Dietary fat absorption relies on the presence of the primary bile salts cholate and chenodeoxycholate. Both primary bile salts are synthesised in the liver with cholate making up around 80% of the total. Conjugation of bile salts to glycine or taurine lowers the pKa and results in an ionised form that is secreted to the small intestine via the gall bladder. Here the primary bile salts aid the absorption of dietary fat by forming an emulsion and decreasing the fat droplet size, before being reabsorbed via a specific transporter system in the distal ileum. They are then recycled by the liver and thus remain in the enterohepatic circulation. Bile salts that are not reabsorbed in the terminal ileum pass to the colon where bacterial degradation and metabolism result in the formation of the secondary bile salts. Deoxycholate and lithocholate, the secondary bile salts formed from cholate and chenodeoxycholate respectively, can then be partly reabsorbed and re-enter the enterohepatic circulation.
The maintenance of the bile salt pool is regulated such that any loss in the faecal matter is compensated for by de novo synthesis in the liver. Because bile salt synthesis requires the use of cholesterol as a substrate, bile salt turnover can contribute to the regulation of cholesterol homeostasis in the body. This may have implications for whole body cholesterol homeostasis through modulation of the cholesterol concentrations in plasma. Bile salt synthesis varies from around 250 mg per day up to nearly 900 mg per day, with the variation resulting from both genetic and environmental factors. Diet may play a role in regulating the synthesis of new bile salts, although exact mechanisms are not fully understood. For example, high fat diets may effect the primary bile salt synthesis rates by inducing an enhanced capacity for primary bile acid re-absorption in the terminal ileum1.
Dietary fibres from fruits, vegetables and cereals have been shown to have cholesterol lowering properties2. In particular, soluble fibre such as pectin from fruit appears to be beneficial. The exact mechanism of action is unknown but evidence suggests that different fibres have slightly different effects. It is likely that soluble fibres are able to increase excretion of bile acids in the faecal matter through binding, alter the ratios of the primary to secondary bile acids, increase faecal cholesterol and fatty acid excretion, as well as having minor indirect effects, such as the replacement of cholesterol containing foods by low cholesterol high fibre alternatives. In addition, some vegetables such garlic and other allium plants, may decrease cholesterol by reduction of hepatic cholesterol synthesis via inhibition of the enzyme hydroxymethylglutaryl-CoA (HMG-CoA) reductase. Tocotrienols, the unsaturated forms of vitamin E, may also inhibit HMG-CoA reductase.
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