Marie-Anne Levrat

Effect of propionate on fatty acid and cholesterol synthesis and on acetate metabolism in isolated rat hepatocytes

In the present study the actual role of propionic acid in the control of fatty acid and cholesterol synthesis was investigated in isolated liver cells from fed rats maintained in the presence of near-physiological concentrations of glucose, glutamine and acetate. Using 3H2O for lipid labelling, propionate appears as an effective inhibitor of fatty acid synthesis and to a lesser extent of cholesterol synthesis, even at the lowest concentration used (0.6 mmol/l). Butyrate is a potent activator of both synthetic pathways, and the activating effect was not counteracted by propionate. Using 1-[14C]acetate, it was observed that propionate at a moderate concentration, or 1 mmol oleate/l, are both very effective inhibitors of 14C incorporation into fatty acid and cholesterol. This incorporation was drastically inhibited when propionate and oleate were present together in the incubation medium. The net utilization of acetate by rat hepatocytes was impaired by propionate, in contrast to oleate. 1-[14C]butyrate was utilized at a high rate for fatty acid synthesis, but to a lesser extent for cholesterol synthesis; both processes were unaffected by propionate. Intracellular citrate concentration was not markedly depressed by propionate, whereas it was strongly elevated by butyrate. In conclusion, propionate may represent an effective inhibitor of lipid synthesis when acetate is a major source of acetyl-CoA, a situation which is encountered with diets rich in readily-fermentable fibres. The present findings also suggest that propionate may be effective at concentrations close to values measured in vivo in the portal vein.

Resistant starch is more effective than cholestyramine as a lipid-lowering agent in the rat.

Amylase-resistant starch (RS) represents a substrate for the bacterial flora of the colon, and the question arises as whether RS shares with soluble fibers common mechanisms for their lipid-lowering effects. It is uncertain whether a cholesterol-lowering effect depends basically on an enhanced rate of steroid excretion or whether colonic fermentations also play a role in this effect. In the present study, the effect of RS (25% raw potato starch), of a steroid sequestrant (0.8% cholestyramine), or both were compared on bile acid excretion and lipid metabolism in rats fed semipurified diets. RS diets led to a marked rise in cecal size and the cecal pool of short-chain fatty acids (SCFA), as well as SCFA absorption; cholestyramine did not noticeably affect cecal fermentation. Whereas cholestyramine was particularly effective at enhancing bile acid excretion, RS was more effective in lowering plasma cholesterol (−32%) and triglycerides (−29%). The activity of 3-hydroxy-3-methylglutaryl-CoA reductase was increased fivefold by cholestyramine and twofold by RS. This induction in rats fed RS diets was concomittant to a depressed fatty acid synthase activity. In rats fed the RS diet, there was a lower concentration of cholesterol in all lipoprotein fractions, especially the (d=1.040−1.080) fraction high-density lipoprotein (HDL1), while those fed cholestyramine had only a significant reduction of HDL1 cholesterol. In contrast to cholestyramine, RS also depressed the concentration of triglycerides in the triglyceride-rich lipoprotein fraction. There was no noticeable synergy between the effects of RS and cholestyramine when both were present in the diet. This suggests that the cholesterol-lowering effect of RS is not limited to its capacity to enhance bile acids excretion. The difference between RS and cholestyramine could relate to the capacity of fermentation end-products to counteract the upregulation of cholesterol and bile acid biosynthesis. Thus, in the absence of fermentation in the large intestine, a high rate of bile acids excretion is not always sufficient to elicit a cholesterol-lowering effect.

