of the residual nitrogenous fraction in the large intestine, that is after the sites of amino acids release and absorption, is surveyed. A fundamental knowledge of these physiological processes may lead to development of methods estimating the yield of protein utilization in the digestive tract, based on the measurement either of amino acid disappearance from the lumen or of amino acid appearance in the portal blood. Digestion is a key step in the availability of amino acid, but it only gives a partial picture of this process. Therefore, its study should also be related to that of amino acid metabolism and overall protein synthesis.