Effects of Fasting on Blood Non-Protein Amino Acids in Humans

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Abstract
Fasting in adult humans led in 48 hours to increased blood levels of leucine and valine. Blood levels of 5 other amino acids, namely lysine, threonine, methionine, arginine and tryptophan were simultaneously reduced by fasting. The response of adult humans to fasting, in terms of blood non-protein amino acid levels, was entirely different from that found in chicks up to 6 weeks of age, in that the blood levels of different amino acids were increased as a result of fasting. Examination of the literature revealed a correspondence, for the human species, between amino acids exhibiting a blood level rise during fasting and those not metabolically available by tissue conversion of structural analogues. A similar relationship had been found earlier in chicks. Hence it is suggested that a correspondence, between amino acids exhibiting increased blood levels due to fasting and those metabolically unavailable by precursor amination, may hold true for a variety of species.