Cholesterol-lowering properties of oats: Effects of processing and the role of oat components
Abstract: The cholesterol-lowering effect of oats has been studied for almost fifty years. The effect has hitherto essentially been associated with its content of β-glucans, the primary soluble dietary fibre in oats. Few studies have been published regarding the extent to which other oat components may contribute to the effect and how processing of oats may influence the cholesterol-lowering properties. The objective with the current thesis was to increase the knowledge in this area, and was intended to provide a basis for the development of new functional foods from oats, with optimised cholesterol-lowering properties. For this purpose, different oat products or fractions of oats were produced, characterised and evaluated in mice. The wild-type mouse strain C57BL/6 fed an atherogenic diet was shown to be an appropriate tool for evaluation of effects on plasma cholesterol, although substrain differences in response to oats were found. When oat bran was processed to different average molecular weight of β-glucans and then evaluated in C57BL/6NCrl mice, all products were found to lower plasma cholesterol to the same degree as untreated oat bran. This was also seen for a second mouse model (LDL-receptor deficient mice), although in this case there was a small but statistically significant decline in effect with reduced average molecular weight of β-glucans. It could, however, not be excluded that other components, apart from the β-glucans, might have contributed to the effects on plasma cholesterol in these studies. Indeed, the cholesterol-lowering effect of oat bran, was found to be attributed to two complementary fractions: an oat bran oil and a β-glucan containing fraction, which supported previously published studies that components other than β-glucan may be of importance. The effect of the oat bran oil was shown not to be caused by the content of unsaturated fatty acids, but by other components. An extraction method was developed in order to purify β-glucans from oat bran for studies in mice. The highest yield was obtained after a combined heat treatment and a starch digestion step followed by treatment with pancreatin. Using this method, about 80% of the β-glucans could be extracted. When purified β-glucans from oats were studied, the difference in purity, viscous properties and average molecular weight, seemed not to be crucial parameters for the cholesterol-lowering effect. Further studies are needed to investigate whether purified β-glucan products, with average molecular weights lower than 285 kDa, are able to lower plasma cholesterol and which components in the oat bran oil fraction that cause the effect on plasma cholesterol. The results should also be confirmed in human intervention studies.
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