Karl H. Pegel

Department of Chemistry and Applied Chemistry of Natal, Durban, 4041 South Africa

Published in the South African Journal of Science Vol 93 June 1997

Sitosterol, the principal phytosterol in most higher plants and hence in plant- derived food products, is found in the serum and tissue of healthy humans in concentrations 800-1000 times less than the endogenous cholesterol. The glucoside of the sitosterol (sitosterolin) is present in mammalian serum at even lower concentrations. In many animals, sitosterol and sitosterolin concentrations relative to cholesterol are considerably higher than in humans . Only plants can synthesize these compounds and humans and animals obtain them from their diet. Even though their absorption efficiency is low (0.1 and 0.02 for sitosterol and sitosterolin respectively relatively to cholesterol), their apparent synergistic stimulatory effect on the immune system and prophylactic effect on a variety of diseases of civilization indicates their importance in human and animal nutrition. Since modern food processing tends to reduce their concentration in processed plant-food products, and eating habits also effect their consumption adversely, it is desirable to eat sufficient unrefined or unprocessed plant foods or resort to food supplements containing sitosterol and sitosterolin

Epidemiological studies have shown that diets rich in vegetables and fruit reduce the risk of developing various types of cancer, ¹ cardiovascular disease, ² diabetes ³ and other common ailments of civilization,⁴ Ubiquitous components of such diets identified as disease preventing agents are Vitamin E (tocopherols), B-carotene (carotenes), Vitamin C (ascorbic acid ), flanonoids (vitamin P, rutin, naringen, etc.) and various phyto-oestrogens (genistein, daidzein, biochanin A, formonotion, coumestrol, etc.) However, none of these substances, except for Vitamin C and Vitamin E has shown any great improvement health when given individually in its pure form and in certain instances caution may even be desirable when some of them are taken over protracted periods (e.g.b-carotene ⁵ and phyto-oestrogens ⁶). Many of these studies also report that plant constituents other than those mentioned above , for example sitosterol, may contribute to or be responsible for the observed health promoting effect of plant diets.^4,7.

Animal synthesize only cholesterol, but plants have the biosynthetic ability to produce , besides small amounts of cholesterol, a number of plant sterols of which the most common are campesterol, sitosterol stigmasterol, and in some plant families the D⁷-sterol analogues in which the D⁷=stigmasterol and spinasterol are most abundant. ^8,10 These plant sterols contain a n extra alkyl group at C-24 in the side chain, providing more bulk (volume) to the molecule in comparison to cholesterol. One of the most important functions of cholesterol in animals and phytosterols in plants presence as components in the bilamellar (endofacial and exofacial) cell membranes.¹¹ In addition, cholesterol is converted in animal and plants to Vitamin D₃¹² and pregnenolone which, particularly in animals, then gives rise to various sex steriods and corticosteroids and in plants( and perhaps animals also),^13,14 the cardenolides and bufadienolides.^8,9,15 Plants also use cholesterol to produce spiroketal steriods (diosgenin, solasodine, etc.).¹⁵ Sitosterol n be degraded to pregnenolone in animals ^16,17 and plants ¹⁵ and hence to all the steroid hormones derived from pregnenolone and it’s C-21 analogues, but plants do not seem effectively to utilize their 24-alkylsterols for vitamin D or spiroketal steriods synthesis ^,9,15

