The relative role of protein synthesis and degradation in determining
postprandial net protein deposition in human muscle is not known. To this aim, we
studied forearm leucine and phenylalanine turnover by combining the arteriovenous
catheterization with tracer infusions, before and following a 4 h administration
of a mixed meal in normal volunteers. Forearm amino acid kinetics were assessed
in both whole blood and plasma. Fasting forearm protein degradation exceeded
synthesis (P < 0.01) using either tracer, indicating net muscle protein loss. The
net negative forearm protein balance was quantitatively similar in whole blood
and in plasma. After the meal, forearm proteolysis was suppressed (P < 0.05- <
0.03), while forearm protein synthesis was stimulated (P < 0.05- < 0.01).
However, stimulation of protein synthesis was greater (P < 0.05- < 0.01) in whole
blood (leucine data: +50.4 +/- 7.8 nmol/min x 100 ml of forearm; phenylalanine
data: +30.4 +/- 11.6) than in plasma (leucine data: +17.8 +/- 5.6 nmol/min x 100
ml of forearm; phenylalanine data: +5.7 +/- 2.1). Consequently, the increment of
net amino acid balance was approximately two to fourfold greater (P < 0.04- <
0.03) in whole blood than in plasma. In conclusion, meal ingestion stimulates
forearm protein deposition through both enhanced protein synthesis and inhibited
proteolysis. Plasma data underestimate net postprandial forearm protein
synthesis, suggesting a key role of red blood cells and/or of blood mass in
mediating mealenhanced protein accretion.
