Contrasting effects of water-soluble and water-insoluble dietary fibers on bile acid conjugation and taurine metabolism in the rat

The effect of the type of dietary fiber on the bile acid and taurine metabolism was examined in rats. Diets containing 10% of various water-soluble fibers (citrus pectin, konjak mannan, guar gum) as compared to a fiber-free diet increased biliary excretion of total bile acids. In contrast, water-insoluble dietary fibers (cellulose, corn bran, chitin; 10% in the diets) as well as cholestyramine (5% in the diet) considerably, decreased bile acid excretion. Water-soluble dietary fibermediated increases in bile acid excretion were totally attributable to increases in glycine-conjugates. Thus, these fibers greatly increased by the bile acid glycine-to-taurine ratio (G/T). Excretio of glycine conjugates decreased more than that of taurine conjugates in rats fed various water-insoluble dietary fibers. As a results, G/T in rats fed water-insoluble fibers was significantly lowered as compared to G/T in animals fed a fiber-free diet. Cholestyramine did not affect the G/T ratio of bile acids. Fecal bile acid excretion and the activities of hepatic cholesterol 7α-hydroxylase (EC 1.14.13.17) in rats fed various water-soluble dietary fibers approximately doubled as compared to the respective values for rats fed a fiber-free diet. Whereas cholestyramine greatly increased these parameters, water-insoluble fibers did not significantly affect them. Various water-soluble fibers decreased hepatic concentration and urinary excretion of taurine as well as the activity of hepatic cysteine dioxygenase (EC 1.13.11.20). In contrast, water-insoluble fibers considerably increased hepatic taurine concentrations and enzyme activities. The parameters for taurine metabolism were unaffected by cholestyramine. It was suggested that the types of dietary fiber affected hepatic taurine synthesis and thus modified bile acid glycine/taurine ratios.     

Cholesterol binding capacity of fiber from tropical fruits and vegetables

The cholesterol binding capacity of 28 fiber samples from a variety of the more common tropical fruits and vegetables was determined. The binding capacity of cholestyramine, cellulose, lignin, guar gum and citrus pectin were also determined. Capacities were evaluated by an in vitro method that simulates the effect of the human digestive system on fiber using a series of enzymatic treatments before the binding was determined. Binding values varied from 3% for a soluble fraction of cassava to 84% for cholestyramine. Values for most fruit and vegetable fiber samples were less than or ca. equal to cellulose or lignin (20% and 16%, respectively). Apart from cholestyramine, sweet potato was the most effective binder (30%). Citrus pectin, at 8%, was a relatively poor binder. The capacity of guar gum (17%) was slightly less than cellulose. These data do not support the conclusion from in vivo studies that the hypocholesteremic effects observed for citrus pectin and guar gum are the result of the direct binding of cholesterol or bile acids in the large intestine. Southern Region, U.S. Department of Agriculture, Agricultural Research Service. Mention of a Trademark or proprietary product is for identification only and does not imply a guarantee or warranty of the product by the U.S. Department of Agriculture over other products that may also be suitable.     

Dietary fibers: V. Binding of bile salts,phospholipids and cholesterol from mixed micelles by bile acid sequestrants and dietary fibers

Mixed micelles were prepared containing combinations of either taurocholate or taurochenodeoxycholate, monoolein, oleic acid, dioleylphosphatidylcholine (lecithin) and cholesterol. These were incubated with commercial bile-acid-sequestering resins, cholestyramine and DEAE-Sephadex, or various dietary fibers and fiber components including wheat bran, cellulose, alfalfa, lignin and 2 viscosity grades of guar gum. Binding was determined as the difference between the radioactivity of each micellar component added and that recovered in the centrifugal supernatant after incubation. In general, the extent of bile salt sequestration was characteristic and reproducible for each bile salt, and was largely unaffected by the presence of one or more additional components of the micellar mixture, including the other bile salt. Cholestyramine bound 81–92% of the bile salts and 86–99% of the phospholipid and cholesterol present in micelles. DEAE-Sephadex sequestered only 49% of the taurocholate and 84% of the taurochenodeoxycholate, but completely removed all of the phospholipid and cholesterol from micelles containing either bile salt. Among the dietary fibers, guar gum of either viscosity bound between 20–38% of each micellar component, whereas lignin, alfalfa, wheat bran and cellulose were progressively less effective in sequestratin of individual components of mixed micelles. The extent of sequestration of micellar components by these resins and fibers is reasonably correlated with the effects of these same materials on lymphatic absorption of lipids and to their suggested hypocholesteremic properties.     