Dietary sterols absorbed by all animals and most of those investigated (including dog, pig, mouse, rat, and sheep contain about 10-20 times more sitosterol in their serum and tissues than humans (-5mM)^22-27 In healthy humans the sitosterol to cholesterol ratio on a molar basis is about 1 to 800-1000 ^28,32 In humans the absorption of cholesterol is about 50% while that of sitosterol is about 5%^17,33 Campesterol (24cc-methylcholesterol), with its smaller C24 alkyl group, is absorbed more readily (+20%) than sitosterol (24&-ethylcholesterol) but less efficiently than cholesterol.^17,29 A small percentage of humans are hyperabsorbers of plant sterols (called phytosterolaemics, with 5-10 times the normal serum sitosterol and lower campesterol levels)with 10-100 times the normal serum sitosterol and higher campesterol levels) and there are a few individuals in whom sitosterol exceeds the absorption of campesterol (sitosterolaemics,with 10-100 times the normal serum sitosterol and higher campesterol levels), reaching sitosterol serum levels found on many animals but accompanied in humans by serious health consequences.^17,21,34 It is known however that sitosterols and sitosterolin on oral administration are not toxic,^36,38 Moreover ,sitosterols acts as a plasminogen activator ^39,40 and promotes the formation of polyunsaturated fatty acids from linoleic acid,⁴¹ whereas high serum cholesterol levels seem to reduce plasminogens activator activity ^39,42 and the ability to convert linoleic acid to the essential polyunsaturated fatty acids,⁴³ needed for prostaglandin and leukotriene synthesis important in cell-mediated immune functions.⁴⁴

promotes chemically induced colon carcinogenesis in rats ^57,58 and inflammation,⁵⁹ but this does not necessarily apply to its glucoside, cholesterolin. ³⁸ Sitosteryl glucoside has a protective effect on saponin-induced haemolysis at a dose(PD₃₀) of 1.7 mg or (3.0 mM), whereas sitosterol showed no such effect.⁶⁰ Intravenous administration of SG at 50 mg kg^-1 protects against histamine induced vascular permeability (up to 50%) in rats ²¹ and in guinea pigs ⁶¹ and it increases haemostatic activity at 25 mg kg^-1 by about 18% in mice; this latter effect is slightly reduced (-10%) on oral administration (2 mg kg^-1 ).²¹ It has also been shown that, individually, sitosterols and its glucoside have a proliferating effect on T-cell production in vitro, still noticeable at the remarkable low level of 10 pg l^-1 and 1 pg l (24 fM and 1.7 fM, respectively, with a synergistic enhancement when both are given together . Also, when these two compounds were given to human volunteers on a normal diet, an enhanced T-cell proliferative response was observed after 4 weeks of daily oral supplementation with 60 mg sitosterol and 0.6 mg of its glucoside.⁶² Interestingly, cholesterol showed rather a T cell suppression reponse in the in vitro test, even at 1 mg l^{-1 63} This strongly suggests that plant sterols and sterolins, and particularly sitosterols and sitosterolin at surprisingly low concentrations, have a beneficial effect on the immune system and that their low absorption rate is of little consequence in relative terms. The important factor is an adequate maintenance of their body pools via a sufficient and constant dietary supply in view of the fact that body pools of both sitosterol and sitosterolin are rapidly diminished on a diet devoid of either compound.^21,33,62 Reports exist about anti-inflammatory.^61,69 anti-ulcer, ^70-74 anti-diabetic ^75,77 and anti-cancer activity ^{49,56,65,67,78-82} of both sitosterol and sitosterolin.

