Absorption kinetics of amino acids, peptides, and intact proteins.

The small intestine acts as interface and regulator between the gut lumen and the rest of the body and controls the degree and rate of transport of amino acids coming from dietary protein via the portal vein to the liver and the systemic circulation. To measure protein absorption, kinetics multicatheter animal (pig) models in combination with amino acid tracer technology are available. Dietary factors influence the absorption rates from the lumen to the gut, metabolism of dietary component in the gut, and the release of amino acids to the portal circulation from digested protein. In a balanced-protein meal, the gut dietary amino acid utilization (30-50%) for gut protein synthesis will result in a labile protein pool in the gut that can be beneficial during the postabsorptive state. To enhance gut retention, amount and quality of protein and the presence of carbohydrate are major factors. Besides this the use of a slowly digestible protein or the presence of fiber in the meal can increase retention further. During the absorption of low-quality protein meals, fewer amino acids are utilized by the gut, resulting in higher amounts of amino acid release to the portal circulation. Malnutrition or starvation, protein depletion, deficiencies of specific nutrients, or illness such as sepsis all inhibit the growth and change protein turnover of the intestinal mucosa and therefore affect absorption kinetics. Therefore, the kind of protein meal that has the most optimal absorption kinetics (the most beneficial) for gut and for the rest of the body depends on these (patho)physiological circumstances. Despite the absence of different absorption kinetics between protein, peptides, and amino acids, they could be beneficial in specific circumstances.