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close this bookProtein-Energy Interactions (International Dietary Energy Consultative Group - IDECG, 1991, 437 pages)
close this folderAmino acid oxidation and food intake
View the document(introductory text...)
View the document1. Introduction
View the document2. Nitrogen balance and amino acid oxidation
View the document3. Amino acid oxidation during periods of positive or negative energy balance
View the document4. Interactions between energy and protein metabolism
View the document5. Amino acid degradation and gluconeogenesis
View the document6. Summary
View the documentReferences

5. Amino acid degradation and gluconeogenesis

It is often believed that gluconeogenesis from amino acids is induced during starvation to provide glucose and that the need to provide glucose is a driving factor in the increased gluconeogenesis taking place after trauma and during disease (KINNEY and ELWYN, 1983). This view may or may not include the belief that gluconeogenesis from amino acids is a minor process in the fed state. In fact, conversion of the glucogenic moieties of the degraded amino acids to glucose occurs even in the fed state. A detailed quantitative analysis of the energy exchanges associated with the degradation of amino acids in man (JUNGAS et al., 1992) demonstrates that gluconeogenesis and export of glucose is essential, because complete oxidation of the amino acid mixture by the liver would provide much more ATP than needed by this organ. Gluconeogenesis from amino acids must thus be regarded as a normal process associated with amino acid degradation, occurring at higher rates under conditions of normal food intake than during fasting.

The activation of pyruvate dehydrogenase by insulin is one of the mechanisms allowing insulin to promote carbohydrate oxidation (DENTON et al., 1987). The first irreversible steps in the degradation of the branched-chain amino acids are catalyzed by a-ketoacid oxidizing enzyme complexes which are similar to PDH and can also be activated by insulin (HARPER et al., 1984). It seems that glucose availability and the readiness to oxidize pyruvate which are enhanced by insulin explain the carbohydrate-sparing effect of exogenous carbohydrate (Figure 3). However, high insulin levels also open the pathway for the irreversible degradation of the BCAA, notably in skeletal muscle. Since the BCAA are indispensable amino acids, their loss by the irreversible oxidation of their a-keto derivatives condems the other amino acids to being degraded as well, since they cannot be used for protein synthesis without enough BCAA.