Plant Sterols and Cholesterol Reduction

Plant sterols, which are not synthesized by humans, are naturally occurring plant compounds. The most common plant sterols are 3-sitosterol, campesterol and stigmasterol, from which 3-sitosterol is the most abundant one (constituing 90 per cent of the total). The hydrogenation of unsaturated bonds in plant sterols results in plant stanols. Plant sterols and stanols are structurally related to cholesterol, but contain an extra methyl or ethyl group on the cholesterol side chain. Plant sterols and stanols occur naturally in food as free alcohol, esterified with long-chain fatty acids (25±80 per cent of total plant sterols) or conjugated as glycosides (usually in small amounts).

The main sources of plant sterols in the basic diet are cooking oils and margarine. Bread and cereals can also contribute significantly to total plant sterol intake (Morton et al., 1995). The majority of plant oils contains 0.1±0.5 per cent, while some germ oils (rice bran, wheat germ, oats) contain up to 4 per cent total plant sterols. Reduced-fat health spreads on the market contain approximately 0.3±0.4 per cent plant sterols or stanols. Individual concentrations of plant sterols in cereal grains, vegetables and fruits are reviewed by Piironen et al. (2000). Vegetables and fruits contain <0.05 per cent (based on the edible portions), except for seedlings of barley, beans, and peas, which contain 0.1-0.2 per cent plant sterols. Some seeds are also rich sources: sunflower and sesame seeds contain 0.5-0.7 per cent and legumes can contain 0.22 per cent plant sterols.

In the Netherlands an average daily intake of plant sterols of 263 (women) to 307 mg (men) has been estimated (NormeÂn et al., 2001). In the United Kingdom, this intake is estimated to vary between on average 150 and 200 mg (Morton et al., 1995). For the United States, average daily intakes of plant sterols are reported between ca. 180 and 250 mg (Connor, 1968) and for Japan about 400 mg (Hirai et al., 1986). The Tarahumara Indians of Mexico, who consume a diet containing unusually high amounts of beans and corn, ingest over 400 mg of phytosterols per day (Cerqueira et al., 1979). In general the intake of adult vegetarians and their children is higher (up to 40 per cent) than the average for the population as a whole (Nair et al., 1984; Miettinen et al., 1990; Ling and Jones, 1995).

The plant sterols used in the 1950s were poorly soluble in fat and high amounts (10±20 g per day) were needed to obtain substantial reductions in serum cholesterol levels. This limited the practical use of plant sterols as cholesterol-lowering agents for many years. Esterification of plant sterols and stanols with fatty acids increases this solubility in fat, thus allowing lower concentrations to be incorporated in foods and increasing its functionality at lower dietary intakes. This renewed the interest in plant sterols and stanols as blood cholesterol-lowering agents. Plant sterol mixtures from various sources have been isolated and modified, i.e. hydrogenated and or esterified, for application as a functional food ingredient. Furthermore, new technologies have been developed to improve solubility and functionality of unesterified plant sterols and stanols. At present many plant sterol and plant stanol-enriched food products are on the market and claim cholesterol-lowering activity.

 

Plant sterols and stanols are known to have a marked effect on human blood cholesterol, specifically a lowering of low-density lipoprotein (LDL) cholesterol which is known to be the risk factor for cardiovascular diseases. It has been shown that the main mechanism to lower blood cholesterol is by inhibiting dietary cholesterol absorption as well as biliary cholesterol absorption (Grundy and Mok, 1976; Heinemann et al., 1991; Gylling et al., 1997). The precise mode of action is not yet clear, but the postulated mechanism is that plant sterols and stanols are able to displace cholesterol in bile acid micelles due to competition for micellar solubilization (Grundy and Mok, 1976; Ikeda et al., 1989). As plant sterols and stanols are more hydrophobic than cholesterol, they have a higher affinity for micelles. As a consequence the micellar cholesterol concentration will be reduced, resulting in a decrease of cholesterol absorption.

