Increasingly, research suggests that both insoluble and soluble fibres are beneficial to long-term health. Soluble fibre has some interesting metabolic effects, with perhaps the most researched and most important being the ability to normalise elevated blood lipid levels. Insoluble fibre in contrast, may have important properties for microbial growth and gastrointestinal motility, but research has not found it to be beneficial at lowering blood lipids. With regard soluble fibres, there appears to be slight differences in their effects on human physiology, with some types producing better blood lipid lowering effects than others. Amongst the soluble fibres, beans and oats appear to have the greatest lipid lowering effects in humans. Research has compared the effects of soluble and insoluble fibres on blood lipids and repeatedly found that soluble fibre is significantly more effective, when compared to insoluble fibre.
For example, researchers1 have investigated the blood lipid lowering effects of oat bran and wheat bran on 20 hypercholesterolaemic men. The subjects received a control diet for 7 days and were then randomly allocated to receive an identical diet that differed only in the form of fibre it contained. Both diets contained 34 g of total dietary fibre, but one diet contained 13.4 g of soluble fibre from oat meal, whereas the other provided 7.8g of soluble fibre and the rest as insoluble fibre from breakfast cereal and wheat bran muffins. After 21 days on the two treatment diets, the subject consuming oat bran had a significant 12.8 % reduction in total cholesterol, a significant 12.1% reduction in low density lipoprotein (LDL), and significant 13.7 % reductions in apolipoprotein B-100. In contrast, the wheat fibre had no significant effects over the baseline levels. However there was a significant 10 % reduction in serum triglycerides in the wheat bran groups.
These results support other studies showing lipid lowering effects from soluble fibre of around 10 to 20 %. In this study the cholesterol lowering effect of the soluble fibre from oats was 13 %, with the oat bran diet providing 5.6 g more soluble fibre than the wheat bran diet. With oats containing around 7.4 g of fibre per 100g of cereal, a ~13 % reduction in serum total cholesterol appears to be easily achievable. Soluble fibre may lower serum cholesterol because it increases bile acid excretion and therefore increases flux of cholesterol to the faeces. In addition, soluble fibre preferentially lowers LDL cholesterol and this may be because it can increase hepatic LDL receptors. Soluble fibre is also fermented in the gut to form the short-chain fatty acids such as acetate, propionate and butyrate and these may inhibit hepatic cholesterol synthesis. In addition, reductions in digestive speed may lower insulin levels and this may inhibit hepatic production of cholesterol.
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