Correction factors for C-13-labelled substrate oxidation at whole-body andmuscle level

The oxidation of fatty acids, carbohydrates and amino acids can be measured by quantifying the rate of excretion of labelled CO2 following administrat ion of C-14- or C-13-labelled substrates at whole-body and tissue level. Ho wever, there is a theoretical need to correct the oxidation rates for the p roportion of labelled CO2 that is produced via oxidation but not excreted. Furthermore, depending on the substrate and position of the C label(s), the re may also be a need to correct for labelled C from the metabolized substr ate that does not appear as CO2, but rather becomes temporarily fixed in ot her metabolites. The bicarbonate correction factor is used to correct for t he labelled CO2 not excreted. Recently, an acetate correction factor has be en proposed for the simultaneous correction of CO2 not excreted and label f ixed in other metabolites via isotopic exchange reactions, mainly in the tr icarboxylic acid cycle. Changes in metabolic rate induced, for example, by feeding, hormonal changes and physical activity, as well as infusion time, have been shown to affect both correction factors. The present paper explai ns the theoretical and physiological basis of these correction factors and makes recommendations as to how these correction factors should be used in various physiological conditions.