Effects of soluble corn bran arabinoxylans on cecal digestion, lipid metabolism, and mineral balance (Ca, Mg) in rats

The effects of soluble corn bran arabinoxylans on cecal digestion, lipid metabolism, and mineral utilization [calcium (Ca) and magnesium (Mg)] were investigated in rats adapted to semipurified diets. The diets provided either 710 g/kg wheat starch alone (control) or 610 g/kg wheat starch plus 100 g/kg corn soluble fiber (arabinoxylans) and either 0 or 2 g/kg cholesterol (control + cholesterol and arabinoxylans + cholesterol, respectively). Compared with rats fed the control diets, rats fed the arabinoxylan diets had significant cecal hypertrophy (+50% after 3 days of the fiber adaptation) and an accumulation of short-chain fatty acids, especially propionic acid (up to 45% in molar percentage). Arabinoxylans enhanced the cecal absorption of Ca and Mg (from 0.07 to 0.19 micromol/min for Ca and from 0.05 to 0.23 micromol/min for Mg). Mg balance was enhanced by arabinoxylans (+25%). The arabinoxylan diet markedly reduced the cholesterol absorption from 50% of ingested cholesterol in controls up to approximately 15% in rats adapted to the arabinoxylans diet. Arabinoxylans were effective in lowering plasma cholesterol (approximately -20%). There was practically no effect of the diets on cholesterol in d > 1.040 lipoproteins (high density lipoproteins) whereas arabinoxylans were very effective in depressing cholesterol in d < 1.040 lipoproteins (especially in triglyceride-rich lipoproteins). Corn fermentable fiber decreased the accumulation of cholesterol in the liver. In parallel, the arabinoxylan diet counteracted the downregulation of 3-hydroxy-3-methylglutaryl-CoA by cholesterol. These data suggest that arabinoxylans may have a great impact on intestinal fermentation, mineral utilization, and cholesterol metabolism.

Fiber fermentability in the rat cecum and its physiological consequences

Abstract The aim of the study was to compare the fermentability of complex fiber sources in the rat large intestine. The studied sources ranged from predominantly structural fiber (oat fiber and external pea fiber) to more soluble plant storage fibers (beet fiber and internal pea fiber) and were included at levels of about 13% or 26% TDF in the diet. Rats fed the diets containing the more insoluble fibers had a compensatory increase in intake, resulting in similar weight gains in all groups. Beet and internal pea fibers were better fermented, and caused a more pronounced hypertrophy of the cecum and greater absorption of short-chain fatty acids (SCFA). The SCFA profile varied with a higher proportion of butyrate in the cecal contents of rats fed the less soluble fibers and a higher proportion of propionate in those fed the internal pea fiber. Feeding fiber consistently enhanced the transfer of urea nitrogen into the cecum, enhancing fecal nitrogen excretion. The fiber diets elicited an accumulation of Ca and Mg in the cecum, the Ca and Mg absorption was closely correlated with the fermentability of the fibers. Except oat fiber, the studied fibers carried bile acids into the large intestine but also enhanced their cecal reabsorption. It appears that, besides analytical data on fiber solubility, the fermentability of complex fiber source is effectively appreciated by their effects on the parameters of cecal digestion.

Effectiveness of resistant starch, compared to guar gum, in depressing plasma cholesterol and enhancing fecal steroid excretion

Amylase-resistant starch (RS) represents a substrate that can be administered in substantial amounts in the diet, in contrast to gel-forming polysaccharides, such as guar gum (GG). The aim of this work was thus to compare the effects of GG and RS on cholesterol metabolism in rats adapted to 0.4% cholesterol diets, using dietary GG or RS levels (8 or 20%, respectively) that led to a similar development of fermentations, as assessed by the degree of enlargement of the cecum. The RS diet elicited a marked rise in the cecal pool of short-chain fatty acids, especially acetic and butyric acid, whereas the GG diet favored high-propionic acid fermentations. Both polysaccharides markedly altered the cholesterol excretion, from 50% of ingested cholesterol in controls, up to about 70% in rats adapted to the RS or GG diets. With these diets, the fecal excretion of bile acids was enhanced (67 and 144% with the RS and GG diets, respectively). RS and GG diets were effective in lowering plasma cholesterol (about −40%) and triglycerides (−36%). There was practically no effect of the diets on cholesterol in d>1.040 lipoproteins (high density lipoproteins), whereas RS (and to a larger extent, GG) were very effective to depress cholesterol in d<1.040 lipoproteins (especially in triglyceride-rich lipoproteins). Fermentable polysaccharides counteracted the accumulation of cholesterol in the liver, especially cholesterol esters. In parallel, liver acyl CoA:cholesterol acyltransferase was depressed in rats fed the RS or GG diets, whereas only the GG diet counteracted the downregulation of 3-hydroxy-3-methylglutaryl-CoA by cholesterol. These data suggest that RS may be practically as effective as a gel-forming gum, such as GG, on steroid excretion and on cholesterol metabolism.

