225-OR: Key Role for Glucose-Alanine Cycling in the Regulation of Hepatic Mitochondrial Oxidation during Starvation in Humans

We examined whether starvation alters rates of hepatic mitochondrial oxidation in humans by applying a novel ¹³C Positional Isotopomer NMR Tracer Analysis (PINTA) method to assess rates of hepatic mitochondrial oxidation in healthy volunteers (n=15) following an overnight (12h) and a 60h fast. A 3h intravenous infusion of [3-¹³C] lactate (4.3 μmol/Kg-min), [1,2,3,4,5,6,6-²H₇] glucose (0.22 μmol/Kg-min), and [¹³C₄]-β-hydroxybutyrate (BOHB) (0.01 mg/Kg-min) was administered and blood was collected for GC-MS, LC-MS/MS and ¹³C MRS PINTA analyses. Following the 60h fast rates of endogenous glucose production decreased by 27% (p=0.0002) and rates of hepatic mitochondrial oxidation decreased by 52% (p=0.0014) whereas rates of hepatic pyruvate carboxylase flux remained unchanged. To ascertain whether decreased rates of alanine turnover during starvation play a role in promoting decreased rates of hepatic mitochondrial oxidation we infused [3-¹³C] alanine in a subgroup (n=5) of subjects and found that 60h of fasting promoted a 28% reduction in alanine turnover (p=0.044 vs. 12h fast). In order to determine whether this reduction in alanine turnover was responsible for the reduced rates of hepatic mitochondrial oxidation we infused unlabeled alanine to a subgroup (n=9) of the 15 subjects in a repeat 60h fasting study to increase rates of alanine turnover to rates measured after 12h of fasting and found that this perturbation increased rates of hepatic mitochondrial oxidation by 63% (p=0.009 vs. 60h fast) to rates similar to the 12h fasting state (60h+ala: 232±21 vs. 12h: 295±44 μmol/min, p=0.303).

Conclusion: These studies demonstrate that 60h of starvation in humans induces marked reductions in rates of hepatic mitochondrial oxidation, which in turn can be attributed to reduced rates of glucose-alanine cycling.