Soluble fibres and cholesterol lowering

In the Brit Journal of Nutrition ( 2007, 97, 1049-1050 ) there is an interesting review by Poppitt on the cholesterol lowering properties of soluble oat fibre and β-glucans. The potential for a reduction in cardiovascular risk with a high intake of dietary fibre was originally suggested by Ancel Keys and colleagues in the Journal of Nutrition in 1960. An intake of whole; grains may decrease the incidence of fatal and nonfatal coronary heart disease, which may be affected through a hypocbolesterolaemic action effect of the viscous soluble fibre component of whole-grain foods such as oat and barley cereals rather than the insoluble fibre component of whole-grain foods such as wheat and rice. However, not all published studies of high soluble fibre oat and barley products show cholesterol-lowering effects.
Soluble fibre is found in many legumes (including peas, beans), some fruits (including apples, pears) and the plaintain seed husk psyllium (Plantago psyliutm). However it is the β-glucan soluble fibre of cereals, with linear unbranched polysaccharides of linked β(1→3)-(1→4)D glucopyranose units, which appears to be important in the cholesterol-lowering properties of viscous soluble fibre.
It is differences in factors such as solubility, viscosity and molecular weight of β-glucans which may be the reason for conflicting outcomes. The mechanisms by -which β-glucan may act includes increased intestinal viscosity and/or bile acid binding which in turn may (i) decrease reabsorption of bile acids and drive bile acid synthesis from hepatic cholesterol, hence depleting the body’s cholesterol pool and/or (ii) decrease absorption of intestinal cholesterol. Another mechanism could be that bile acids entering the colon are sequestrated by bacteria, of which there increased activity when the colon is exposed to fermentable fibre.
There have been mixed results form trials with of β glucans. The differences may be due to unfavourable structural changes during commercial purification, (depolymcrisation of the linear structure, decreasing molecular weight and viscosity ). Mild extraction conditions may not deactivate endogenous 6-glucanases and hence increase depolymerisation: cooking processes may decrease peak MW. and freezing and storage may reduce the extractability of β-glucan in the intestine.
In 1997 the US Food and Drug Administration (FDA) concluded that B-glucan soluble fibre (3g/d) from oat bran and rolled oats or from whole-grain barley and dry-milled barley products are efficacious in lowering total and LDL-cholesterol concentrations.
Poppitt S 2007 Soluble oat and barley β-glucan enriched products: can we predict cholesterol lowering effects
Gelissen and Eastwood 1995 Taurocholic acid adsorption during non-starch polysaccharide fermentation : an in vitro study 74, 221-228

Martin Eastwood
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