Richard J. Illman

The adaptive effects of dietary fish and safflower oil on lipid and lipoprotein metabolism in perfused rat liver

To elucidate the mechanisms underlying the plasma triacylglycerol-lowering effects of certain fish oils, livers from male rats fed either a standard commercial diet (controls) or diets supplemented with 15% (w/w) fish or safflower oils were perfused with undiluted rat blood. Rates of hepatic lipogenesis, measured by the incorporation of 3H2O into fatty acids, followed the order: control greater than safflower oil greater than fish oil. Secretion of newly synthesized fatty acids in very-low-density lipoproteins was also inhibited by the feeding of both oil-supplemented diets with the greater suppression being seen in livers from animals fed fish oil. The hepatic release of very-low-density lipoprotein triacylglycerol mass was also significantly depressed in animals fed the fish oil-supplemented diet but not in those fed safflower oil. Ketogenesis did not differ between livers from rats fed the control and safflower oil diets but was significantly raised in the fish oil group. Increased ketogenesis with fish oil was paralleled by a decrease in the sensitivity of carnitine palmitoyl transferase of isolated mitochondria to inhibition by malonyl-CoA. The inhibitory effect of malonyl-CoA in the safflower oil group was intermediate between that in the fish oil and control groups. Activities of glycerophosphate acyltransferase with either palmitoyl-CoA or oleyl-CoA were increased by feeding oil-supplemented diets. Activity with palmitoyl-CoA that was suppressible by N-ethylmaleimide was also considerably diminished in both groups. The results indicate that the lowering of plasma triacylglycerols by fish oil reflects: (a) diminished lipogenesis; (b) increased fatty acid oxidation possibly in peroxisomes; and (c) diminished secretion of triacylglycerols by the liver.

Hypocholesterolaemic Effects of Dietary Propionate: Studies in Whole Animals and Perfused Rat Liver

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 mumol/ml in the stomach contents of control rats and 148 mumol/ml (of which propionate contributed 116 mumol/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 mumol/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 mumol/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 mumol/ml did not alter cholesterol synthesis but that inhibition occurred at 18 mumol 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.

Comparative effects of dietary wheat bran and its morphological components (aleurone and pericarp-seed coat) on volatile fatty acid concentrations in the rat.

Adult male rats were fed on diets containing 100 g dietary fibre/kg either as alpha-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. After 10 d, concentrations of total volatile fatty acids (VFA) in caecal fluid were significantly different between groups and fell in the order: aleurone greater than wheat bran greater than pericarp-seed coat greater than 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. Because of the differences in the volume of caecal 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. 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. VFA concentrations in hepatic portal venous plasma were proportional to caecal concentrations with very high (greater than 3 mM) values being recorded in the aleurone group. The findings are discussed in relation to the apparent susceptibility of the morphological components of wheat bran to fermentation by large bowel bacteria.

Effects of dietary oat bran on faecal steroid excretion, plasma volatile fatty acids and lipid synthesis in rats

Abstract Two mechanisms have been proposed for the hypocholesterolaemic effects of dietary oat bran, i.e. acceleration of cholesterol catabolism or inhibition of cholesterol biosynthesis. The latter is proposed to occur via propionate formed through large bowel microbial fermentation of fibre. We have examined both of these mechanisms in rats fed dietary oat bran and have found that faecal bile acid and neutral sterol excretion was enhanced compared with animals fed cellulose. In contrast, concentrations of propionate in the hepatic portal vein, although raised by dietary oat bran, were less than 2% of those at which inhibition of hepatic cholesterogenesis has been observed in vitro. When hepatic cholesterol synthesis was measured with 3 H 2 O, it was increased in oat branfed rats. Therefore, it appears most likely that the hypocholesterolaemic effects of this cereal fibre preparation can be explained by increased faecal steroid excretion and not through inhibition of cholesterogenesis by volatile fatty acids of large bowel origin.

Plasma lipids and large bowel volatile fatty acids in pigs fed on white rice, brown rice and rice bran

Adult male pigs were fed on a diet containing (% of energy) fat 25 starch 55 from white rice and providing 20 g fibre/pig d (diet WR). In two other groups rice bran was added to the diet to provide 43 g fibre/d. One group received the diet unmodified (diet RB), but in another (diet RO) heat-stabilized unrefined rice oil replaced the palm oil. In a further group brown rice replaced white rice and provided 37 g fibre/pig per d (diet BR). Plasma cholesterol concentrations were similar with diets WR, RB and BR. With diet RO the concentration was significantly lower than with diets WR and BR but was not different from diet RB. Plasma high-density-lipoprotein-cholesterol and plasma triacylglycerols were unaffected by diet. In all groups, digesta mass rose from the caecum to the proximal colon but fell in the distal colon. Diet WR gave the lowest digesta mass while diet BR gave a significantly higher mass along the large bowel length. RB- and RO-fed pigs had equal masses of digesta which were intermediate between BR- and WR-fed pigs at all sampling sites. Pools of individual and total volatile fatty acids (VFA) in the proximal large bowel were unaffected by diet. Pools of total and individual VFA in the median and distal colon were lowest with diets WR and RB and significantly higher with diet BR. In these regions of the colon pools of acetate in RO-fed pigs did not differ from those in the BR-fed group but were higher than in other groups. However, pools of propionate and butyrate with the RO diet were significantly lower than with diet BR and the same as with diets WR and RB. Portal venous VFA concentrations were unaffected by diet. The higher large bowel digesta masses and VFA with diet BR may reflect the escape of starch from the small intestine.

