Epidemiological studies suggest that fibre consumption is associated with lower plasma cholesterol concentrations. It has been suggested that the plasma cholesterol lowering effects of high fibre diets may be responsible for their cardioprotective effects. For example, it is estimated from meta-analyses that addition of around 10 grams of dietary fibre per day is associated with a 10 to 30 % reduction in the risk of coronary deaths. Of particular benefit appear to be the water soluble or mucilaginous fibres although the exact reason for this is not clear. One suggestion is that high fibre diets are beneficial because high fibre diets are also high plant diets, and plants contain other cardioprotective nutrients. Another suggestion is that dietary fibre is able to bind bile acids and this increases the excretion of cholesterol from the liver because cholesterol is required for the synthesis of replacement bile acids. Indeed there is evidence to support this contention, and lupin kernel fibre appears to be particularly effective in this regard.
Another suggestion is that the non-digestible components of plants, including dietary fibre, pass to the colon where they are fermented by gut bacteria to form a number of short chain fatty acids including acetate, butyrate and propionate. These short chain fatty acid may in turn then be absorbed where they interfere in some way with cholesterol metabolism in the liver. Researchers have assessed the effects of citrus pectin and lupin kernel fibre in hypercholesterolaemic individuals using a double-blind crossover trial1. Subjects consumed either 25 grams of lupin kernel fibre, 25 grams of citrus pectin fibre or a low fibre diet for 4 weeks in a random order with 2 weeks between each trial. Total cholesterol, LDL cholesterol and triglycerides were 9, 12 and 10 % lower following the lupin fibre diet compared to the control diet. In addition the LDL to HDL ratio was also significantly lower following the lupin fibre diet compared to the control diet. However the citrus fibre diet did not differ significantly from the control diet.
During the lupin fibre diet, there was an increases in the main short chain fatty acids in the faeces of the subjects, but only acetate increased following the citrus fibre diet. Interestingly both high fibre diets causes improved satiety in the subjects and caused modifications to the subjects nutritional behaviour. In particular both high fibre diets resulted in significantly lower body weights, lower body mass indices and lower waist circumferences in the subjects. As the energy intake of the subjects was not reduced in this study, it is further evidence that calorie restrictive diets are not necessary in order to cause weight loss. Excretion of total bile acids remained constant throughout the different diets, but plasma cholesterol was lowered by the lupin fibre diet, suggesting that the cholesterol lowering effect was not as a result of bile acid binding. Therefore it could be said that the cholesterol lowering properties of the fibre were as a result of the increased production of short chain fatty acids in the colon.
Dr Robert Barrington’s Nutritional Comments: It is not clear if the cholesterol lowering effects of dietary fibre are a cause of the cardioprotective effects of dietary fibre or are related to some other mechanisms of cardioprotection. Fibre for example, is known to have beneficial glycaemic effects. In this regard soluble fibre forms a viscous gel in the intestinal tract through absorption of water, and this inhibits the digestion of starch and the absorption of glucose, slowing the postprandial rise in blood glucose seen after a carbohydrate meal. Such beneficial glycaemic effects are associated with improvements to insulin sensitivity, which could in turn reduce the risk of developing metabolic syndrome, a known risk factor for cardiovascular disease. As insulin resistance is associated with both detrimental changes to plasma cholesterol and with an increased risk of cardiovascular disease, and because fibre improves insulin sensitivity, it could be that the effects of dietary fibre on cardioprotection are confounded by at least one variable.
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