Response of human muscle protein synthesis to feeding: a comparison of methods

Abstract

Feeding has been shown to stimulate muscle protein synthesis rates (FSR) in humans. However, while a substantial stimulation (+60–100%) of muscle FSR in response to feeding has been shown in studies using the constant infusion method, a much smaller response (+30%, P=NS) has been detected with the flooding method. The aim of this study was to assess whether the response of muscle protein synthesis to feeding depends on the technique used for measurement. Muscle FSR were measured both in the fasting and feeding states in two groups (n=8) of healthy volunteers (Age 31±2 y), with the same tracer and feeding regimen. However, in one group FSR were assessed with a flooding amount of L-[2H5]phenylalanine (45 mg/kg BW) and in the other one with a primed constant infusion of the same labeled amino acid (6 μmol/kg/h). Fasting FSR obtained with the flooding method were similar to that obtained with the constant infusion method calculated with phenylalanine within the tissue taken as the precursor pool for protein synthesis (1.86±0.13 vs. 1.94±0.15 %/day). The results also show that feeding is associated with a stimulation of FSR, whether assessed by the flooding method (+22%, P<0.01) or constant infusion (+13%, P<0.05). Lower fasting FSR (1.31±0.07 %/day, P<0.01) and a greater stimulation after feeding (+25%, P<0.05) were observed with the constant infusion method when plasma was used as a surrogate precursor pool. This study demonstrates that human muscle FSR measured with the flooding method is comparable to that obtained with the constant infusion, and that, under similar experimental conditions, the two methods give comparable estimates of stimulation from feeding. Supported by NIH grants AG017446 and 5-MO1-RR-10710