Ketogenesis supports hepatic polyunsaturated fatty acid homeostasis via fatty acid elongation.

Therapeutic interventions targeting hepatic lipid metabolism in metabolic dysfunction-associated steatotic liver disease (MASLD) and steatohepatitis (MASH) remain elusive. Using mass spectrometry-based stable isotope tracing and shotgun lipidomics, we established a novel link between ketogenesis and MASLD pathophysiology. Our findings show that mouse liver and primary hepatocytes consume ketone bodies to support fatty acid (FA) biosynthesis via both de novo lipogenesis (DNL) and FA elongation. Analysis of 13 C-labeled FAs in hepatocytes lacking mitochondrial D-β-hydroxybutyrate dehydrogenase (BDH1) revealed a partial reliance on mitochondrial conversion of D-βOHB to acetoacetate (AcAc) for cytoplasmic DNL contribution, whereas FA elongation from ketone bodies was fully dependent on cytosolic acetoacetyl-CoA synthetase (AACS). Ketone bodies were essential for polyunsaturated FA (PUFA) homeostasis in hepatocytes, as loss of AACS diminished both free and esterified PUFAs. Ketogenic insufficiency depleted liver PUFAs and increased triacylglycerols, mimicking human MASLD, suggesting that ketogenesis supports PUFA homeostasis, and may mitigate MASLD-MASH progression in humans.