Phytosterols are cholesterol-like steroidal compounds which occur in plants and are found naturally in the diet. It includes saturated (stanols) and unsaturated plant sterols. Phytosterols have similar chemical structure as that of cholesterol and vary with cholesterol only in carbon side chains where an extra methyl or ethyl group is present in the side chain of the former compound while the latter consists of a double bond in the steroid ring structure.
For decades, phytosterol enriched foods, dietary supplements, milk products like cream, cheese and yoghurt, spreads, bread, salad dressings and drinks have been marketed.
Owing to its structural similarity with cholesterol, it shares the same function as well. As cholesterol exhibits in animal membranes, Phytosterol stabilises the phospholipid bilayers constituting the plant cell membranes.
Why are phytosterols unique?
More than 200 different phytosterols and related structures have been identified and the most common ones are stigmasterol, sitosterol, campesterol, brassicasterol which are unsaturated and sitostanol and campestanol, as the name suggests are saturated compounds. Currently, the focus of the clinical studies has been emphasised more on free phytosterols and esters of phytosterols.
Absorption efficiency of plant sterols is much lesser than cholesterol (about 60%) in humans which is around 2-5%. Studies reveal that percent absorption of β-sitostanol and β-sitosterol are <1% and around 5% respectively. This leads us to the conclusion that unsaturated sterols are more efficiently absorbed than the saturated stanols. However, the effectiveness of both of them in lowering serum cholesterol was found out to be the same while stanols differed from sterols only in terms of chances of adverse effects which were less due to lesser absorption in the body.
Health Benefits of Phytosterols
It Lowers serum cholesterol levels: Numerous clinical trials with well documented results have shown that daily consumption of products rich in phytosterols, phytostanols and its derived products such as esters lower the serum total and LDL (low density lipoproteins) cholesterol concentrations by 10-14%. They exhibit this action by reducing the absorption of cholesterol by the body which results in an increased synthesis of hepatic cholesterol. They may also affect the cellular cholesterol metabolism within intestinal enterocytes. While talking about the LDL cholesterol level, the process of up-regulation of LDL receptor synthesis is responsible for the reduction of serum LDL cholesterol. It has also been noted that the serum HDL cholesterol or triglycerides level can’t be altered by phytosterols as phytosterols have no effect on them.
This effect has been studied in more than 60 human trials which has taken into account the subjects with normal lipid concentration, hypercholesterolaemia and diabetes mellitus. These trials suggested that by consuming an average dose of 2.4 g daily, the mean reduction of LDL cholesterol levels observed was 9.9%.
Many researches have been carried out to study the effects of phytosterols by administering them with different vehicles. A trial in an outpatient setting suggested that effective management of hypercholestrolaemia can be achieved by following a routine prescription of 25g daily fat spread constituting 2 g phytosterols. In addition to the fat spread, by administering phytosterols with other vehicles like orange juice, soymilk, yoghurt, low fat cheese or egg white also showed reduction in serum cholesterol.
The issue of the better efficacy between stanol esters and sterol esters remains debatable and lacks supportable evidence.
Anti-cancer activity: Preliminary evidences have been found which suggest that Phytosterols may inhibit lung, breast, ovarian and stomach cancers.
Anti-inflammatory effects: Insufficient data from cell culture and animal studies indicate the attenuation of the inflammatory activity of the immune cells including macrophages and neutrophils by phytosterols.
Various in vitro and animal studies have been carried out which although provided promising results but theres a lack of confirmation in controlled clinical trials in humans. According to these studies pytosterols also exhibit anti-oxidant, anti-atherogenic and anti-cancer activity.
Rich Sources of Phytosterols
Phytosterol’s sole source is diet as it is not produced in our body. On an average, 250 mg/day of phytosterol is obtained from a conventional Western diet while a vegetarian diet provides twice of the amount stated above. They are present in all plant foods mainly as the structural components of the cell membrane. Among naturally occuring richest sources of phytosterols are vegetable oils and products derived from them. Other foods having high content of fat and/or fibre content are nuts, seeds, grains and legumes. Fruits, vegetables, berries and cereal products are the ones having low phytosterol content.
Among nuts, Pistachios are the richest source of phytosterols (279mg/100g), almonds (143mg/100g), cashews (158mg/100g), with Brazil nuts and English walnuts having the lowest content (95mg/100g and 113mg/100g respectively). According to a survey carried out, for every weekly serving of nuts, the risk of coronary heart disease reduces by 8.3 percent. Among seeds, sesame seeds and wheatgerm have the highest concentration (>400mg/100g), followed by sunflower seed kernels (270mg/100g). Smooth peanut butter contains 135mg/100g. Good choices among plant oils are olive, corn, canola, sesame, linseed, cottonseed and soyabean. A recent study stated that the phytosterol content of refined oil greatly varies depending on how the product is refined. Other sources are broccoli, avocados, brussel sprouts and fortified food including yoghurt, milk, chocolate, salad dressings, soymilk etc.
Recommended Dietary intake of Phytosterols
The daily intake can vary between 100 and 400 mg unlike few million years ago which was quite higher, around 1d/day due to the increased consumption of fruit, vegetables and other plants.
Recommended dose of Phytosterols
Food and Drug Administration (FDA) in 2003 announced that the lowest effective daily dose of free phytosterols is 800mg, though further evidence is required to obtain the optimal dose which would provide relative efficacy when incorporated in different foods and tablets.
Phytosterols : Precautions
When is Phytosterol contra-indicated?
- Patients suffering from Phytosterolaemia (rare genetic defect) are susceptible from high phtosterol intakes as there is increased absorption of phytosterols which results in atherosclerosis and cell fragility.
- No data has been provided for the use of phytosterols in children with normocholesterolaemia less than 5 years.
- No long-term data has been acquired in case of consumption of phytosterols in pregnancy and breast-feeding.
Potential adverse effects that might be observed:
Phytosterols have proven to be clinically safe when consumed within the recommended intake range. However, along with the reduction of serum cholesterol levels, they also might reduce the absorption of other lipid soluble compounds like carotenoids (β-carotene and lycopene) and Vitamin E.
Possible drug interactions of Phytosterols
Statins – Combination treatment of statins and phytosterols show an additive effect in lowering the serum LDL cholesterol levels as both have different mechanism of action thereby causing a risk of hypocholesterolaemia. Therefore, it is adviced that you avoid this combination treatment.
Lipid-soluble nutrients – As stated above, phytosterols consumption can decrease the absorption of substances like carotenoids and vitamins thus posing a defeciency threat.
It is recommended that you consume phytosterols in your diet on a daily basis. Choose foods fortified with phytosterol, phytostanol and their derivatives. You can also add Phytosterol supplements directly to your regimen after consulting it with your physician. Foods and beverages with added phytosterols and phytostanols are now available in many countries throughout the world, for which countries also allow health claims for such products.