Whole-body protein turnover in peritoneal dialysis patients: a comparison of the [N-15]glycine end product and the [C-13]leucine precursor methods

Background. Two well-described methods for measuring whole-body protein turnover (WBPT) are the precursor method using a primed continuous infusion of [1-C-13]leucine and the end-product method with a single oral dose of [N-15]glycine. We previously measured the effects of amino acid (AA)-containing dialysate on protein anabolism in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) using the [1-C-13]leucine technique. Here, we examine whether the less invasive [N-15]glycine method could also be appropriate for studying nutritional interventions. Methods. We compared the results of WBPT measurements using a single oral dose of [N-15]glycine with those obtained with the primed continuous infusion of [1-C-13]leucine during AA and glucose (G) dialysis and G-only dialysis in 12 CAPD patients in the fed state. Results. The end-product method showed a wide variation for protein synthesis and breakdown measurements. It did not detect a small but significant increase in protein synthesis with AA-containing dialysate as shown by the precursor method. However, a significant relation was found between both methods for net protein synthesis (i.e. protein synthesis minus breakdown) during AA and G (r = 0.75, P = 0.005) or during G-only dialysis (r = 0.86, P < 0.001). The agreement between the two methods for the net protein balance was good [intra-class correlation coefficient (ICC) = 0.88] with G-only dialysate and moderate (ICC = 0.70) with AA and G dialysate. Conclusion. While the precursor method shows less variation, the more convenient end-product method may be useful in larger groups of selected patients including those on PD.