According to proponents of the cholesterol theory of cardiovascular disease, dietary cholesterol causes elevations in plasma (blood) levels of cholesterol and this in turn causes atherosclerosis, plaques which occlude blood flow in arteries and lead to heart attacks. Based on this theory it is often suggested that limiting dietary cholesterol is effective at lowering the risk of cardiovascular disease because it lowers the amount of dietary cholesterol in the blood, which in turn reduces the formation of atherosclerotic plaques. Based on the suggestion that limiting dietary cholesterol is beneficial, plant sterols have been proposed as an effective solution to those wishing to lower plasma cholesterol levels. Sterols are plant compounds with similar structures to animal steroids such as cholesterol (and have a similar role in plants), and evidence suggests that when they are present in the gut concomitantly, the plant sterols reduce the absorption of dietary cholesterol perhaps through competitive inhibition.
For example, in one study1, researchers assessed the ability of plant sterols to inhibit the absorption of dietary cholesterol. Subjects were admitted to a medical ward and fed a number of controlled meals in three separate studies. In the first study, subject consumed a meal containing 500 mg of cholesterol from eggs. In a second study, they were fed an identical meal, but the food also contained 1 gram of the plant sterol β-sitosterol. In a third study, the subjects received the same egg meal but this time with 2 grams of β-sitosterol oleate ( a version of the sterol bound to an oleic acid molecule). When the authors analysed the cholesterol excreted in the faeces of the subjects they observed that the addition of β-sitosterol to the test meal decreased the absorption by 42 % and the presence of β-sitosterol oleate decreased the absorption of cholesterol by 33 %. These results are in agreement with other studies that show that plant sterols limit the absorption of dietary cholesterol in humans.
Stanols are similar in structure to the sterols in plants. Studies suggest that stanols too can lower the absorption of dietary cholesterol in animals and humans. The mechanisms by which sterols and stanols can inhibit the absorption of cholesterol is not fully understood, but is thought to involve one of two mechanisms. Firstly, it is possible that the sterols and stanols take up the physical space in the micelles that are needed for the transport of lipids into enterocytes before they are absorbed to the circulation. Alternatively, the coprecipitation theory suggests that stanols and sterols in the gut may increase precipitation of cholesterol and thus limit its absorption. As fats are absorbed from the gut, the relative concentration of the less well absorbed stanols and sterols increases. As the concentration reaches a critical threshold, the stanols and sterols precipitate out of solution and with them some of the cholesterol (due to its similar structure) also precipitates, making it less likely to be absorbed.
While this is all interesting from a biochemical perspective, is it important nutritionally? Should we care that high concentrations of isolated dietary plant sterols limit the absorption of cholesterol? Well that really depends if you believe that dietary cholesterol is a cause of cardiovascular disease. Certainly, evidence is lacking in the nutritional literature for any detrimental health effects of dietary cholesterol amongst healthy people, although people with particular genetic disorders are affected. Further, while the use of plant sterols has been shown to lower the absorption of cholesterol, there is no real evidence that this brings health benefits particularly against cardiovascular disease. In fact the opposite may be true, because evidence (here) suggests that sterols and stanols themselves, when administered in an isolated form and in high concentrations, may cause detrimental health effects to the cardiovascular system.
Dr Robert Barrington’s Nutritional Recommendation: Most plant based diets naturally contain plant sterols and stanols but the levels are too low to have an effect on the absorption of cholesterol. In order to have an effect plant sterols and stanols must be administered in a concentrated form mixed within the food matrix. However, there is a lack of evidence for the long term safety of administering plant sterols to humans in this way. In addition, there is only weak and circumstantial evidence that dietary cholesterol is involved in the aetiology of cardiovascular disease. Diets high in plant foods can be protective of cardiovascular disease, but the reason for this is not known. However, it is clearly not because of the plant sterols and stanols within the plant foods and as such consuming concentrated forms of these compounds in order to limit cholesterol absorption is not recommended.
RdB
