Baseline measurements for stable isotope studies: an alternative to biopsy.

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 (delta 13CPDB) 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.