Dietary fibres are a heterogeneous group of plant polysaccharides. Unlike starches, fibres possess glycosidic bonds that are not hydrolysable by human enzymes. As a result the sugars within the polysaccharides are not digestible and are not absorbed in the small intestine. Traditionally dietary fibres have been considered as providing roughage to food, and in this regard their metabolic importance was downplayed. However, evidence has been accumulating for the last 30 years to indicate that dietary fibres not only have important metabolic roles in human nutrition, but that they may be essential to human health if starch is present in the diet. One of the metabolic roles of dietary fibres is in the regulation of lipid absorption and the modulation of lipid metabolism. This ability to regulate lipid metabolism may relate to the ability of fibre to bind to bile acids in the gut and increase their excretion rates, or may result from the physical barrier presented by the fibre to lipid absorption.
Animal experiments have been performed using rats in order to assess the effects of fibre on lipid metabolism in mammals. In one study, rats were fed selected diets containing 15 % wheat bran, alfalfa, yeast cell wall glycan, cellulose or pectin to investigate the effects of different fibre types1. Another group of rats were fed the cholesterol lowering drugs cholestyramine. The results showed that cholestyramine, pectin, alfalfa and cellulose all resulted in significant reductions in plasma cholesterol and triglycerides. However the wheat bran only had minimal effects and the yeast cell wall glycan was not effective at reducing lipid absorption. Transit times through the gut were significantly slowed by both wheat bran and cellulose. Both cholestyramine and alfalfa showed bile acid binding ability, although the cholestyramine was much more effective in this role. The test substances that inhibited the absorption of lipids also resulted in significant decreases in hepatic triglyceride production, but increases in hepatic phospholipids.
The authors suggested that the test substances were able to prevent the absorption of lipids because they could either bind bile acids or prevent the interaction of the lipids with the enterocytes of the gut. With regards to the former mechanisms, cholestyramine is a known bile acid sequestrant, and the reason for its incorporation into this study. Interestingly, none of the test fibres or the drug had any effect on plasma cholesterol. This the authors concluded was due to the test diet being a healthy diet which did not cause elevations in the plasma levels of cholesterol in the rats. Therefore dietary fibres and cholestyramine appear to modulate cholesterol levels, but will not lower levels if they are within the normal range. None of the substances tested had any effects on liver levels of 7α-hydroxylase activity, suggesting that none of the substances could directly block the synthesis pathway for cholesterol at the rate limiting step. This study supports the contention that dietary fibre is heterogeneous in its effects.
Dr Robert Barringtons Nutritional Recommendation: The ability of fibre to lower plasma levels of cholesterol and triglycerides is well reported. In this respect dietary fibres of different kinds may have differing effects and some may be more efficacious than others. The suggestion that dietary fibre can inhibit the absorption of lipids through a bulking action that provides a physical barrier between the dietary lipids in the gut lumen and the enterocyte is interesting. This may be a similar effect that is observed when soluble dietary fibres of particular types are able to inhibit glucose absorption through formation of a physical barrier in the unstirred layer of the enterocytes. Dietary fibres clearly have beneficial effect on the metabolism of lipids and in this regard the consumption of high fibre diets are recommended. However, the types and amounts of fibres that provide optimal effects are not known. Therefore the current recommendation is to eat a varied diet containing a range of plant foods that provides a wide range of plant fibres in the diet. This should include sources of soluble and insoluble fibre.
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