Physiological Functions of Phytosterols and Their Application in Food

Litian ZHANG Sining TANG Suyan GAO Yi ZHANG Ruihuan DU Aili XIANG Lei WANG

Abstract Phytosterols are a kind of active ingredients in plants with various physiological functions. Phytosterols have a good effect in antagonizing cholesterol and preventing cardiovascular diseases. They also have physiological functions including regulating growth， promoting protein synthesis， resisting inflammation， oxidation and cancer， and regulating immune and parahormones. In this paper， the physicochemical properties of phytosterols， the mechanism of lowering cholesterol， their application in food field and their development prospects were systematically reviewed.

Key words Phytosterol; Physiological function; Food

Phytosterols are a kind of natural organic compounds widely found in living tissues. They are one of the components that make up cell membranes in plants， as well as precursors for the synthesis of various hormones， vitamin D and sterol compounds， having important physiological functions. There are many kinds of natural phytosterols， with generally four molecular structures： brassicasterol， stigmasterol， campesterol and sitosterol[1]. At present， more than 250 phytosterols have been identified from plants， mainly in the form of free state， steryl glycoside and acylated steryl glycoside， sterol ester （fatty acid ester and phenolic acid ethers）. Phytosterols can control glycogen and mineral metabolism， maintain environmental stability inside organisms， and regulate stress responses. In recent years， with the rapid development of biological science， oil science and engineering technology， phytosterols have received much attention in the fields of food， medicine， feed and chemical industry.

Sources and Properties of Phytosterols

Sources of phytosterols

Phytosterols are widely found in various vegetable oils， their processing by products， nuts and plant seeds， and also exist in other plant foods such as vegetables and fruits， among which wheat germ oil， maize germ oil， rice bran oil， sesame oil and safflower oil have a high content. Large amounts of sterols are enriched in the distillates of rapeseed oil， soybean oil and rice bran oil， mainly stigmasterols， sitosterols and campesterols[2].

Sitosterols （including campesterols） are used in fermentation to produce androst 1，4 diene 3，17 dione （ADD） and androst 4 ene 3，17 dione （4AD）. Stigmasterols are chemically broken from the C 22 double bond and then structurally modified to serve as a variety of steroidal corticosteroid medicines.

Properties of phytosterols

Phytosterols can react with fatty acids to form fatty acid esters， can be oxidized to ketones and can react with other strong acids such as sulfuric acid. In addition， they can also complex with organic acids （such as oxalic acid， malic acid and succinic acid）， urea and digitonin.

The purified phytosterol mixture is a flake or powdery white solid which is white scaly or acicular crystals by solvent crystallization. The physicochemical properties of phytosterols are mainly hydrophobic， but they are also hydrophilic because of their hydroxyl group structure. Therefore， on the one hand， having both a hydrophilic group and a lipophilic group in the same substance structure means that the substance is emulsifiable， and the emulsifiability of the phytosterol can be improved by chemically modifying the hydroxyl group. Phytosterols have amphoteric characteristics that make them capable of regulating and controlling the fluidity of reversed phase membranes. On the other hand， phytosterols are mainly hydrophobic， but they have certain hydrophilicity due to the hydroxyl group in their structures. The larger the side chain， the stronger the hydrophobicity of sterols. Phytosterols have a slightly larger specific gravity than water， is insoluble in water， and is soluble in various organic solvents[3].

Physiological functions and Effects of Phytosterols

Phytosterols have good physiological functions， and are capable of affecting the metabolism of cholesterol， improving the anti tumor effect of the organism， affecting the synthesis of parahormones， resisting oxidation， resisting inflammation and reducing fever， and protecting skin （Fig. 1）.

Effect of phytosterols on cholesterol metabolism

At present， phytosterols exact action mechanism of lowering cholesterol is not fully understood. There are several main possibilities： ① inhibiting intestinal absorption of cholesterol， ② promoting cholesterol dissimilation and ③ inhibiting the biosynthesis of cholesterol in the liver. Phytosterols are similar in structure to cholesterol and are absorbed in the same way as cholesterol in living organisms， while the absorption rate of sterols in animals is less than 10% of cholesterol. As sterols can inhibit cholesterol biosynthesis in the liver， promote cholesterol dissimilation， and inhibit cholesterol in the intestines， phytosterols are good cholesterol lowering drugs. Studies have shown that phytosterols can  even  lower cholesterol level in organisms by 9%-17% for patients who use statins， but different structures of phytosterols have different cholesterol lowering effects. For instance， sitosterols and stigmasterols have a quite obvious effect of lowering cholesterol， while the effects of campesterols to lower cholesterol are relatively weak[4].

Anti tumor effect of phytosterols

Phytosterols have been shown to have good anti tumor effects. Metabolites produced by the action of intestinal microbes are easy to induce colon cancer and inflammation. On the contrary， phytosterols promote the direct excretion of cholesterol itself， reducing the microbial catabolic products of cholesterol and preventing tumors. Some phytosterols can directly inhibit tumors. For example， sitosterols have been shown to be effective in inhibiting the growth of human colon cancer cells HT29， epithelial cell proliferation and chemically induced rat colon tumors， thereby being used as an antitumor drug. Studies have found that when β sitosterols and their glucosides are mixed at a ratio of 100∶1， they can stimulate the proliferation of peripheral lymphocytes in vitro. Bouic et al.[5] reported that such mixtures can stimulate the activity of Th 1 type CD4 cells and increase the phagocytic  ability  of NK cells to tumor NK562. De et al.[7] conducted a study on the relationship between the intake of β sitosterols， campesterols， stigmasterols and total phytosterols and the occurrence of gastric cancer of the Uruguayan population， and the results showed that the intake of total phytosterols and the occurrence of gastric cancer were in negative correlation in the Uruguayan population. Vivancos et al.[6] found that β sitosterols can improve the activity of glutathione peroxidase and superoxide in macrophages by studying the regulation of β sitosterols on the antioxidant enzyme activity in RAW 264.7 macrophages. The improvement of the antioxidant capacity of macrophages is of great significance to the improvement of organisms defense ability and phagocytic ability to tumor cells.

