What are Sterols? - Definition, Structure, Function, Examples and Uses

Sterols are a class of lipids that play an important role in the structure and function of cells in living organisms. They are a type of steroid with a specific chemical structure characterized by a hydroxyl group (-OH) attached to a carbon atom of the A ring. Sterols are found in all eukaryotic cells, where they play an important role in regulating membrane fluidity, serving as a precursor for the synthesis of hormones and vitamin D, and aiding in the digestion of fats. The most common sterol is cholesterol, which is abundant in animal tissues and plays an important role in maintaining cell membrane integrity, regulating cholesterol levels in the blood, and serving as a precursor for the synthesis of steroid hormones.

Sterol Definition

Steroids are a general term for a large class of cyclopentyl perhydrophenanthrene derivatives that are widely distributed in the biological world. Steroids include sterols (such as cholesterol, lanosterol, sitosterol, stigmasterol, ergosterol), bile acid and bile alcohol, steroid hormones (such as adrenocortical hormones, androgens, estrogens), insect molting hormones, cardiac glycosides and saponin glycosides, and toad venom, etc. In addition, there are also synthetic steroid drugs such as anti-inflammatory agents (prednisolone, dexamethasone), steroid drugs that promote protein synthesis, and oral contraceptives. Among them, sterols, as a type of steroid, are widely distributed in the biological world. When lipids in animal and plant tissues are extracted with fat solvents, there are often different amounts of substances that cannot be saponified by alkali. They all have cyclopentane polyhydrophenanthrene as the basic structure and contain alcohol groups, so they are called sterol compounds. Sterols mainly include animal sterols, plant sterols and yeast sterols. Many animal hormones belong to sterols, such as progesterone, testosterone, estradiol in sex hormones and corticosterone in adrenal hormones.

Sterol Structure

Plant sterols are a class of bridged ring compounds composed of multiple aliphatic six-membered rings and five-membered rings. The main skeleton structure is a cyclopentane octahydrophenanthrene skeleton, also known as the sterol backbone. Steroid compounds usually exist in three forms: sterols, alkanoic acid sterols or glycosides. Among them, under saponification conditions, alkanoic acid esters and glycosides will decompose into free plant sterols. As shown in Fig. 1, carbon 3 of sterol is connected to the alcohol hydroxyl group and carbon 17 is connected to different substituents R. It will form various types due to the stereoisomerism of the carbon atom of the alcohol hydroxyl group and the different substituents R2. Whether carbon 5 is saturated determines whether it is a sterol or a stanol. The five common sterols are β-sitosterol (Fig. 1A), stigmasterol (Fig. 1B), campestanol (Fig. 1C), campesterol (Fig. 1D) and cholesterol (Fig. 1E). Among them, the first four are plant sterols, and the fifth is cholesterol. When the alcohol hydroxyl group of carbon 3 undergoes esterification reaction with fatty acids, the corresponding plant sterol esters are formed. For example, it combines with oleic acid to form plant sterol oleate.

Sterol Function

• Lowering cholesterol: A large number of medical clinical experiments, experiments on hyperlipidemia mice and human trials on volunteers have shown that plant sterols (esters) have the effect of lowering cholesterol and blood lipids. Related mechanism studies have shown that plant sterols reduce the body's intake of cholesterol by competing with cholesterol for absorption in the stomach and intestines.
• Anti-inflammatory effect: In 2019, Vilahur Gemma et al. wrote an article "Plant Sterol Esters and Inflammation", which recorded many in vitro studies and experimental animal models of plant sterol esters for anti-inflammatory effects. It shows that plant sterols can effectively reduce the secretion of inflammatory factors.
• Reduce the risk of colon cancer: Araceli Urquiza-López et al. found that a plant cell metabolite can effectively inhibit the proliferation of human HT-29 colon cancer cells. Material analysis showed that plant sterol esters play a role in it.
• High melting point and low solubility: In order to solve the difficulty of application, researchers have made various physical and chemical modifications to plant sterols. Physical modifications include microemulsification and self-emulsification, microencapsulation, biopolymer-based, surfactant-based and lipid-based nanocarriers, etc. Chemical modifications include esterification, ethoxylation, and polyethylene glycol (PEG) modification, which respectively increase the fat solubility and water solubility of phytosterols.
• Safety: Phytosterols are safe for the human body, but the safety of modified phytosterols is not absolutely safe. In response to the above-mentioned physical and chemical modifications, Feng et al. studied their safety and came to the following conclusions: phytosterol esters are considered safe, but phytosterol oxides that may be produced during the synthesis process show toxicity in animal models. The toxicity of nanocarriers needs further study.

Sterols and Stanols

Stanols are derivatives of sterols, which are obtained by saturation process. They are more hydrophobic and more fat-soluble than sterols. Studies have shown that the absorption rate of plant sterols is much lower than that of cholesterol, and the absorption rate of plant stanols is even lower than that of plant sterols, only one-tenth of plant sterols and only one-thousandth of cholesterol. Like plant sterols, plant stanols can also help lower cholesterol levels by blocking the absorption of dietary cholesterol. One of the most well-known types of stanols is sitostanol, and studies have shown that adding it to the diet is particularly effective in lowering cholesterol levels.

What are Plant Sterols?

Plant sterols, also known as phytosterols, are natural alcohol compounds with cyclopentane polyhydrophenanthrene as the skeleton. Common ones include β-sitosterol, stigmasterol, campesterol and rapeseed sterol. They are widely found in vegetables, fruits, beans, nuts, grains and other plants. Plant sterols have outstanding lipid-lowering, anti-oxidation, anti-inflammatory, inhibiting cancer cell proliferation, repairing skin barrier, soothing skin and other functions, and are often used in food health care, cosmetics, medicine, agriculture and animal husbandry. At present, the synthesis methods of plant sterol esters are mainly divided into two categories, chemical synthesis methods and enzyme catalysis methods. Chemical synthesis methods mainly include direct esterification and ester exchange methods. The enzyme catalysis method selects a suitable lipase for catalysis, mainly Candida lipase.