Effects of dietary fiber in Vervet monkeys fed “Western” diets

Male Vervet monkeys (7/treatment) were fed a “Western” diet containing 46.2% calories as fat, 39.8% as carbohydrate and 14.0% as protein. The diet was augmented with 10% cellulose or 10% pectin. A third (control) group of seven monkeys was fed a commercial ration augmented with fruit and bread. After 34 weeks, serum cholesterol levels were elevated significantly in the two test groups compared with the controls but there was no difference between the two fiber-fed groups. Serum triglycerides were unaffected. Liver cholesterol levels were the same in all three groups but liver triglyceride levels were lower in the monkeys fed cellulose. Biliary lipids were similar in all three groups as were the calculated lithogenic indices. The average aortic sudanophilia (percent of total area) in the three groups was cellulose, 10.6±2.5; pectin, 8.1±2.5; and control, 1.1±0.4. One animal in each of the groups fed “Western” diet exhibited an atherosclerotic plaque. The results indicate that there is no difference between pectin and cellulose with regard to their effects on either lipidemia or aortic sudanophilia in Vervet monkeys fed a Western-type diet.     

Dietary Guar Gum Reduces Lymph Flow and Diminishes Lipid Transport in Thoracic Duct-Cannulated Rats

Guar gum has a well-recognized hypolipidemic effect. This effect is thought to be due to the physicochemical properties of guar gum, which may cause changes in adsorption of lipids or the viscosity of the intestinal contents. Guar gum is a non-specific absorption inhibitor of any type of lipid-soluble compound. Permanent lymph duct cannulation was performed on rats to investigate the effects of dietary guar gum on lymph flow and lipid transport. Rats fed a 5% guar gum diet were compared with those fed a 5% cellulose diet, and lymph was collected after feeding. The water-holding capacity (WHC), settling volume in water (SV), and viscosity of guar gum were compared with those of cellulose. Rats fed with the guar gum diet had significantly lower lymph flow and lymphatic lipid transport than did rats fed with the cellulose diet. The WHC, SV, and viscosity of guar gum were significantly higher than those of cellulose. We propose that dietary guar gum reduces lymph flow and thereby diminishes lipid transport by means of its physicochemical properties related to water behavior in the intestine.     

Psyllium, not pectin or guar gum, alters lipoprotein and biliary bile acid composition and fecal sterol excretion in the hamster

Different soluble dietary fibers known to alter cholesterol metabolism were fed to golden Syrian hamsters, and their specific impact on lipoproteins, biliary bile acid profile, and fecal sterol excretion was evaluated. Semipurified diets containing 20% fat; 0.12% cholesterol; and 8% of psyllium (PSY); high (hePE) and low (lePE) esterified pectin; or high (hvGG) and low (lvGG) viscous guar gum were fed for 5 wk. Compared to control, PSY caused a significant reduction in plasma cholesterol (2.9±0.5 vs. 5.5±0.5 mmol/L), whereas hePE, lePE, hvGG, or lvGG had no apparent effect on plasma lipids. Hepatic total and esterified cholesterol were substantially decreased with PSY, pectin and guar gum, whereby PSY produced the most pronounced effect. Distinctive changes existed in the bile acid profile related to the different fibers. In contrast to pectin and guar gum, PSY caused a significant increase in the cholate:chenodeoxycholate and the glycine:taurine conjugation ratio. Pectin and guar gum did not alter daily fecal neutral sterol excretion while PSY caused a 90% increase due to a higher fecal output. Daily fecal bile acid excretion and total fecal bile acid concentration were significantly increased by PSY, whereas hePE, lePE, hvGG, and lvGG revealed no or only minor effects. Taken together, the disparate hypocholesterolemic effects of PSY, pectin, and guar gum on cholesterol and bile acid metabolism in the hamster are possibly related to different physicochemical properties, e.g., viscosity and susceptibility to fermentation, affecting the fiber-mediated action in the intestine.     

