A large body of evidence suggests that dietary fibre is beneficial to the health. Traditionally fibre was thought of as roughage, and that this provided bulk to the food to aid gastrointestinal transit. This traditional viewpoint is likely true, but as studies have accumulated data it has become clear that fibre plays a much more complex role in human metabolism. More recently, investigations into the effects of various dietary fibres have shown that the effects of fibre are far more pronounced than was previously thought. Further, the range of effects exerted by fibre on human metabolism is diverse. Fibre can be classified into soluble and insoluble fibre depending on its ability to absorb and interact with water. However, within these broad categories exist a number of subcategories, and it appears that within these subcategories fibre can have varying effect. However, despite the beneficial effects of fibre being well reported, controversy still surrounds the exact mechanism by which fibre can benefit human health.
Studies have shown that fibre may have the ability to alter the absorption of fat. This has been suggested as a possible explanation as to why fibre may favourable affect blood lipids. For example, guar gum and pectin, two soluble fibres found in beans and fruit respectively, have been shown to exert significant beneficial effects of plasma lipoprotein levels. The insoluble bran fibres found in cereal grains too may possess the ability to alter plasma lipoprotein levels albeit in a different way. In this regard, insoluble bran fibres may inhibit the release of pancreatic lipase and reduce the absorption of triglycerides from the small intestine. For example, in one study1 an in vitro model of the human gut was used to assess the ability of wheat bran, hemicellulose (xylan), slightly methylated pectin (42 %) and cellulose to bind to pancreatic lipase and inhibit enzymatic hydrolysis of triglycerides. In the study addition of wheat bran to a mixture of fat, colipase and human bile resulted in a reduction in triglyceride hydrolysis at a 1 % concentration.
However, the cellulose, hemicellulose and pectin did not inhibit the hydrolysis of the triglycerides. When the authors compared the effects of wheat bran to those of cholestyramine, a potent inhibitor drug of triglyceride hydrolysis, they found the effect of the wheat fibre comparable (66 to 82 % for cholestyramine versus 77 to 94 % for wheat bran fibre). The authors suggests that the effects of wheat bran on triglyceride hydrolysis may be due to the presence of a pancreatic lipase inhibitor in the wheat bran, but was not due to the binding of the wheat bran fibre to the triglycerides themselves. In contrast, cholestyramine bound both to the triglycerides and the pancreatic lipase, suggesting that the inhibitory effect of cholestyramine is different to that of wheat bran. Pancreatic lipase may therefore be inhibited by a component in wheat bran, explaining its triglyceride lowering effects. This may explain in part some of the health benefits of insoluble fibre, although it is likely other mechanisms also exist.
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