Techniques for assessing protein and glucose kinetics

Isotopic labelling techniques can lead to a better understanding of the changes in substrate flow resulting from trauma and other pathological conditions. This article describes the basic approaches used to measure rates of substrate flow, especially those using stable isotopes, and their application to the study of glucose and protein kinetics. Methods for measuring glucose turnover and gluconeogenesis in the whole body by constant infusion of different labelled forms of glucose are explained. The advantages of measuring regional rates of glucose metabolism, using arteriovenous balance of tracer and tracee, are illustrated with results demonstrating the role of gluconeogenesis by the kidney. Similar approaches are used to measure protein turnover rates in the whole body and in specific regions, with labelled amino acids such as [1-¹³C]leucine. In addition, rates of protein synthesis in individual tissues can be assessed by measuring the incorporation of tracer into protein of a biopsy sample. The relative merits of two methods of giving the tracer, by constant infusion or by flooding injection, are explained, with examples of studies of muscle protein synthesis in surgical patients.