Plants contain a rich cocktail of biochemical compounds, many of which have been shown to interact with mammalian physiology. Plant sterols and stanols are two such groups of compounds that are bioactive in humans and animals. Stanols and sterols have structures similar to cholesterol (figure 1), but differ in their side chains. In addition, whereas stanols have saturated structures, the structures of sterols are unsaturated as they are in cholesterol. Only plants can synthesise sterols and stanols, but they are significant in human nutrition because of their widespread abundance in food. Common sterols such as β-sitosterol and stimasterol are more abundant in human diets when compared to common stanols such as campestanol and sitostanol. Plant sterols have a lower bioavailability than dietary cholesterol, and it is believe that they may be able to displace cholesterol from mixed micelles in the small intestine, thus reducing dietary cholesterol absorption.
Figure 1. A comparison of the structure of cholesterol, to that of the campesterol, sitosterol, campestanol and sitostanol.
The ability of plant stanols and sterols to modify plasma cholesterol has been extensively investigated, and generally significant reductions have been reported. However, much of the research has centred on the use of subjects with elevated levels of plasma cholesterol, including those due to genetic defects. In one meta-analysis, researchers1 reviewed the randomised control trials investigating the effects of plant sterols and stanols on plasma lipid levels of individuals with familial hypercholesterolaemia. The results included 6 studies that met the criteria set by the authors and included subject aged 2 to 69 years. Consumption of vegetable spread enriched with plant stanols and sterols at a concentration equivalent to 2.3 g /d over 6.5 weeks, significantly reduced low density lipoprotein (LDL) cholesterol by 10 to 15 % with a mean decrease of 0.64 mmol/L when compared to a control. No adverse side effects were reported.
In another meta-analysis2, researchers reviewed the randomised controlled trails evaluating plant sterols and stanols for their ability to lower plasma lipids in hypercholesterolaemic individuals. The studies administered stanols and sterols at intakes of between 1.7 and 6 g/d in combination with and compared to statins. From the eight studies included, the authors reported that plant stanols and sterols when in combination with statins were more effective at lowering total and LDL cholesterol compared to a statin alone (0.36 mmol/L and 0.35 mmol/L, respectively). The authors suggested that this might reduce the risk of a coronary event by 1%. However, other authors have shown that plant sterols significantly lower plasma cholesterol to the same extent whether alone or in combination with a statin. This suggests that statin use may be no better that a placebo, and that in this study stanols and sterols were responsible for the lipid lowering effects.
The hypothesis that plant sterols and stanols lower plasma levels of cholesterol by inhibiting the absorption of cholesterol, reported to be between 26 and 36 %, seems plausible. However, this line of reasoning may not explain the lipid lowering effects seen in studies. If reducing gastrointestinal absorption of cholesterol was able to lower plasma levels of cholesterol, then dietary restriction of cholesterol would have a similar effect. However, restricting dietary cholesterol has no significant effect on the plasma levels of cholesterol, because as dietary levels fall, endogenous production increases. Further, if preventing the absorption of dietary cholesterol decreased plasma cholesterol levels, increasing dietary levels of cholesterol should increase plasma levels. However, again, increase cholesterol consumption from, for example eggs, does not raise plasma cholesterol levels. Sterols and stanols must therefore lower plasma cholesterol by some other mechanism, rather than inhibition of intestinal cholesterol absorption.
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