In addition, some other mechanisms for the cholesterol-lowering effect have been proposed. It has been suggested that plant sterols and stanols influence the cholesterol metabolism in the enterocyte, resulting in an increased excretion of cholesterol by the enterocyte back into the intestinal lumen (Plat and Mensink, 2002). At present, scarce evidence for this effect is available. Also effects of plant sterols and stanols on cholesterol excretion via the enterohepatic cycle have been suggested. However, no consistent evidence is available. Bile salt excretion was reduced in transgenic mice after consumption of plant stanols (Volger et al., 2001) but remained unchanged after consumption of plant stanols by colectomized patients (Miettinen et al., 2000) or plant sterols in ileostomy patients (NormeÂn et al., 2000).

In 1981 Pollak and Kritchevsky reviewed published studies on the clinical use of plant sterols. The authors estimated that clinical data on the cholesterol-lowering action of plant sterols was available in about 1800 subjects at that time. A significant reduction of plasma cholesterol from the pre-treatment level was reported after consumption of extremely high amounts of plant sterols (10±20 g/ day) in the unesterified form. From the time of this review until 2003 many clinical trials were performed on the cholesterol-lowering capacity of plant sterols and stanols at lower intake levels and mainly in its esterified form. Published papers on this topic, specifically from the mid-1990s until mid-2003, are discussed in this post.

Plant sterols have lowered serum total and LDL-cholesterol under a wide range of different study conditions. The dosages of plant sterols and plant stanols given in these studies ranged from 0.7 to 3.4 g per day. In four of the reviewed studies, plant sterols or stanols were in the unesterified form (Denke 1995; Christiansen et al., 2001; Vanstone et al., 2002; Jones et al., 2003). Total cholesterol was decreased by up to 13.4 per cent and LDL-cholesterol by up to 16.0 per cent. Levels of serum HDL-cholesterol and triacylglycerols remained more or less unchanged. In two out of the 25 reviewed studies the cholesterol-lowering effect of either unesterified plant stanols (Denke, 1995) or unesterified plant sterols (Jones et al., 2003) could not be demonstrated.

Recently, a meta-analysis was performed by Katan et al. (2003). They demonstrated that at intake levels higher than 1.1 g/day the mean reduction in the LDL-cholesterol level was 10.1 per cent (95 per cent confidence interval 8.9±11.3 per cent) in 27 trials testing stanols (mean dosage 2.5 g/day) and 9.7 per cent (95 per cent confidence interval 8.5±10.8 per cent) in 21 trials testing sterols (mean dosage 2.3 g/day).

Stanols and sterols are effective in lowering plasma lipoproteins within a few weeks, the effect remaining stable in studies during 1 year. Miettinen et al. (1995) reported a reduction 8.5 per cent of LDL­cholesterol in a hypercholesterolaemic Finnish population after consumption of 1.8 g/day plant stanol esters during one year. Another study of one year con­ducted by Hendriks et al. (2003) showed a sustained cholesterol-lowering effect of daily consumption of 20 g spread enriched with 1.6 g plant sterol esters during one year in Dutch normocholesterolaemic and mildly hypercholesterolaemic men and women. This cholesterol-lowering effect was consistent throughout the study: cholesterol was reduced on average by about 4 per cent and LDL­cholesterol on average by about 6 per cent. The differences in response between both studies are probably not caused by differences in efficacy between stanols and sterols but by differences in, for example, blood cholesterol levels between study populations.