Examples of Sterols

Sterols are a diverse class of lipids that play key roles in the structure and function of cell membranes in living organisms. Examples of sterols include cholesterol, plant sterols, and ergosterol, each of which has unique properties and biological functions. While cholesterol is best known for its role in human health and disease, plant sterols and ergosterol also play important roles in the health and survival of plants, fungi, and yeast.

Cholesterol

Cholesterol is a sterol found in animal cell membranes and synthesized in the liver. It is an important component of cell membranes and helps regulate membrane fluidity and permeability. Cholesterol is also a precursor for the synthesis of steroid hormones such as estrogen, testosterone, and cortisol, as well as vitamin D. While cholesterol is necessary for many biological functions, high levels of cholesterol in the blood can increase the risk of heart disease and stroke.

Plant Sterols

Another example of a sterol is plant sterols, which are plant-derived sterols that are structurally similar to cholesterol. Plant sterols are found in the cell membranes of plants and are thought to play a role in regulating membrane fluidity and permeability. Plant sterols have been shown to have many health benefits, including lowering LDL (bad) cholesterol levels and reducing the risk of heart disease. They are often added to margarine and other foods as a means of lowering cholesterol levels. Campesterol and stigmasterol are two types of plant sterols commonly found in plant foods such as nuts, seeds, and vegetables. These compounds are structurally similar to cholesterol and are thought to compete with cholesterol for intestinal absorption, thereby lowering cholesterol levels in the blood. Sitosterol is another plant sterol commonly found in vegetable oils such as corn oil and soybean oil. Like other plant sterols, sitosterol has been shown to have cholesterol-lowering properties and may help reduce the risk of heart disease.

Yeast Sterols

In addition to cholesterol and plant sterols, ergosterol is another sterol found in fungi and yeast. Ergosterol is an important component of the cell membranes of these organisms and is essential for their survival. Ergosterol is also a precursor for the synthesis of vitamin D in fungi and yeast.

What are Sterols Used For?

Sterols for Cosmetics

In the Catalog of Used Cosmetic Raw Materials (2021 Edition), 66 sterol raw materials can be retrieved. Among them, there are about 36 phytosterol raw materials, and the rest are cholesterol raw materials. Cholesterol plays a vital role in maintaining the structure and function of the skin. It is a key component of cell membranes and helps regulate the fluidity and permeability of cell membranes. Cholesterol is also a precursor for the synthesis of vitamin D, which is essential for skin health. In addition, cholesterol is also believed to have anti-inflammatory properties that can help calm and soothe irritated skin. By reducing inflammation, it can help relieve symptoms such as redness, itching, and sensitivity. Phytosterols have a proliferative effect on fibroblasts, promote the production of collagen, and have an inhibitory effect on melanocytes. UV protection, moisturizing, antioxidant, anti-inflammatory, and hair growth effects mean that it has a significant impact on skin health. It has the effects of inhibiting skin inflammation, repairing the skin barrier, slowing skin itching, preventing skin aging, improving skin elasticity, and reducing skin roughness. Therefore, phytosterols are best used in anti-aging creams and sunscreen lotions. Additionally, their anti-inflammatory effects make them an ideal ingredient in products that can be used to treat atopic eczema and protect baby's skin.

Sterols for Food and Health Products

Based on the cholesterol-lowering effect of plant sterols, a health craze for plant sterols was formed in Europe and the United States as early as 1950. In order to prevent chronic diseases, people look for and eat foods containing plant sterols to increase their intake of plant sterols. Later, they gradually began to add plant sterols to food and develop foods with high plant sterol content. As a result, the application and research of plant sterols in the food field are increasing. In addition, around the functions of lowering blood lipids, protecting blood vessels, and clearing blood vessels, different forms of sterol-based health products such as protein powder, capsules, and tablets have been developed.

Sterols for Pharmaceutical Industry

The application and development of plant sterols in the pharmaceutical field mainly focuses on regulating blood lipids, resisting digestive tract tumors, the synthesis of steroid hormones, and pharmaceutical liposomes. They are used to treat non-alcoholic fatty liver disease, protect gastric mucosa, and fight digestive tract tumors. Steroid hormone drugs are a class of drugs second only to antibiotics. In industrial production, the target product is obtained by structurally modifying natural substances with steroid skeletons. Liposomes are widely used in various pharmaceutical carriers because of their biodegradability, compatibility, targeting, and reduced toxicity. Plant sterols can improve the stability and targeting of liposome membranes, so they are often used in the preparation of pharmaceutical liposomes.

Sterols for Agriculture and Animal Husbandry

In livestock and poultry breeding, sterols are widely added to feed. As a functional feed additive, plant sterols can reduce the cholesterol content of livestock and poultry meat, eggs and milk products, promote animal growth, help animal health and increase production. Among plant growth regulators, brassinolide, with a chemical formula of C₂₈H₄₈O₆, is a new type of green and environmentally friendly plant growth regulator. It can promote the growth of crops such as vegetables, melons, and fruits through seed soaking and stem and leaf spraying at appropriate concentrations, improve crop quality, and increase crop yields. At present, the main source of brassinolide is stigmasterol as the main raw material, which is synthesized through sulfonylation, skeleton rearrangement, oxidation, double bond rearrangement, dihydroxylation and ring expansion reactions.