In Vitro Interactions of Dietary Fibre Enriched Food Ingredients with Primary and Secondary Bile Acids

Dietary fibres are reported to interact with bile acids, preventing their reabsorption and promoting their excretion into the colon. We used a method based on in vitro digestion, dialysis, and kinetic analysis to investigate how dietary fibre enriched food ingredients affect the release of primary and secondary bile acids as related to viscosity and adsorption. As the main bile acids abundant in humans interactions with glyco- and tauroconjugated cholic acid, chenodesoxycholic acid and desoxycholic acid were analysed. Viscous interactions were detected for apple, barley, citrus, lupin, pea, and potato derived ingredients, which slowed the bile acid release rate by up to 80%. Adsorptive interactions of up to 4.7 μmol/100 mg DM were significant in barley, oat, lupin, and maize preparations. As adsorption directly correlated to the hydrophobicity of the bile acids the hypothesis of a hydrophobic linkage between bile acids and dietary fibre is supported. Delayed diffusion in viscous fibre matrices was further associated with the micellar properties of the bile acids. As our results indicate changes in the bile acid pool size and composition due to interactions with dietary fibre rich ingredients, the presented method and results could add to recent fields of bile acid research.

Mechanisms of Interactions between Bile Acids and Plant Compounds-A Review

Plant compounds are described to interact with bile acids during small intestinal digestion. This review will summarise mechanisms of interaction between bile acids and plant compounds, challenges in in vivo and in vitro analyses, and possible consequences on health. The main mechanisms of interaction assume that increased viscosity during digestion results in reduced micellar mobility of bile acids, or that bile acids and plant compounds are associated or complexed at the molecular level. Increasing viscosity during digestion due to specific dietary fibres is considered a central reason for bile acid retention. Furthermore, hydrophobic interactions are proposed to contribute to bile acid retention in the small intestine. Although frequently hypothesised, no mechanism of permanent binding of bile acids by dietary fibres or indigestible protein fractions has yet been demonstrated. Otherwise, various polyphenolic structures were recently associated with reduced micellar solubility and modification of steroid and bile acid excretion but underlying molecular mechanisms of interaction are not yet fully understood. Therefore, future research activities need to consider the complex composition and cell-wall structures as influenced by processing when investigating bile acid interactions. Furthermore, influences of bile acid interactions on gut microbiota need to be addressed to clarify their role in bile acid metabolism.

Differentiation of Adsorptive and Viscous Effects of Dietary Fibres on Bile Acid Release by Means of In Vitro Digestion and Dialysis

To explain the cholesterol-reducing effects of dietary fibres, one of the major mechanisms proposed is the reduced reabsorption of bile acids in the ileum. The interaction of dietary fibres with bile acids is associated with their viscous or adsorptive effects. Since these fibre characteristics are difficult to investigate in vivo, suitable in vitro methodologies can contribute to understanding the mechanistic principles. We compared the commonly used centrifugal approach with a modified dialysis method using dietary fibre-rich materials from different sources (i.e., barley, citrus, lupin, and potato). Digestion was simulated in vitro with oral, gastric, and small intestinal digestion environments. The chyme was dialysed and released bile acids were analysed by high-performance liquid chromatography. The centrifugation method showed adsorptive effects only for cholestyramine (reference material) and a high-fibre barley product (1.4 µmol taurocholic acid/100 mg dry matter). Alternatively, the dialysis approach showed higher values of bile acid adsorption (2.3 µmol taurocholic acid/100 mg dry matter) for the high-fibre barley product. This indicated an underestimated adsorption when using the centrifugation method. The results also confirmed that the dialysis method can be used to understand the influence of viscosity on bile acid release. This may be due to entrapment of bile acids in the viscous chyme matrix. Further studies on fibre structure and mechanisms responsible for viscous effects are required to understand the formation of entangled networks responsible for the entrapment of the bile acids.

Liver-specific transgenic expression of cholesteryl ester hydrolase reduces atherosclerosis in Ldlr-/- mice

The liver plays a central role in the final elimination of cholesterol from the body either as bile acids or as free cholesterol (FC), and lipoprotein-derived cholesterol is the major source of total biliary cholesterol. HDL is the major lipoprotein responsible for removal and transport of cholesterol, mainly as cholesteryl esters (CEs), from the peripheral tissues to the liver. While HDL-FC is rapidly secreted into bile, the fate of HDL-CE remains unclear. We have earlier demonstrated the role of human CE hydrolase (CEH, CES1) in hepatic hydrolysis of HDL-CE and increasing bile acid synthesis, a process dependent on scavenger receptor BI expression. In the present study, we examined the hypothesis that by enhancing the elimination of HDL-CE into bile/feces, liver-specific transgenic expression of CEH will be anti-atherogenic. Increased CEH expression in the liver significantly increased the flux of HDL-CE to bile acids. In the LDLR(-/-) background, this enhanced elimination of cholesterol led to attenuation of diet-induced atherosclerosis with a consistent increase in fecal sterol secretion primarily as bile acids. Taken together with the observed reduction in atherosclerosis by increasing macrophage CEH-mediated cholesterol efflux, these studies establish CEH as an important regulator in enhancing cholesterol elimination and also as an anti-atherogenic target.