Effect of cholestyramine on bile acid metabolism in conventional rats

Effects of cholestyramine on biliary secretion of cholesterol, phospholipids and bile acids and fecal excretion of sterols and bile acids were examined in Wistar male rats. Six rats were fed a basal diet, and the other six were fed a basal diet supplemented with 5% cholestyramine for eight days. Bile flow and biliary secretion of bile acids and phospholipids (per hour per rat) decreased with cholestyramine treatment, while biliary cholesterol secretion (per hour per rat) remained unchanged. In the biliary bile acid composition, a marked increase of chenodeoxycholic acid with a concomitant decrease of β-muricholic acid was observed in cholestyramine-treated rats. Fecal excretion of total sterols and bile acids increased about three-and four-fold, respectively, after cholestyramine treatment. The increase of fecal bile acids derived from cholic acid was more predominant than that derived from chenodeoxylcholic acid, resulting in an increase of the cholic acid group/chenodeoxycholic acid group ratio.     

The cholesterol-lowering effect of guar gum is not the result of a simple diversion of bile acids toward fecal excretion

The effects of partially hydrolyzed, nonviscous, guar gum (PHGG) on cholesterol metabolism and digestive balance have been compared with those of native guar gum (GUAR) in rats adapted to 0.4% cholesterol diets. Both types of guar gum elicited acidic fermentations in the large intestine, but only GUAR effectively lowered plasma cholesterol (P<0.001), chiefly in the triglyceride-rich lipoprotein fraction. The biliary bile acid excretion was significantly enhanced in rats fed GUAR (P<0.05), as well as the intestinal and cecal bile acid pool (P<0.001). In rats fed GUAR and to a lesser extent in those fed PHGG, the fecal excretion of bile acids and neutral sterol was higher than in controls (P<0.01). The digestive balance (cholesterol intake-steroid excretion) was positive in control rats (+47 μmol/d), whereas it was negative in rats fed GUAR (−20 μmol/d), which could involve a higher rate of endogenous cholesterol synthesis. In rats fed PHGG, the steroid balance remained slightly positive. Liver 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase activity was very low (22 pmol/min/mg protein), owing to cholesterol supplementation, in control rats or in rats fed PHGG, whereas it was markedly higher (+463%) in rats fed GUAR. In conclusion, even if PHGG does alter some parameters of the enterohepatic cycle of cholesterol and bile acids, its effects are not sufficient to elicit a significant cholesterol-lowering effect. The intestinal (ileal or cecal) reabsorption of bile acids was not reduced, but rather increased, by GUAR; nevertheless the intestinal capacities of reabsorption were overwhelmed by the enlargement of the digestive pool of bile acids. In the present model, induction of HMG-CoA reductase probably takes place in the presence of elevated portal bile acid concentrations.     

The cholesterol-lowering effect of guar gum in rats is not accompanied by an interruption of bile acid cycling

A viscous hydrocolloid (guar gum, GG; 2.5% of the diet) or a steroid sequestrant (cholestyramine; 0.5% of the diet) was included in semipurified diets containing 0.2% cholesterol to compare the cholesterol-lowering effects of each agent in rats. In the present model, GG significantly lowered plasma cholesterol (−25%), especially in the density <1.040 kg/L fraction, whereas cholestyramine was less potent. Bile acid fecal excretion significantly increased only in rats fed cholestyramine, similar to the cecal bile acid pool; the biliary bile acid secretion was accelerated by GG, but not their fecal excretion, whereas GG effectively enhanced neutral sterol excretion. As a result, the total steroid balance (+13 μmol/d in the control) was shifted toward negative values in rats fed the GG or cholestyramine diets (−27 or −50 μmol/d, respectively). Both agents induced liver 3-hydroxy-3-methylglutaryl-CoA reductase, but cholestyramine was more potent than GG in this respect. The present data suggest that, at a relative low dose in the diet, GG may be more effective than cholestyramine in lowering plasma cholesterol by impairing cholesterol absorption and by accelerating the small intestine/liver cycling of bile acids, which is interestingly, accompanied by reduction of bile acid concentration in the large intestine.     

