Starvation protein catabolism during

During starvation, following depletion of hepatic glycogen, amino acids become the major source for glucose homeostasis because of the decrease in plasma insulin 

Amino acid metaboli.sm following dietary protein intake. Digestion of protein produces amino acids. Within the intestinal cell, amino acid interconversions form alanine, which is delivered by the portal blood to liver, where it serves as the source of cr-amino nitrogen and pyruvate, which is converted to lipid and glucose. Excess nitrogen is converted to urea. Branched-chain amino acids (BCAAs) are not taken up by the liver but enter peripheral tissues such as muscle where they serve as an important fuel source. Their a-amino nitrogen is transported to liver in the form of alanine. 

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Other factors may affect zinc excretion. For example, low dietary intake of zinc or malnutrition can increase the urinary excretion of zinc. This release of zinc is a result of tissue breakdown and catabolism during starvation and elevated urinary excretion of zinc may persist after intake levels return to normal (Spencer et al. 1976). Administration of histidine or high-protein diet may increase urinary zinc excretion however, a corresponding increase in zinc absorption may maintain zinc balance in the body (Henkin et al. 1975b Hunt et al. 1991). 

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

The uniqueness of leucine can be further demonstrate by examining muscle protein synthesis. We have shown that leucine has the ability to stimulate protein synthesis in muscles during catabolic conditions such as starvation (25). Using a large dose of leucine, protein synthesis can be stimulated 50 in muscles from starved rats. The significance of this effect remains controversial however, the in vitro activity is clear and emphasizes the unique metabolic potential of leucine. 

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