Influence of inulin on urea and ammonia nitrogen fluxes in the rat cecum: consequences on nitrogen excretion

Abstract The effect of the addition of 15% chicory inulin on the nitrogen balance and the processes of N exchanges between blood plasma and the large intestine have been investigated in rats adapted to diets having a normal (15%) or high (45%) casein level. Inulin diets elicited a marked enlargement of the cecum, together with an acidification of the cecal content and an increase of the cecal volatile fatty acids pool. The net N balance was not affected by the various dietary conditions, but N digestibility was apparently depressed in rats fed inulin. This merely reflected a shift of N excretion from the renal to the intestinal site; this last effect was more marked in rats adapted to the normal protein level. Inulin depressed cecal ammonia only in rats adapted to the normal protein level. The balance between urea N transfer (blood plasma → cecum) and ammonia N transfer (cecum → blood plasma) was positive only in rats fed inulin, reflecting a net urea N retention in the cecum. In rats fed the inulin-45% casein diet, urea N transfer was very high, but very large amounts of N were reabsorbed as ammonia, suggesting a relatively poor incorporation of this N source in bacteria. The total concentrations of free amino acids in the cecum were significantly enhanced by inulin, but the cecal reabsorption of amino acids was quantitatively marginal. It appears that inulin is very effective in increasing fecal N excretion, concomitantly to depressed renal N excretion, without altering protein bioavailability. This shift seems more evident when the dietary protein level is moderate.

Cholesterol-lowering effects of guar gum: Changes in bile acid pools and intestinal reabsorption

Soluble fibers such as guar gum (GG) may exert cholesterol-lowering effects. It is generally accepted that bile acid (BA) reabsorption in portal blood is reduced, thus limiting the capacity of BA to down-regulate liver cholesterol 7α-hydroxylase, the rate-limiting enzyme of BA synthesis. In the present work, rats were adapted to fiber-free (FF) or 5% GG diets (supplemented or not with 0.25% cholesterol), to investigate various aspects of enterohepatic BA cycling. GG in the diet at a level of 5% elicited a significant lowering of plasma cholesterol during the absorptive period, in cholesterol-free (−13%) or 0.25% cholesterol (−20%) diet conditions. In rats adapted to the GG diets, the small intestinal and cecal BA pools and the ileal vein-artery difference for BA were markedly enhanced; reabsorption in the cecal vein was also enhanced in these rats. [14C]Taurocholate absorption, determined in perfused ileal segments, was not significantly different in rats adapted to the FF or GG diet, suggesting that a greater flux of BA in the ileum might support a greater ileal BA reabsorption in rats adapted to the GG diet. In contrast, capacities for [14C]cholate absorption from the cecum at pH 6.5 were higher in rats adapted to the GG diet than to the FF diet. Acidification of the bulk medium in isolated cecum (from pH 7.1 down to pH 6.5 or 5.8) or addition of 100 mM volatile fatty acids was also found to stimulate cecal [14C]cholate absorption. These factors could contribute to accelerated cecal BA absorption in rats fed the GG diet. The effects of GG on steroid fecal excretion thus appear to accompany a greater intestinal BA absorption and portal flux to the liver. These results suggest that some mechanisms invoked to explain cholesterol-lowering effect of fibers should be reconsidered.

