In adult male rats fed a non-purified diet supplemented with 5 % sodium propionate, plasma cholesterol concentrations were significantly depressed. Although liver cholesterol was increased by feeding propionate, rates of hepatic cholesterol and fatty acid synthesis were unchanged. Tissue concentrations and rates of synthesis of cholesterol were also unaffected by dietary propionate in stomach, small intestine and caecum. Concentrations of propionate in hepatic portal venous plasma were raised by feeding the supplemented diet but the increase was low in comparison to the dietary intake. Examination of the gut contents revealed concentrations of total volatile fatty acids (VFA) of 19 µmol/ml in the stomach contents of control rats and 148 µmol/ml (of which propionate contributed 116 µmol/ml) in those fed the supplemented diet. Duodenal and ileal concentrations of VFA were very low and were only slightly raised in the propionate-fed rats while caecal VFA were the same in both groups with a combined mean of 159 µmol/ml. These data indicate that in the rat, the absorption of dietary propionate appears to occur in the stomach. In pigs fed a standard ration hepatic portal venous VFA remained low for the first 4 h after feeding but then rose with the onset of large bowel fermentation. Feeding the diet supplemented with propionate caused hepatic portal venous plasma concentrations to rise by approximately 0.4 µmol/ml. This increase was apparent 30 min after feeding and was sustained for 3 h but subsequently there was no difference to controls. As in the rat, the absorption of dietary propionate appeared to occur in the upper gastrointestinal tract. The transport of propionate via the porcine hepatic portal vein also appeared insufficient to account for the dietary intake and suggests metabolism of the acid by the upper gastrointestinal tract. Further studies with perfused livers from fed rats indicated that propionate at a concentration of 1 µmol/ml did not alter cholesterol synthesis but that inhibition occurred at 18 µmol of propionate/ml. It appears that a redistribution of cholesterol from the plasma to the liver, rather than inhibition of hepatic and intestinal cholesterol synthesis, is responsible for the hypocholesterolaemic effects of dietary propionate. Because the absorption and transport of dietary propionate appears to follow a time course which differs considerably to that of the acid produced by the large bowel microflora, we conclude also that VFA produced by such fermentation would not seem to be responsible for the hypocholesterolaemic effects of certain water-soluble plant fibres.

1. Adult male rats were fed on diets containing 100 g dietary fibre/kg either as a-cellulose or wheat bran or the pericarp-seed coat or aleurone layers prepared from that bran by sequential milling and air elutriation and electrostatic separation.2. After 10 d, concentrations of total volatile fatty acids (VFA) in caecal fluid were significantly different between groups and fell in the order: aleurone > wheat bran > pericarp-seed coat > cellulose. This ranking probably reflected the ease of fermentation of fibre polysaccharides by colonic bacteria which also resulted in a considerably higher faecal bacterial mass in the aleurone group.3. Because of the differences in the volume ofcaecal digesta, the mass of caecal VFA was considerably the highest in the aleurone group, intermediate with wheat bran and equally low in the pericarp-seed coat and cellulose groups.4. The diet based on aleurone gave a relatively higher proportion of propionate but with both pericarp-seed coat and wheat bran the contribution of butyrate was raised.5. VFA concentrations in hepatic portal venous plasma were proportional to caecal concentrations with very high (> 3 mM) values being recorded in the aleurone group.6. The findings are discussed in relation to the apparent susceptibility of the morphological components of wheat bran to fermentation by large bowel bacteria.