1. The effect of a protein-free meal and a protein-rich meal on the concentration of free amino acids in plasma and muscle tissue was studied in eight healthy subjects. The energy content of the protein-free meal was 3800 kJ. The protein-rich meal was identical in composition except that 50 g of bovine serum albumin was added. Plasma and samples from the quadriceps femoris muscle (percutaneous muscle biopsy) for amino acid determination were collection before and at 1, 3, 5 and 7 h after the meal.

2. After the protein-free meal the concentrations of most essential amino acids and of some non-essential amino acids in plasma decreased continuously below basal levels at 5–7 h. The muscle concentration of essential amino acids fell too, reaching its nadir 3–5 h after the meal. The decrease in plasma amino acid concentration was smaller than the decrease in muscle concentration for all essential amino acids except phenylalanine.

3. The concentrations of most amino acids in plasma increased transiently 1 and 3 h after the protein-rich meal; histidine and several non-essential amino acids fell below the basal levels at 5–7 h after the meal. In muscle, threonine, valine, leucine, lysine and alanine were increased at 1 and 3 h after the protein-rich meal; isoleucine, serine and glycine fell below the basal level after 5 and 7 h. For the essential amino acids except threonine and lysine, the increase in plasma concentration was greater than the increase in muscle concentration.

4. Correlations were observed between the relative content of the essential amino acids in the bovine serum albumin and the increase in concentration of these amino acids in plasma and muscle. Methionine and isoleucine, which had the lowest content in bovine serum albumin, fell below basal levels in plasma and (for isoleucine) in muscle 5–7 h after the meal, suggesting that these two amino acids might have been limiting for protein synthesis.

5. Amino acid analysis in plasma and muscle samples taken postprandially should be evaluated with caution considering the strong influence of meal composition on plasma and muscle free amino acid profiles.

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