Dietary fibre is a heterogenous group of non-digestible plant polysaccharides. Due to the structural heterogeneity, dietary fibre from different sources shows physiological and biochemical differences in humans. One of the effects of fibre in human nutrition is that of modification to the lipid metabolism in the liver. In this regard dietary fibre can influence the lipoprotein concentrations in the blood through its regulatory role in hepatic lipid metabolism. The cholesterol lowering effects for some dietary fibres have been extensively reported, but these effects appear to differ between different fibre types. Soluble fibre has for example, been shown to lower plasma cholesterol levels, particularly the low density lipoprotein (LDL) particle, to a greater extent that some insoluble fibres. This explains the cholesterol lowering effects of legumes and oats, both of which are good sources of soluble fibre. Soluble fibre may lower plasma cholesterol levels because of its ability to increase bile acid secretion.
Citrus pectin is soluble fibre that is found in many fruits and vegetables and evidence suggests that it may have cholesterol modifying effects. To these ends the physiological and biochemical effects of citrus pectin have been investigated in healthy volunteers1. Subjects received a controlled diet for 2.5 weeks that contained low levels of dietary fibre. Following this the subjects were fed either the same low fibre diet (18 grams total fibre per day), a high fibre diet rich in fruits and vegetables (43 grams total fibre per day), a diet supplemented with powdered pectin fibre (28 grams total fibre per day) or a diet supplemented with powdered wheat bran fibre (37 grams total fibre per day). The protein, fat and carbohydrate content of the diet was controlled to make the diets similar in their macronutrient content. The results of the study showed that plasma total cholesterol decreased by 0.34 and 0.17 mmol/L in the subjects consuming the fruit and vegetable diet and the pectin supplement diet.
As has been shown previously, the change in plasma total cholesterol could be explained by a significant increase in faecal steroids, suggesting that the pectin fibre had increased the excretion rate of bile acids. However this change was relatively small and the authors suggested that it could not explain all of the changes to the plasma cholesterol levels. A 15 gram portion of grapefruit pectin per day has been shown to lower cholesterol by around 10 %. Pectin therefore produces a modest reduction in plasma cholesterol levels, but it does this without producing side effects. Some evidence (here) suggests that components of pectin can interact with the low density lipoprotein particle electrostatically, although the significance of this is not fully understood. All of the fibre diets in this study shortened the transit time for chyme through the gut and increase faecal wet weight compared to the low fibre diet. Therefore soluble fibre appear to lower plasma cholesterol levels, but all fibre types may improve colonic function.
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