PROTEIN DIGESTION & AMINO ACID ABSORPTION

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Protein Meal


Protein Meal

Blood levels of Glucose, Insulin, Glucagon

Carbohydrate Meal


Carbohydrate Meal

Blood levels of Glucose, Insulin, Glucagon

A High Protein Meal
Following ingestion of a high protein meal, the gut and the liver utilize most of the absorbed amino acids. Glutamate and aspartate are utilized as fuels by the gut, and very little enters the portal vein. The gut may also use some branched chain amino acids. The liver takes up 60 - 70% of the amino acids present in the portal vein. These amino acids, for the most part, are converted to glucose.

After a pure protein meal, the increased levels of dietary amino acids reaching the pancreas stimulate the release of glucagon above fasting levels, thereby increasing amino acid uptake into the liver.

Insulin release is also stimulated, but not nearly to the levels found after a high carbohydrate meal (figure at the right; click the image for a comparison with a carbohydrate meal and note the differences in the blood glucose, insulin and glucagon levels between a low or no carbohydrate protein meal and a high carbohydrate meal). In general, the insulin released after a high protein meal is sufficiently high that net protein synthesis is stimulated, but gluconeogenesis in the liver is not inhibited. The higher the carbohydrate content of the meal, the higher the insulin/glucagon ratio and the greater the shift of amino acids away from gluconeogenesis into biosynthetic pathways in the liver, such as the synthesis of plasma proteins.

Most of the amino acid nitrogen entering the peripheral circulation after a high protein meal or a mixed meal is present as the branched chain amino acids (leucine, isoleucine, valine). Because the liver has low levels of transaminases for these amino acids, it cannot oxidize them to a significant extent and they enter the systemic circulation. The branched chain amino acids are taken up slowly by skeletal muscle and other tissues. These peripheral non-hepatic tissues utilize the amino acids derived from the diet principally for net protein synthesis.