Because sitosterol is absorbed less efficiently (1/10) than cholesterol, it has unsurprisingly been found to lower the absorption rate of cholesterol when given at relatively high oral doses (3-30 gM) and even the normal daily dietary supply of plant sterols at 200-240 mg (-130-160 mg sitosterol) seems to have an effect on cholesterol serum levels. ^81,84 For this reason sitosterols was and still is used in some countries for the treatment of mild cases of hypercholesterolaemia.^17,57,83,84 The side effects of these high doses (other than occasional constipation ⁸⁵ and diarrhoea ⁸⁶) have never been reported nor have any health promoting effects been observed or particularly looked for other than an improvement in serum cholesterol status. ⁸⁶ Since 1974, sitosterol in combination with its glucoside (S, SG in the ratio 10 : 0.1 mg per capsule Harzol ™) has been used in Germany for the treatment of benign prostrate hypertrophy (BPH), usually at a daily dose of 3 x 2 capsules or 60 mg plant sterol per day (-44 mg sitosterol plus campesterol, campestanol, dihydrosistosterol and stigmasterol. ^86,84 BPH patient response is in general positive, although other health benefits have been reported .^86,89 A similar product used to be available in Germany for the treatment of soft-tissue rheumatism (Flemun^®,10 mg sitosterol with 0.1 mg glucoside) with moderate success,⁹⁰ but it is no longer marketed. Sitosterol and its glucoside ,either singly or in combination , are found in BPH remedies from Serenoa repens (Saba verrulata),^91,92 Pygeum africanum (Prunus africana) ^97,98 and as the D⁷ analogues in pumpkin seed, ^94,97 Sitosterol glucoside and hence sitosterol are also present in many popular plant remedies such as Haragophynum procumbens (devils claw), commonly used for the treatment of rheumatic complaints, ^95,96 Silybum nutrianum (milk thistle) extracts used for the treatment of liver complaints ^97,98 Gingko bilibu extracts for the treatment of cardiovascular illnesses ^99,100 Panax ginseng popular as a universal tonic ^96,101 and Eleutherococcus senticosus (Siberian ginseng) also used as a general tonic in which sitosterol glucoside has been identified as, one of the ‘Adaptogens’ (eleutheroside A).^96,102,103 Adaptogens are a group of natural plant products which promote overall health without the rapid response normally elicited by a drug and without the side effects associated with any drug used.^102,104

Plant sterols are obligatory metabolites in all higher plants, ferns and many algae together with their glucosides. Since sitosterol is in the most instances the major plant sterol it is not surprising that this compound, often together with its glucoside, has been isolated from or identified in many plants and is occasionally mentioned in epidemiological reports as a possible contributor to the health promoting of vegetable and fruit diets ^4,105,106,. Indeed, sitosterol and its glucoside have been evaluated for a variety of biological activities, ^{21,36,42,45,56,60,90,104,108.} However, it has been stated that ‘the ubiquitous occurrence of sitosterol , plant sterols and in general, and their glucosides in all vegetables and fruit makes it highly unlikely that they have any drug related properties and many reports of their medicinal properties are based on in vitro or unrealistically high in vovo doses, which make a therapeutic application of these compounds highly unlikely.^76,77.

In a way this statement is correct, since sitosterol and sitosterolin are not drugs in the accepted sense, but rather slow acting essential micronutrients or adaptogens better considered as minor but nevertheless important cell membrane constituents. Nevertheless, popular micronutrients such as Vitamin E, Vitamin C and b-carotene belong to this category and have found wide acceptance associated with a considerable volume of research and promotional literature publicizing their use and health advantages These three compounds (Vitamin E, Vitamin C and b-carotene). Like plant sterol and sterolins.are primary plant metabolites and, in the course of evolution, animal and human have become adapted to their dietary availability and integrated them more or less with their own metabolic needs,¹⁰⁹ Animals cannot synthesize carotenoids, but they need a small steady supply of b-carotene to produce vitamin A (retinol). Animals make their own vitamin E analogue, ubiquinone / ubiquinol. ¹¹⁰ which is more hydrophilic (water soluble) than vitamin E, and which provides antioxidant protection in fatty tissues/environments.This is perhaps not so important in a lean health population, but may be of vital consequence in a affluent adipose society. Vitamin C can be synthesized by most animals, but in humans this ability has been lost probably because the larger portion of their evolutionary diet was based on processed plant food, as it still is in primitive societies. Any prolonged reduction in serum and tissue levels of Vitamin C will result in serious health problems. Vitamin C supplementation of the diet then becomes a necessity. This may also apply to the plant sterols and their glucosides (sterolins).^{36,37,107,108} A good quality diet providing mainly unprocessed plant food will readily supply a daily amount of 200-300 mg plant sterol (containing -65% sitosterol, 130-195 mg ^{70,86, 111,113} and roughly 10% that value of their glucosides,²⁰ However, the actual intake of plant sterol in different countries and among population groups may differ considerably and range from 40-400 mg per day ^{17,27,32,37,51,88,111-118} Surprisingly, some vegetarians have daily intakes of below 100 mg (-65 mg sitosterol).⁵¹