Influence of dietary fiber on lipids and aortic composition of vervet monkeys

A semipurified, cholesterol-free diet containing 40% carbohydrate can produce aortic sudanophilia or aortic atherosclerosis in vervet monkeys (Ceroopithecus aethiops pygerethrus) depending on the particular carbohydrate fed. Four groups of vervet monkeys (three males and three females per group) were fed semipurified diets containing lactose. Two of the groups were also fed 15% cellulose (C) or 15% cellulose plus 0.1% cholesterol (CC); the two other groups were fed 15% pectin (P) or 15% pectin plus 0.1% cholesterol (PC). The average serum total cholesterol and low density lipoprotein cholesterol levels over the entire feeding period (mg/dl±SEM) were, for C, 156±14 and 95±5; for P, 173±15 and 112±8; for CC, 187±27 and 122±21; and for PC, 155±11 and 108±7. Cholesterol levels at autopsy (mg/dl±SEM) were, for C, 103±6; for P, 108±16; for CC, 92±9; and for PC, 106±7. Aortic sudanophilia (percentage of area) was, for C, 5.9±2.7; for P, 13.5±9.4; for CC, 5.3±2.1; and for PC, 21.6±10.3. Dietary pectin led to more severe sudanophilia (increased by 129% in the absence of cholesterol and by 308% in its presence) than did cellulose. Analysis of aortic glycosaminoglycans (GAG) revealed that dermatan sulfate levels fell in both cholesterol-fed groups, and chondroitin sulfate fell in aortas of group CC. Heparan sulfate levels were unaffected by cholesterol feeding. Hexuronic acid, galactosamine and hexosamine levels were elevated in the pectin-fed monkeys, but levels were unaffected by dietary cholesterol. Pectin may contribute galactosamine and glucuronic acid towards aortic GAG.     

Fiber,hypercholesteremia, and atherosclerosis

Epidemiological data suggest that populations subsisting on high fiber diets are free of a number of the diseases of Western civilization, among them coronary heart disease. Studies in animals and man show that each type of fiber exerts it own specific influence. Thus, in man bran has no effect on serum lipids, but pectin lowers cholesterol levels. In animals fed atherogenic diets, alfalfa and pectin exert some measure of protection, but cellulose does not. To fully understand the mode of action of dietary fiber, it is necessary to ascertain the mechanism(s) of action of each chemical component of that fiber.     

Partially Hydrolyzed Guar Gums Reduce Dietary Fatty Acid and Sterol Absorption in Guinea Pigs Independent of Viscosity

This study investigated the effect of two partially hydrolyzed guar gums (PHGG) on fatty acid and sterol excretion. PHGG were obtained by chemical hydrolysis of guar gum (GG) with H₂O:EtOH (1:1) at 100 °C for 1 h (PHGG1) or 2 h (PHGG2). The viscosity of the PHGG in a 1 % (w/v) aqueous solution corresponded to that of a pseudoplastic fluid and was higher for PHGG1 than PHGG2. Guinea pigs (n = 8 per group) were fed high fat diets (17/100 g) that contained 12/100 g of cellulose, PHGG1, or PHGG2 for 4 weeks. Despite the differences in viscosity, the two PHGG exerted similar physiological effects. Compared to the control cellulose group, the body weight gain was lower in animals fed PHGG, although no effect on food consumption was observed. PHGG increased the excretion of fatty acids and neutral sterols, but not bile acids. Consumption of PHGG did not alter the fecal fatty acid profile, while intestinal bioconversion of sterols tended to increase in response to PHGG2. A reduction in the viscosity within the range tested did not correlate with losses in the hypocholesterolemic capacity of PHGG as both were effective in reducing plasma cholesterol. Thus, we conclude that the chemical hydrolysis of guar gum renders the gum suitable for inclusion in food products without significantly altering its beneficial health effects.     

The combined effects of dietary proteins and fish oil on cholesterol metabolism in rats of different ages

Male Sprague-Dawley rats at the ages of four weeks and nine months were fed purified diets containing 20% proteins either as casein (CAS), milk whey protein (WHY), or soybean protein (SOY) with 5% sardine oil for four weeks. The hypocholesterolemic effect of SOY was not statistically evident as compared to milk proteins at both ages, although serum cholesterol tended to be low in the SOY groups. A significant agedependent increase in serum cholesterol was observed in all dietary groups. Liver cholesterol concentrations were comparable in young rats, whereas in adults they were significantly lower in the SOY than in the CAS or WHY groups. At both ages, the activity of liver 3-hydroxy-3-methylglutaryl coenzyme A reductase tended to be higher in the SOY than in the other groups. Fecal steroid excretion was significantly higher in rats fed SOY than those fed either CAS or WHY, especially in adult rats. Significant age- and dietary protein-effects were observed in fatty acid profiles of liver microsomal phospholipids. Thus, the effects of dietary proteins on various lipid parameters were essentially maintained even when fish oil served as the source of dietary fat.     

