Previously it was considered that dietary cholesterol and saturated fat were the cause of elevated plasma lipoproteins, but this has since proven to be false. More recent evidence suggests that consumption of high intakes of refined carbohydrates and sugars causes metabolic abnormalities such as insulin resistance, and this in turn raises plasma lipoprotein levels beyond normal. The reason why refined carbohydrates are detrimental to the health possibly stems from their lack of fibre and micronutrients, normally present in the whole grain bran. The lack of fibre in refined grains increases the glycaemic index and decreases enterohepatic recirculation, and the lack of micronutrients causes metabolic dysfunction. Epidemiology shows that fibre intake is inversely associated with cardiovascular disease and clinical trials have shown that fibre supplementation can lower plasma levels of low density lipoproteins (LDL), some sub-fractions of which may be associated with cardiovascular disease.
The ability of dietary fibre to lower plasma lipoprotein levels has been extensively researched. However, controversy surrounding the effects of fibre has resulted from some negative findings. This may relate to the large variety of fibres in various plant foods, that may have differing effects on human physiology. Generally, research appears to show benefits for dietary fibre in terms of lipoprotein lowering effects, but the exact benefit varies between trials. A meta-analysis of 67 controlled trials was performed by researchers1 in order to attempt to quantify the lipoprotein lowering effects of dietary fibre. The results showed that 2 to 10 g/d fibre was associated with a 0.045 mmol/L decrease in total cholesterol per gram of soluble fibre, and a 0.057 mmol/L decrease in low density lipoprotein (LDL) cholesterol per gram of soluble fibre. The researchers reported that fibre from oats, psyllium and pectin all showed the same effects.
The authors concluded that soluble fibre was able to significantly lower total and LDL cholesterol, and that a small benefit was possible with as little as a 28g serving of oats containing 3 g soluble fibre. This 3 g of fibre was able to lower total and LDL cholesterol by ≈0.13 mmol/L. Although this figure may seem small, it is common practice in studies involving lipoproteins to report the relative risk, which is the ratio of the absolute risk in two differing groups. This magnifies the effects of the changes and makes the figures look larger than they are in absolute terms. In fact, beans and oats have shown to be as effective as statin drugs at lowering lipoprotein levels in hyperlipidaemic subjects. However, the benefits of dietary fibre go beyond cardiovascular health, as high intakes can cause weight loss and are inversely associated with blood sugar disorders.
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