A viscous fibre (methylcellulose) lowers blood glucose and plasma triacylglycerols and increases liver glycogen independently of volatile fatty acid production in the rat

1. Adult male rats were maintained on diets containing 80 g methylcellulose/kg of low (25 cP), medium (400 cP) and high (1500 cP) viscosity. 2. After 10 d, the viscosity of stomach and caecal contents was found to have increased in proportion to that of the dietary fibre. Concentrations of volatile fatty acids in caecal digesta were lowest with the high-viscosity fibre but acetate was the major acid present with all three diets. Acetate was the only acid found in significant quantities in hepatic portal venous plasma and concentrations of this acid were unaffected by diet. 3. Concentrations of glucose in arterial blood were low with the medium- and high-viscosity diets while the content of liver glycogen was high. These effects of fibre were not directly on glucose absorption as the intestines were net removers of the hexose at the time of sampling. 4. Hepatic lipogenesis and plasma triacylglycerol concentrations were both higher in rats fed on the low-viscosity fibre. Plasma cholesterol concentrations, hepatic cholesterol synthesis and faecal bile acid excretion were not altered by dietary fibre viscosity. 5. We conclude that the effects of dietary fibre on carbohydrate absorption and storage and fatty acid synthesis are a function of the viscosity of the fibre in solution, high viscosity slowing the digestion and absorption of nutrients in the small intestine. Large-bowel microbial fermentation is not of direct significance to these events. In contrast, effects of fibre polysaccharides on sterol metabolism seem not to be related to their rheological properties.

A trial of the effects of soya-bean flour and soya-bean saponins on plasma lipids, faecal bile acids and neutral sterols in hypercholesterolaemic men.

1. The hypothesis that soya-bean saponins, by binding bile salts in the gastrointestinal lumen, are responsible for some of the plasma-cholesterol-lowering effect of soya-bean preparations, was tested. In a double-blind crossover study 50 g soya-bean flour/d, containing either 22 or 4 g saponins/kg (adjusted by ethanol extraction) was incorporated in biscuits as a substitute for biscuits or bread into the diet of ten outpatient hypercholesterolaemic men over two consecutive 4-week study periods. The diet was monitored to ensure constancy, saponin-rich foods excluded, faeces collected for bile acid and neutral sterol analysis, and blood taken for plasma lipoprotein lipid analysis. 2. Neither diet had any effect on cholesterol in any plasma lipoprotein fraction, on fasting plasma triglyceride, or on faecal bile acids and neutral sterols. 3. These results suggested that soya-bean saponins are not responsible for the hypocholesterolaemic effect of soya-bean products.

Volatile fatty acids in the human intestine: Studies in surgical patients

Abstract Volatile fatty acids (VFA) were measured in digesta taken from the stomata of 39 patients who had undergone large bowel surgery. Total VFA were highest in patients with transverse colostomy and lowest with ileostomy. Levels were intermediate in sigmoid colostomy patients and the same as in faeces from normal controls. The only acid present at the ileostomy site was acetate but in all other samples propionate and butyrate together contributed 30–40% of the total. Neither total nor individual VFA concentrations at any stomal site were related to total fibre intake.

Effects of dietary oat bran and diabetes on plasma and caecal volatile fatty acids in the rat

Abstract Diets high in plant fibre, particularly viscous gums, are of potential benefit in the control of diabetes in man. This study compares the effects of oat bran, one such fibre preparation, with purified cellulose on volatile fatty acid metabolism in rats fed a high carbohydrate diet. Concentrations of total volatile fatty acids were significantly higher both in hepatic portal venous plasma and caecal contents of rats fed oat bran. Major differences were also found in the relative contributions of individual acids with proportionately less acetate and more propionate and butyrate in these animals compared with rats fed cellulose. In rats fed a standard laboratory diet, diabetes induced by streptozotocin produced a sustained increase in caecal and hepatic portal venous concentrations of acetate, propionate and butyrate. Because of the high simple carbohydrate content of the experimental diet, the tolerance of diabetic rats for the bran and cellulose diets was poor and no differences in VFA metabolism were observed after 24 hr of diabetes. It is concluded that volatile fatty acid concentrations were in proportion to the probable digestibility of the source of dietary fibre and are significant metabolic fuels liable to altered metabolism by insulin insufficiency.

Effects of Wheat Bran and Porridge Oats on Hepatic Portal Venous Volatile Fatty Acids in the Pig

Adult male pigs (40-60 kg of body weight) of the Kangaroo Island strain were surgically implanted with chronic indwelling hepatic portal venous cannulae. After a 24-hour fast the animals were given meals containing 500 g of either wheat bran or porridge oats and 200 g of sucrose and 2 litres of milk. With both cereal preparations plasma volatile fatty acids rose in the hepatic portal vein but the increase was significantly greater with wheat bran. Omission of sucrose and milk did not alter the response to porridge oats but diminished the response to wheat bran. These changes in plasma volatile fatty acids were unaffected by prior cooking of the cereals with hot water. With all test meals acetate and propionate were the major acids found, with butyrate contributing less than 8% of the total. This compositional profile was also found when the pigs were fed a commercial ration. The absence of butyrate differed from observations in the rat and reflected low concentrations of this acid in large bowel digesta. The difference in the response of the concentration of volatile fatty acids to feeding porridge oats and wheat bran in the pig was also the reverse of that found in the rat. These species differences may be of significance in relation to the choice of animal models for human fibre metabolism.