Effects of dietary n−3 fatty acid-enriched chicken eggs on plasma and tissue cholesterol and fatty acid composition of rats

The purpose of this study was to examine the effects of feeding n−3 polyunsaturated fatty acid (PUFA)-enriched chicken eggs on plasma and liver cholesterol levels and fatty acid composition in rats. Eggs were collected from laying hens fed diets containing 10% flax seed (Hn−3), 12% sunflower seed (Hn−6), or wheat and soybean meal control (CON). Yolk powders were prepared and fed at the 15% level to weanling female Sprague-Dawley rats for 28 days. Consumption of n−3 PUFA-enriched yolks significantly reduced both plasma and liver total cholesterol. Liver total lipids and phospholipids of rats fed Hn−3 diet were enriched with linolenic, eicosapentaenoic, and docosahexaenoic acids with a concomitant reduction of arachidonic acid in liver phospholipids. The plasma cholesterol of rats fed yolk powders enriched with n−6 PUFA (mainly linoleic acid) was reduced to the same extent as in those fed the n−3 enriched, but the liver cholesterol was significantly increased, indicating differential effects of dietary n−3 and n−6 PUFA. The results demonstrated that the cholesterolemic and tissue lipid modulating properties of chicken eggs could be modified in a favorable way by altering the fatty acid composition of yolk lipids through manipulation of laying hen diets.     

Rice bran oil and hypocholesterolemia in rats

Rats fed rice bran oil at 10% level for a period of eight weeks showed significantly lower levels of total cholesterol, low density lipoprotein and very low density lipoprotein cholesterol, both on cholesterol-containing and cholesterol-free diets. High density lipoprotein cholesterol was increased but triglyceride showed a decrease that was not statistically significant. Liver cholesterol and liver triglycerides were also reduced. Fecal excretion of neutral sterols and bile acids was increased after ingestion of rice bran oil.     

The effect of alfalfa-corn diets on cholesterol metabolism and gallstones in prairie dogs

Cholesterol gallstones were present in prairie dogs fed alfalfa plus corn with and without exogenous cholesterol (0.4%). The diets fed to the animals for eight weeks contained alfalfa plus corn in fixed proportions of 50∶50, 85∶15 and 15∶85 (w/w). At sacrifice, all animals were healthy but had not gained weight; no deaths occurred during the experiment. Cholesterol gallstones were present in all groups. In the absence of exogenous cholesterol, the highest stone incidence was found in the animals which received the lowest fiber (highest corn) diets (alfalfa plus corn, 50∶50, 67%; alfalfa plus corn, 15∶85, 83%). Cholesterol gallstone incidence was 100% when exogenous cholesterol was added to the alfalfa plus corn diets (50∶50 and 15∶85). No pigment gallstones were detected in any animal. Liver and plasma cholesterol concentrations were highest in the animals receiving alfalfa plus corn (15∶85) plus 0.4% cholesterol (4.29 mg/g, and 356 mg/dl, respectively). These values were lowest in animals receiving 85% alfalfa plus 15% corn without cholesterol (2.19 mg/g and 88 mg/dl, respectively). Lithogenic indices were below 1.00 in all groups. Biliary bile acids were mainly amidates of cholic acid and chenodeoxycholic acid, with the former predominating. Thus, gallstones can be formed in prairie dogs in the absence of exogenous cholesterol; gallstone incidence is reduced by dietary fiber.     

Secondary regulatory sites in rat liver cholesterol biosynthesis: Role of 5-pyrophosphomevalonate decarboxylase

The activity of 5-pyrophosphomevalonate decarboxylase in 43,000 g supernatant fractions from livers and kidneys of male adult rats has been determined. Enzyme activity in liver is significantly increased when rats are fed a diet containing 3% cholestyramine (268% of control rats) and decreased when fed a diet containing 2% cholesterol (25% of control rats). No circadian rhythm of enzyme activity is found in liver or kidneys. These results show that variations in hepatic cholesterogenesis affect the activity of 5-pyrophosphomevalonate decarboxylase in a similar way as other enzymes involved in the biosynthesis of cholesterol.     

