Baseline measurements for stable isotope studies: An alternative to biopsy

Abstract

Studies of protein metabolism with stable isotopes require determination of the ‘natural’ isotopic enrichment in tissues. This has previously been determined by taking a pre-test muscle biopsy or by using the isotopic enrichment of a separate control group of subjects. In this study we have measured and compared the “natural” ¹³C enrichment of leucine in plasma protein and muscle protein in 14 subjects. The mean enrichment of leucine (δ¹³C_PDB) in muscle protein, −26.627, was not significantly different from that in plasma protein, −27.152. The data indicate that the ¹³C enrichment of leucine in plasma protein reflects that of muscle protein and provides an attractive alternative to an additional muscle biopsy in studies of protein metabolism with stable isotopes.