Whole body protein turnover was measured in six normal adults using a model based on a primed constant infusion of [2H5]phenylalanine and, independently, by an established method of a primed constant infusion of [1-13C]leucine. Isotopic plateau in plasma was achieved within 2 h for [2H5]phenylalanine and, in four of the subjects who received a priming dose of [2H4]tyrosine, for [2H4]tyrosine. In all subjects whole body protein turnover measured with the phenylalanine model (mean protein synthesis, 2.65 +/- (SD) 0.16 g.kg-1.24 h-1; catabolism, 3.58 +/- 0.26 g.kg-1.24 h-1) was similar to that measured using the leucine model (synthesis, 3.09 +/- 0.27 g.kg-1.24 h-1; catabolism, 3.70 +/- 0.35 g.kg-1.24 h-1). Mean forearm fractional muscle protein synthesis calculated by the phenylalanine model was 0.06 +/- 0.03%/h, which compares closely with literature values derived by other methods. The phenylalanine model allows the rapid assessment of whole body and muscle protein turnover from plasma samples alone, obviating the need for measurement of expired air CO2 production or enrichment.

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