Highly hydrogenated dietary soybean oil modifies the responses to polychlorinated biphenyls in rats

The effects of dietary highly hydrogenated soybean oil (HSO) upon the changes caused by dietary polychlorinated biphenyls (PCBs) were examined in rats. Six groups of rats were fed the following diets for 30 d: a 20% soybean oil-containing diet (control diet), a diet in which a half of soybean oil was substituted with HSO (HSO-A diet), a diet in which cellulose powder was replaced with HSO (HSO-B diet) and these diets supplemented with 100 ppm PCBs (control+PCBs, HSO-A+PCBs and HSO-B+PCBs diets). Hepatic concentration of PCBs and relative liver weight were markedly decreased in rats fed with the HSO-A+PCBs diet compared with those fed with the other diets containing PCBs. Liver lipids and liver cholesterol were considerably decreased with a reciprocal increase in fecal sterol excretion by rats fed the HSO-A+PCBs and the HSO-B+PCBs diets compared with those fed with the control+PCBs diet. The fatty acid composition in hepatic phospholipids showed an independent increase of the saturated fatty acid content induced by dietary HSO and PCBs. Dietary PCBs also caused decreases in the amounts of monounsaturated and n-3 polyunsaturated fatty acids. These results suggest that dietary HSO prevents accumulation of PCBs in the liver and promotes the excretion of lipids stimulated by PCBs, accompanied by a change in fatty acid metabolism.     

Changes in lipids in liver and serum of rats fed a histidine-excess diet or cholesterol-supplemented diets

The influence of dietary excess (5%) L-histidine on serum and liver lipids was examined in rats. Feeding a histidine-excess diet for 3, 6, 14 or 30 days caused growth retardation, hepatomegaly and decreased liver lipids throughout the period of the experiment. Hypercholesterolemia was observed after feeding a histidine-excess diet for 6 days; then serum cholesterol continuously increased for 30 days. Serum triglyceride on day 30 in rats fed the histidine-excess diet showed a significant decrease compared to rats fed the basal diet. Serum phospholipids of rats fed the histidine-excess diet for 7 or 14 days showed a significant increase compared to rats fed the basal diet. When rats were fed a basal, histidine-excess or cholesterol-supplemented diet (0.5% and 1.0% cholesterol) for 6 days, the distribution of serum high density (HDL), low density (LDL) and very low density lipoprotein cholesterol in rats fed the histidine-excess diet was similar to that of rats fed the basal diet, whereas LDL-cholesterol increased and HDL-cholesterol decreased in rats fed the cholesterol-supplemented diet.     

Effect of eicosatetraynoic acid on liver and plasma lipids

Groups of rats were fed a fat-free diet supplemented with 0.5% safflower oil (control) or the control diet containing 0.5% of 5,8,11,14-eicosatetraynoic acid (TYA). Blood was collected weekly and plasma lipids analyzed. After 4 weeks, the animals were killed and the liver lipids were analyzed in detail. The acetylenic fatty acid perturbed plasma neutral lipid and phospholipid class concentrations and reduced growth rates. Liver triglyceride concentrations were reduced dramatically in the TYA fed animals, suggesting interference with complex lipid synthesis. Plasma and liver triglycerides were shifted to higher molecular weight species suggesting that TYA affected fatty acid metabolism. The phospholipids showed an accumulation of 18∶2 and a fall in 20∶4 percentages indicating an inhibition in the conversion of linoleate to arachidonate. All major lipid classes exhibited an increase in 18∶1 levels. Analysis of the octadecenoate positional isomers indicated the proportion of oleate increased substantually in all lipid classes whereas vaccenate proportions had fallen dramatically. All of the data collectively suggest that TYA inhibits the elongation of unsaturated fatty acids. A group of rats bearing hepatoma 7288CTC were also fed the TYA diet. Host liver lipids were affected by TYA similar to normal TYA fed animals, but the effects on hepatoma lipids were marginal.