Comparison of the effects of condensed tannin and pectin on cecal fermentations and lipid metabolism in the rat

Abstract The aim of the study was to compare the effect of a soluble fiber (citrus pectin) and prefermented condensed tannin (Quebracho) on the cecal fermentations, bile acids excretion and lipid metabolism in rats fed semi-purified diets. Analysis of the fermented tannin established than it is rich in monomers and trimers with little highly condensed polymers. Food intake and weight gain were not affected by addition of 5% citrus pectin or 1% Quebracho tannin in the diet. In the cecum, there was evidence of a depolymerization of polymers and of degradation of monomers, together with an accumulation of polar phenolic compounds. There was a significant enlargement of the cecum and of the volatile fatty acids (VFA) pool in rat fed the pectin diet; the Quebracho diet depressed the cecal VFA concentrations, compared to controls. The 1% Quebracho diet enhanced bile acids excretion, but the 5% pectin diet did not. In contrast, plasma cholesterol was depressed more effectively by pectin than by Quebracho. Both diets increased the activity of liver HMG-CoA reductase, a rate limiting enzyme of cholesterogenesis. The data show that, at a relatively low level in the diet, condensed tannins and soluble fibers may affect lipid metabolism by distinct mechanisms, which could be complementary in natural product diets.

Methionine deficiency in rats fed soy protein induces hypercholesterolemia and potentiates lipoprotein susceptibility to peroxidation

A number of studies have provided evidence that plant proteins, especially soy protein, have a cholesterol-lowering effect as compared with casein. However, dietary supply of sulfur amino acids may be deficient when soy protein is present in the diet at a suboptimal level, which could affect lipid metabolism. Accordingly, in rats fed 13% protein diets, soy protein feeding resulted in a cholesterol-increasing effect (+18%), which could be counteracted by methionine supplementation (0.4%). In contrast, soy protein was effective in decreasing plasma triglyceride, as compared with levels in rats fed casein; this triglyceride-lowering effect was entirely abolished by methionine supplementation. The hypercholesterolemic effect of soy protein was characterized by a higher cholesterol content in low-density lipoprotein (LDL) and high-density lipoprotein 1 (HDL1) fractions, together with a marked induction of hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase activity and to a lesser extent cholesterol 7 alpha-hydroxylase. There was practically no induction of these enzymes, as compared with levels in rats fed casein diets, when the soy protein diet was supplemented with methionine. Very-low-density lipoprotein (VLDL) plus LDL susceptibility to peroxidation was higher in rats fed soy protein than in casein-fed rats, which could reflect in part the lack of sulfur amino acid availability, since methionine supplementation led to a partial recovery of lipoprotein resistance to peroxidation. These findings suggest that amino acid imbalance could be atherogenic by increasing circulating cholesterol and leading to a higher lipoprotein susceptibility to peroxidation.

Relationship between Fermentations and Calcium in the Cecum of Rats Fed Digestible or Resistant Starch

The present studies were undertaken to investigate the effects of the dietary calcium (Ca) level on the Ca balance and on large intestine physiology in rats fed diets providing carbohydrate as digestible wheat starch (DS) or resistant starch (RS). Resistant starch was a high-amylose maize starch. The Ca level ranged from a marginally sufficient level (3 g/kg) to a level in excess of nutritional requirements (6 g/kg). The cecal fermentations were more developed in rats fed RS diets and they were affected by the dietary Ca level, compared to rats fed the DS diets. In rats fed a low Ca RS diet, the cecal fermentations were more acidic than with a high Ca level and there was an accumulation of lactic acid together with depressed concentrations of short-chain fatty acids. There was a considerable accumulation of Ca and phosphate (Pi) in the cecum of rats fed the 6 g/kg level; on the other hand, the concentrations of minerals (Ca, Pi) were very low in the cecum of rats fed the low Ca RS diet. The proportion of soluble Ca in the cecum was markedly enhanced by active fermentations, especially in rats fed the high Ca RS diet (16.3 mmol/l). In rats fed the RS diet, there was a substantial absorption of Ca from the cecum, which was relatively proportional to the dietary Ca level. The above data suggest that in these rats, Ca absorption in the large intestine made a substantial contribution to the digestive balance, probably at the expense of absorption in the upper part of the intestine.(ABSTRACT TRUNCATED AT 250 WORDS)