A dual-isotope technique for determination of in vivo ketone body kinetics

'Total ketone body specific activity' has been widely used in studies of ketone body metabolism to circumvent so-called 'isotope disequilibrium' between the two major ketone body pools, acetoacetate and β-hydroxybutyrate. Recently, this approach has been criticized on theoretical grounds. In the present studies, [¹³C]acetoacetate and β-[¹⁴C]hydroxybutyrate were simultaneously infused in nine mongrel dogs before and during an infusion of either unlabeled sodium acetoacetate or unlabeled sodium β-hydroxybutyrate. Ketone body turnover was determined using total ketone body specific activity, total ketone body moles % enrichment, and an open two-pool model, both before and during the exogenous infusion of unlabeled ketone bodies. Basal ketone body turnover rates were significantly higher using [¹³C]acetoacetate than with either β-[¹⁴C]hydroxybutyrate alone or the dual-isotope model (3.6 ± 0.5 vs. 2.2 ± 0.2 and 2.7 ± 0.2 μmol·kg^-1·min^-1, respectively, P < 0.05). During exogenous infusion of unlabeled sodium acetoacetate, the dual-isotope model provided the best estimate of ketone body inflow, whereas ¹⁴C specific activity underestimated the known rate of acetoacetate infusion by 55% (P < 0.02). During sodium β-hydroxybutyrate infusion, [¹³C]-acetoacetate overestimated ketone body inflow by 55% (P = NS), while better results were obtained with ¹⁴C β-hydroxybutyrate alone and the two-pool model. Ketone body interconversion as estimated by the dual-isotope technique increased markedly during exogenous ketone body infusion. In conclusion, significant errors in estimation of ketone body inflow were made using single-isotope techniques, whereas a dual-isotope model provided reasonably accurate estimates of ketone body inflow during infusion of exogenous acetoacetate and β-hydroxybutyrate. In addition, the dual-isotope technique can provide estimates of ketone body interconversion. The dual-isotope model may offer significant advantages over single-isotope methods for studying ketone body metabolism in vivo.