Prolonged Tracer Infusion and Sequential Muscle Biopsies Do Not Affect Human Muscle Protein and Amino Acid Kinetics

Many muscle protein metabolism studies in humans involve a sequential design, with the same subject studied at baseline and during an intervention. However, tracer recycling, prolonged fasting, and sequential muscle biopsies from the same area could theoretically affect muscle protein metabolism. We determined the effect of prolonged tracer infusion with extended fasting and sequential muscle biopsies on parameters of muscle protein and amino acid kinetics in 6 adult men. After a 12 hr overnight fast, a primed continuous infusion of L‐[ring‐ 2 H 5 ]phenylalanine was started. After 120 min, we took the first of a series of 5 hourly muscle biopsies from the same vastus lateralis to measure the fractional protein synthetic rate (FSR). Further, between 150–180, 210–240, and 330–360 min we measured leg phenylalanine kinetics with the two‐pool and the three‐pool arteriovenous balance models. Tracer enrichments were at steady state, and muscle protein FSR and phenylalanine kinetics did not change throughout the experiment (Figure 1). We conclude that prolonged tracer infusion and fasting with up to 5 sequential muscle biopsies do not affect basal muscle protein and amino acid kinetics in human subjects. Thus, when using an acute sequential design it is unnecessary to include a saline only control group to account for these variables. Support: NIH P30 AG17231, R01 AG18311, and R01 AR049877 1