Vegetables contain from 5-40 mg of plant sterols, on average 20 mg, ¹¹⁹ per 100 g and this can greatly increase to 100 mg on drying, since they contain about 80 % water. ¹²⁰ Fruit contains from 2-30 mg plant sterols and average 15 mg per 100 g . All seeds are rich in plant sterols when mature, ranging from 22-714 mg per 100 g - and average value is about 120 mg 100g ^1,119 - but can be considerably less when seeds are immature. 121 This includes all edible seed and culinary products produced from them such as nuts, cereals, beans and seed-derived spices. The plant sterol content in vegetables, fruit and seed embraces the four classes of sterol compounds (S, SE, SG and ASG), of which the glucoside portion, SG and ASG, is usually one tenth ^20,122 but can occasionally be surprisingly high in potatoes, which contain per 100 g about 40 mg plant sterols. of which 16 mg is sitosterol and 28 mg is in the form of SG and ASG.¹²³

The sitosterol content of these plants sterol varies between 40-80% and a good average can be taken as 65% ^{25,88,111-113} Cooking does not destroy these compounds, but boiling water may remove some of the sterolins (SG,and ASG) if the water is discarded, since sterolins have a water solubility of 10 mg-1 at room temperature ^87,122 which is considerable higher at 100ºC. Sterols (S and SE) are practically insoluble even in boiling water,¹²⁴ Slicing, grating, macerating and juicng of fresh plant material may reduce the SG and the ASG content through enzymatic hydrolysis to S, but will not affect the overall S content, the same applies when fruit and vegetables age.^121,123,125 The sterol content of processed refined plant products can alter greatly. Thus while wheat grains contain per 100 g about 4200 mg of plant sterols (S, SE, SG and ASG), of which 1900 mg is in the glucoside fraction, its flour contains per 100 g about 52 mg of total plant sterols of which 15 mg is of the glucoside fraction. The removed bran on the other hand contains about 4500 mg of the plant sterols and the unrefined oil about 2600 mg, of which 740 mg is sitosterol per 100 g,^126,128 The same applies to other cereal products like rice, corn (maize) and rye. Crude plant oils are thus a relatively rich source of phytosterols and their glucosides, but a large proportion, and especially the all glucosides, are removed during refining process.^36,119,126 Thus, while Soya beans contain per 100 g about 160 mg of total plant sterols, of which 90 mg is sitosterol and 50 mg is in the glucoside fraction, the crude oil contains about 350 mg of total sterols which is reduced to 220 mg on refining, completely removing the glucoside fraction; on hydrogenation the sterol content is further reduced to 130 mg per 100 g.^{36,119,129,130} this applies to all plant oils, which even in the refined form are rich source of plant sterols, but lack the sterolins. Plant sterols, but not their glucosides are therefore also present in margarines. ^36,131 but only traces are found in butter obtained from free range cows.¹³²

In order to consume 100 mg of plant sterols a person would have to eat about 500-700 g of fresh vegetables and fruit, about 200 g of flour products without additives or 250 g of potatoes and this amount has to be doubled to reach 200 mg which is acceptable for a normal dietary supply. Even an apparently quality diet may be inadvertently selected in such a manner that the total plant sterol and sterolin intake is just not adequate to maintain a proper serum level of sitosterol.^51,123 and its glucoside relative to serum cholesterol levels apparently necessary for an efficient functioning immune system.⁶² The situation is made worse when daily consumption relies on processed food intake. The first occurs frequently under conditions of stress, while the latter commonly arises during slimming routines, ill health and old age. ¹¹⁶ Especially in these cases dietary plant sterol and sterolin supplementation, either by judicious selection of food products or by plant sterol and sterolin supplements, is indicated. Since many animals have considerably higher sitosterol concentration levels in their serum when living in their natural environment, any artificial feeding may result in plant sterol ^133,132 and sterolin ²¹ deficiencies with serious long- term consequences ⁵² which as yet have never been investigated nor even suggested.