Oats have some interesting physiological properties in humans. Oats have been extensively researched and their effects on plasma cholesterol levels are well studied. Oats are a good source of soluble fibre and this may confer some of the lipid lowering benefits associated with their regular consumption. In particular, the fibre in oats in soluble fibre, a type of fibre that has been shown to have cholesterol lowering effects due to its ability to increase bile acid excretion. As bile acids are synthesised from cholesterol, increased bile acid excretion may divert cholesterol to the synthesis of replacement bile acids, thus lowering plasma levels. Oats are particularly rich in beta-glucan, a component of soluble fibre that is found primarily in the bran layer of the oat grain. Beta glucan fibre comprises around 5 % of the dry weight of the whole oats and may be the component that is responsible for the blood cholesterol lowering effects of oats. Studies investigating isolated beta glucan fibre have shown that it has significant lipid lowering effects.
As well as their ability to modulate plasma cholesterol levels, oats may have weight loss properties. This can be said for all whole grains to some extent, but oats (and legumes) appear to be particularly beneficial. This may relate to their beneficial glycaemic effects that result from the intactness of the original cereal grain. The intact nature of the oat grain, which is usually just rolled as a processing step, leaves much of the original cells structure intact. The fibrous walls of the oats grain, including large amounts of beta glucans fibre, then inhibits the digestion of the starch from the endosperm. The result is that the starch is only slowly digested and absorbed, and the shallow controlled rise in blood glucose that results reduces the requirement for insulin and this improved insulin sensitivity. However, oats may have other effects that contribute to their ability to induce weight loss and this may relate to their ability to increase fat excretion. Such effects are seen in only a short time after eating modest amounts of rolled oats.
For example, in one study1, healthy subjects were fed 125 grams of rolled oats, that were substituted into their diets in place of refined wheat flour. The subjects were then monitored on these controlled diets for 3 weeks, while diets before and after this period were adjusted to match the macronutrient ratios of the treatment period. The results of the study showed that of the 10 subjects, 7 had reductions in total cholesterol levels. However, the effects as a group were not significant. More interesting was the fact that faecal fat excretion increased by 47 % and faecal bile acid by 35 %. This large increase in the excretion of fat is interesting, because it may relate to the fat contained within the oats. Oat lipids have been shown to be relatively poorly absorbed, with only around 65 to 70 % absorption rates. Similarly, the lipids in nuts have also been shown to be poorly absorbed, and this may explain why nuts are also an effective weight loss food. This poor absorption may relate the intact cell structure inherent in oats and nuts.
Although the reductions in cholesterol in this study were not significant, the falls in 7 out of the 10 individuals, combined with a lack of any effect of wheat fibre, suggest that the results are real. Part of the cholesterol lowering ability of oats may come from the polyunsaturated fatty acids (PUFA). This study supports this contention because the PUFA were added to the before and after diets, thereby controlling for the PUFA in the oats, and this negated some of the cholesterol lowering effects. It is therefore likely that the cholesterol lowering effects of oats derive from a number of different factors that combine to produce the overall effect. The ability of oats to increase excretion of bile acids, but not neutral steroids is intermediate between the lack of any effect from wheat fibre, and the ability of pectin to cause an increase in excretion of both bile acid and neutral steroids. Compared to wheat fibre, oats are 4 times as effective at binding bile acid, while chemical differences in pectin also cause binding to neutral steroids.
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