Effects of absorption of large amounts of volatile fatty acids on rat liver metabolism.

The effects of large amounts of volatile fatty acids (VFA) on hepatic metabolism have been investigated in vivo, with rats adapted to a high fiber (HF) diet, or in vitro with isolated hepatocytes. Net absorption of glucose was negligible and lactate production was lower in rats fed the HF diet than in those fed a fiber-free basal diet. VFA (particularly acetate and propionate) were absorbed in very large amounts in rats fed the HF diet. Propionate and butyrate were completely removed by the liver in both groups of rats; the efficiency of acetate uptake was higher in rats fed the HF diet than in those fed the basal diet. Gluconeogenesis was active in rats fed the HF diet, but lactate uptake was very limited in spite of high portal concentrations. Hepatocytes from rats fed the HF diet utilized VFA at different rates: acetate was poorly utilized, propionate utilization plateaued at about 1 mM external propionate, whereas butyrate utilization was utilized about twice as efficiently. Propionate and butyrate displayed opposite effects on lactate utilization. The stimulating effect of butyrate prevailed over the inhibitory effects of propionate, at high concentrations (2 mM). However, the results at lower concentrations (less than or equal to 0.5 mM) suggest that, owing to its higher portal concentrations, the effects of propionate on lactate uptake might prevail in vivo. The effects of acetate in vivo might be greater than on isolated hepatocytes.