Delineation of biochemical, molecular, and physiological changes accompanying bile acid pool size restoration in Cyp7a1(-/-) mice fed low levels of cholic acid

Cholesterol 7α-hydroxylase (CYP7A1) is the initiating and rate-limiting enzyme in the neutral pathway that converts cholesterol to primary bile acids (BA). CYP7A1-deficient (Cyp7a1(-/-)) mice have a depleted BA pool, diminished intestinal cholesterol absorption, accelerated fecal sterol loss, and increased intestinal cholesterol synthesis. To determine the molecular and physiological effects of restoring the BA pool in this model, adult female Cyp7a1(-/-) mice and matching Cyp7a1(+/+) controls were fed diets containing cholic acid (CA) at modest levels [0.015, 0.030, and 0.060% (wt/wt)] for 15-18 days. A level of just 0.03% provided a CA intake of ~12 μmol (4.8 mg) per day per 100 g body wt and was sufficient in the Cyp7a1(-/-) mice to normalize BA pool size, fecal BA excretion, fractional cholesterol absorption, and fecal sterol excretion but caused a significant rise in the cholesterol concentration in the small intestine and liver, as well as a marked inhibition of cholesterol synthesis in these organs. In parallel with these metabolic changes, there were marked shifts in intestinal and hepatic expression levels for many target genes of the BA sensor farnesoid X receptor, as well as genes involved in cholesterol transport, especially ATP-binding cassette (ABC) transporter A1 (ABCA1) and ABCG8. In Cyp7a1(+/+) mice, this level of CA supplementation did not significantly disrupt BA or cholesterol metabolism, except for an increase in fecal BA excretion and marginal changes in mRNA expression for some BA synthetic enzymes. These findings underscore the importance of using moderate dietary BA levels in studies with animal models.

Retention of Primary Bile Acids by Lupin Cell Wall Polysaccharides Under In Vitro Digestion Conditions

Interference of dietary fibres with the enterohepatic circulation of bile acids is proposed as a mechanism for lowering cholesterol. We investigated how lupin hull and cotyledon dietary fibres interact with primary bile acids using an in vitro model under simulated upper gastrointestinal conditions. Cell wall polysaccharides were isolated and extracted to separate pectin-like, hemicellulosic, and lignocellulosic structures. Lupin hull consisted mainly of structural components rich in cellulose. The viscosity of the in vitro digesta of lupin hull was low, showing predominantly liquid-like viscoelastic properties. On the other hand, lupin cotyledon fibre retarded bile acid release due to increased viscosity of the in vitro digesta, which was linked with high contents of pectic polymers forming an entangled network. Molecular interactions with bile acids were not measured for the hull but for the cotyledon, as follows: A total of 1.29 µmol/100 mg DM of chenodesoxycholic acids were adsorbed. Molecular interactions of cholic and chenodesoxycholic acids were evident for lignin reference material but did not account for the adsorption of the lupin cotyledon. Furthermore, none of the isolated and fractionated cell wall materials showed a significant adsorptive capacity, thus disproving a major role of lupin cell wall polysaccharides in bile acid adsorption.

Medicinal Formula Huazhi-Rougan Attenuates Non-Alcoholic Steatohepatitis Through Enhancing Fecal Bile Acid Excretion in Mice

Huazhi-Rougan (HZRG) formula is a Traditional Chinese medicine prescription, and has been widely used to treat non-alcoholic fatty liver disease (NAFLD) and its progressive form non-alcoholic steatohepatitis (NASH). However, the anti-NASH effects and the underlying mechanisms of HZRG have not yet been characterized. Here we showed that 4-week HZRG treatment alleviated methionine-choline-deficiency (MCD) diet-induced NASH in C57BL/6J mice, as evidenced by the improvement of hepatic steatosis and inflammation, as well as the decrease of serum levels of alanine and aspartate transaminases. Fecal 16S rDNA sequencing indicated that HZRG reduced the enrichment of pathogenic bacteria and increased the abundance of bacteria gena that are involved in bile acid (BA) conversation. The alteration of fecal and serum BA profile suggested that HZRG enhanced fecal BA excretion, and reduced the reabsorption of toxic secondary BA species (LCA, DCA, HCA). We further analyzed the BA receptors and transporters, and found that HZRG inhibited the expression of ileal bile acid transporter, and organic solute transporter subunit β, and increased the expression of intestinal tight junction proteins (ZO-1, Occludin, Claudin-2). The modulation of gut dysbiosis and BA profile, as well as the improvement of the intestinal environment, may contribute to the decrease of the p-65 subunit of NF-κB phosphorylation, liver F4/80 positive macrophages, inflammatory cytokine IL-1β and TNF-α expression. In conclusion, HZRG treatment enhances fecal BA excretion via inhibiting BA transporters, modulates BA profiles, gut dysbiosis as well as the intestinal environment, thus contributing to the beneficial effect of HZRG on NASH mice.

Vitamin D Receptor Deletion Changes Bile Acid Composition in Mice Orally Administered Chenodeoxycholic Acid

The vitamin D receptor (VDR) is a nuclear receptor for the active form of vitamin D₃ and also for the secondary bile acid lithocholic acid (LCA). The in vivo role of VDR in bile acid metabolism remains largely uncharacterized. We previously reported that pharmacological VDR activation enhances urinary bile acid excretion, particularly in mice fed chow supplemented with chenodeoxycholic acid (CDCA), which is metabolized to muricholic acid in mouse liver and is also converted to LCA by intestinal bacteria. In this study, we examined the effect of VDR deletion on bile acid composition utilizing VDR-knockout (VDR-KO) mice. VDR deletion did not change total bile acid levels in liver or feces of mice when fed standard chow supplemented with calcium, needed to prevent hypocalcemia in VDR-KO mice. Total bile acid levels in plasma and urine tended to be higher and lower, respectively, in VDR-KO mice. After feeding CDCA-supplemented chow, VDR-KO mice showed decreased hepatic, fecal and urinary total bile acid and CDCA levels compared to wild-type mice. Plasma total bile acids and LCA were relatively high in these mice. These results indicate that VDR deletion influences CDCA metabolism. VDR may play a role in the excretion of excess bile acids.