Ketone bodies are an alternative energy substrate to glucose in brain. Under conditions of oxidative stress, we hypothesize that ketosis stabilizes glucose metabolism by partitioning glucose away from oxidative metabolism towards ketone body oxidation. In this study we assessed oxidative metabolism in ketotic rat brain using stable isotope mass spectrometry analysis. The contribution of glucose and ketone bodies to oxidative metabolism was studied in cortical brain homogenates isolated from anesthetized ketotic rats. To induce chronic ketosis, rats were fed either a ketogenic (high-fat, carbohydrate restricted) or standard rodent chow for 3 weeks and then infused intravenously with tracers of [U-(13)C] glucose or [U-(13)C] acetoacetate for 60 min. The measured percent contribution of glucose or ketone bodies to oxidative metabolism was analyzed by measuring the (13)C-label incorporation into acetyl-CoA. Using mass spectrometry (gas-chromatography; GC-MS, and liquid-chromatography; LCMS) and isotopomer analysis, the fractional amount of substrate oxidation was measured as the M + 2 enrichment (%) of acetyl-CoA relative to the achieved enrichment of the infused precursors, [U-(13)C]glucose or [U-(13)C] acetoacetate.
Results: the percent contribution of glucose oxidation in cortical brain in rats fed the ketogenic diet was 71.2 ± 16.8 (mean% ± SD) compared to the standard chow, 89.0 ± 14.6. Acetoacetate oxidation was significantly higher with ketosis compared to standard chow, 41.7 ± 9.4 vs. 21.9 ± 10.6. These data confer the high oxidative capacity for glucose irrespective of ketotic or non-ketotic states. With ketosis induced by 3 weeks of diet, cortical brain utilizes twice as much acetoacetate compared to non-ketosis.