Phytosterols (also called plant sterols) are a group of steroid alcohol, phytochemicals naturally occurring in plants. They are white powders with mild, characteristic odor, insoluble in water and soluble in alcohols. They have many applications as food additives, and in medicine and cosmetics.

Plants contain a range of phytosterols. They act as a structural component in the cell membrane, a role which in mammalian cells is played by cholesterol.

Due to its presence in terrestrial plant matter and only rare occurrence in unicellar algae, β-sitosterol can be used as a biomarker indicating the amount of terrestrially derived organic matter present in a sample. As these sterols are generally insoluble in water, they will partition onto suspended or settled solid matter (e.g. sediments). Due to grain surface area effects, muds will have greater concentrations by weight than sands or coarser grained sediments. To overcome this effect, ratios of individual sterols to total sterol content or cholesterol are usually used to indicate organic matter source.

The molecule shown on the right is β-sitosterol. By removing carbon 24², campesterol is obtained. By removing carbons 24¹ and 24², cholesterol is obtained. Removing a hydrogen from carbons 22 and 23 yields stigmasterol (stigmasta-5,22-dien-3β-ol). Removing carbon 24² and hydrogens from carbons 22 and 23 yields brassicasterol (ergosta-5,22-dien-3β-ol). Further removal of hydrogens from carbons 7 and 8 from brassicasterol yields ergosterol (ergosta-5,7,22-trien-3β-ol). (Refer to the numbering of steroid skeleton.)

Ergosterol plays a structural role as a sterol in the cell walls of fungi. The presence of ergosterol is one of the few significant differences that fungal cells have from animal cells, which are both eucaryotic. Antifungals often target ergosterol and its synthesis for this reason.

Presence of brassicasterol, together with auxiliary markers α-linolenic acid and erucic acid, is a marker of adulteration of soybean oil and sunflower oil with rapeseed oil. As there is no brassicasterol in sunflower and soybean oil, but its concentration in rapeseed oil is about 1400 mg/kg, the amount of rapeseed oil added can be calculated. ^*

As a food additive, phytosterols have cholesterol-lowering properties (reducing cholesterol absorption in intestines), and may act in cancer prevention. Phytosterols naturally occur in small amount in vegetable oils, especially soybean oil. One such phytosterol complex, isolated from vegetable oil, is cholestatin, composed of campesterol, stigmasterol, and brassicasterol, and is marketed as a dietary supplement.

The mechanism behind phytosterols and the lowering of cholesterol occurs as follows : the incorporation of cholesterol into micelles in the gastrointestinal tract is inhibited, decreasing the overall amount of cholesterol absorbed. This may in turn help to control body total cholesterol levels, as well as modify HDL, LDL and TAG levels. Many margarines, butters, breakfast cereals and spreads are now enriched with phytosterols and marketed towards people with high cholesterol and a wish to lower it.

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Steroids | Alcohols | Food additives | Phytochemicals