Parahormone function

Phytosterols are highly compatible with tissues such as the adrenal gland， liver， testis， and ovary that synthesize steroid hormones， so they are considered to be precursors of steroid hormones for synthesizing steroid hormones. β sitosterols reduce plasma steroid hormones， cholesterol level and in vitro gonadal steroid level in fish[8]. Long term addition of β sitosterols in the feed reduced the number of infertile female minks， while the number of successfully regenerated ones increased significantly[9]. Phytosterols increased the serum testosterone concentration in male rats， and the concentration of serum estradiol in the third generation of female rats increased[10]. In addition， phytosterols can induce an increase in thyroid activities， significantly increasing total thyroxine， total triiodothyronine and free triiodothyronine levels in  serum[11].

Anti oxidation effect

The aging of human body tissues is the main cause of aging. The membrane structure includes phospholipids， glycolipids and sterols， the abnormal changes of which directly lead to the aging of biological tissues. In the membrane structure， sterols play a key scaffolding role. They can limit the free swing of the long chain hydrocarbon groups in fatty acids， reduce membrane fluidity and maintain membrane integrity， thereby retarding membrane aging. When the ratio of sterols in the membrane is lowered， the long chain hydrocarbon groups in fatty acids of the two layer membrane cannot swing freely， and the positions of phospholipid molecules are fixed， which causes the membrane to become rigid due to loss of flexibility and the protein to lose activity， finally resulting in aging or defect characteristics of the membrane which loses adaptability and has the functions greatly damaged. Appropriate intake of phytosterols to supplement the sterol content in cells or tissues of human body can delay aging[12].

Anti inflammatory and analgesic effect

Studies have shown that phytosterols can inhibit the proliferation of bacteria and fungi， and have obvious anti inflammatory and antipyretic effects for treating ulcers. For example， sitosterols have anti inflammatory effects similar to those of hydrocortisone and oxyphenbutazone， and have no side effects. Sitosterols also have an antipyretic effect similar to aspirin （acetylsalicylic acid） with low side effects.

Skin care effect

Skin care is a function of most sterols. Since sterols are W/O type emulsifiers， their emulsification performance and stability are very good， and they have high permeability to the skin， and are excellent skin care ingredients. In addition， phytosterols have weak affinity and act as a regulator in shampoo and hair conditioners， which can enhance hair toughness and protect the scalp. Currently， sterols are widely used in various cosmetics in many countries such as the United States and Japan.

Application of Phytosterols

Application in food

In recent years， more and more countries have strengthened phytosterols into related products such as edible oil products， dairy products， feed additives and cosmetics， which have been strongly supported by the government and welcomed by consumers. In the mid 1990s， in order to enhance the solubility of phytosterols and expand their applications， Finnish scientists esterified them with fatty acids， and the first commercially valuable functional food containing phytosterols appeared （trade name： Benecol）.

Application in drinks

In March 2001， the Kao Corporation of Japan launched Ecoa H&H Cooking Oil， a special food edible fat that added with 4% phytosterols. In December 2003， the US Coca Cola Company developed the "Minute Maid Heart Wise"[13]， a 100% citrus juice added with phytosterols. In March 2004， phytosterols were approved by the European Union Standing Committee and can be sold and used in Europe as new resource foods or food ingredients. In 2006， the Italian company EI launched "Sauscol Bianco"， a probiotic beverage containing phytosterols. In 2008， the European Food Safety Authority （EFSA） issued an announcement which further affirmed that phytosterols have a cholesterol lowering effect[14].

Application in medicine

The application trend of phytosterols in the food field is mainly as a functional component for treating cardiovascular and cerebrovascular diseases. Nowadays， products such as mayonnaise， dessert， milk and edible oil added with phytosterols have been introduced， and foods containing phytosterols are forming an emerging health boom in countries such as Europe and America[15]. In China， since the launch of the first phytosterol containing maize oil in 2009， more and more edible oil producers have begun to produce phytosterol enriched maize oil with an average mass concentration of 10 g/L. At the same time， the heart nourishing dairy products added with phytosterols have gradually entered peoples lives， bringing good news for human health.

Prospects

China has abundant phytosterol resources such as oils， wood pulp leftovers and Chinese herbal medicines， and has favorable conditions for producing plant phytosterols. At present， functional products containing phytosterols are favored by consumers. However， research and development of them in China is still at an early stage， limited to the development of pharmaceuticals， and the research and development on functional food bases is blank. With the recognition of the role of phytosterols in lowering cholesterol， the consumption of functional sterol products has increased significantly. In particular， the improvement of phytosterol content in edible oils and fats by moderate refining or addition has been highly valued by the oil industry. Therefore， it is of great significance to develop a simple and effective phytosterol production process and to study its new use. Only in this way can we participate in international competition and achieve high social and economic benefits.

Agricultural Biotechnology2019

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