Nutritional hyperketonemia by dietary medium-chain fatty acids is driven by the liver without contribution from the...

Dietary medium-chain fatty acids (MCFAs) are absorbed in the intestine and transported to the liver via the portal vein. The rate-limiting enzyme for ketogenesis, 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2), is expressed in both the liver and intestine. While the liver is well established as the primary site of ketogenesis during fasting, the intestine's role in nutritional hyperketonemia from dietary MCFAs is unclear.

To achieve nutritional hyperketonemia, we orally administered medium-chain (C8:0) triacylglycerol (MCT) oil to control and liver- and intestine-specific Hmgcs2 knockout mice and measured β-hydroxybutyrate (β-OHB) levels in the portal vein and systemic circulation. MCFA-driven β-OHB production was also assessed in primary murine hepatocytes and human and murine intestinal cell lines. Expression of enzymes involved in MCFA oxidation and ketogenesis was analyzed using publicly available bulk and single-cell RNA sequencing data from human and mouse tissues.

In MCT-treated mice, β-OHB levels increased four-fold in systemic circulation and statistically more (six-fold) in the portal vein, the latter suggesting intestinal contribution to systemic hyperketonemia. However, circulating β-OHB increased similarly in control mice and those lacking intestinal Hmgcs2 . RNA sequencing data of human and mouse tissues showed that medium-chain acyl-CoA synthetases, enzymes required for MCFA activation, are scarcely expressed in intestinal cells. Consistently, cultured intestinal cells failed to produce β-OHB from MCFA (octanoic acid, C8:0), unlike hepatocytes, which produced substantial levels of β-OHB when treated with MCFA. Finally, MCT-induced nutritional hyperketonemia was completely abolished in mice lacking hepatic Hmgcs2 .

Nutritional hyperketonemia from dietary C8:0-MCFA is mediated by the liver, not the intestine, which appears to lack the enzymes to activate MCFAs. In addition, the common practice of measuring metabolites or other factors in portal vein blood as a readout for intestinal contribution must be used with caution.