It has been suggested that plant stanols are more effective in cholesterol lowering than plant sterols since 3-sitostanol has shown to be more effective than 3-sitosterol in the inhibition of cholesterol absorption (Heineman et al., 1988, 1991; Ntanios and Jones, 1998). Also Katan et al. (2003) suggested a higher efficiency of plant stanols. They indicated that although the difference in LDL-lowering capacity between plant stanols (10.1 per cent) and sterols (9.7 per cent) was not statistically significant `the comparison lacked the statistical power to detect a moderate difference’. There is a wide range in efficacies over the various studies testing either sterols or stanols. Straightforward conclusions on the cholesterol-lowering effect of plant sterols and plant stanols can be drawn only from a direct comparison in the same study. Efficacy of plant sterols and plant stanols has been compared directly in several studies so far (Weststrate and Meijer, 1998; Hallikainen et al., 2000; Jones et al., 2000; NormeÂn et al., 2000). These short-term human trials, either dietary controlled or in free-living conditions, showed a similar (Weststrate and Meijer, 1998; Hallikainen et al., 2000; NormeÂn et al., 2000) or better (Jones et al., 2000) efficacy in cholesterol lowering for spread enriched with plant sterol esters than for spread enriched with plant stanol esters. Although it is difficult to draw firm conclusions from these results, most probably there is no difference in the cholesterol-lowering effect of plant sterols compared with plant stanols.

Besides dosage and hydrogenation (sterols vs stanols), a number of factors have been suggested as influencing the efficacy of plant sterols and stanols. The possible influence of esterification, matrix, diet, blood cholesterol concentration and eating moment are discussed in the following sections.

Esterification of plant sterols and stanols increases solubility, thus providing a technically feasible way of introducing plant sterols into edible fats and oils (Wester, 2000). The cholesterol-reducing effect of plant sterols and plant stanols may be dependent on the physical state (i.e. esterified or unesterified). Jones and Raeini-Sarjaz (2001) reviewed several studies in this field. They concluded that unesterified sterol and stanols can have the same effect on plasma cholesterol as sterol and stanol esters. A great cholesterol-lowering potential of unesterified sterols and stanols when incorporated in high-fat products. Vanstone et al. (2002) examined the effects of unesterified plant sterols and stanols on total and LDL-cholesterol concentrations in 15 people with hypercholesterolaemia. When added to butter in a daily dosage of 1.8 g, total cholesterol decreased up to 13.1 per cent and LDL-cholesterol up to 16 per cent. Also Christiansen et al. (2001) showed that use of spreads enriched with unesterified plant sterols by people with hypercholesterolaema decreased total and LDL-cholesterol concentrations up to 11.3 per cent. However, especially for unesterified sterols and stanols, the matrix seems to be important for their efficacy. For instance, Jones et al. (2003) incorporated unesterified plant sterols in a non-fat or low-fat beverage which were consumed by people with moderate hypercholesterolaemia on a controlled diet for 21 days. They concluded that intake of plant sterols as part of non-fat or low-fat beverages did not exert any greater hypocholesterolaemic effect than a non-fat placebo beverage. Furthermore, Meguro et al. (2001) compared the cholesterol-lowering effects of unesterified plant sterols dissolved in triacylglycerols and given with mayonnaise with the effects in diacylglycerols. No serum total cholesterol-lowering effect was observed when dissolved in triacylglycerol. Free plant sterols dissolved in diacylglycerols reduced LDL-cholesterol more than when dissolved in triacylglycerol (Meguro et al., 2001).

For plant sterol and stanol esters the matrix seems to be less important. Also when incorporated into low-fat products such as bread and cereals or low-fat yogurt they have shown to be effective in cholesterol lowering (Nestel et al., 2001). Also Mensink et al. (2002) demonstrated that daily intake of 3 g plant stanol acid esters reduced LDL-cholesterol by 14 per cent when incorporated into a low-fat yoghurt.

It has been suggested that composition of the diet might influence the cholesterol-lowering capacity of plant sterols and plant stanols. Mussner et al. (2002) demonstrated that the LDL-cholesterol-lowering capacity of margarine enriched with plant sterol esters enriched margarine was more pronounced at higher cholesterol and fat intake. Furthermore, Denke (1995) could not demonstrate a cholesterol-lowering effect of 3 g/day sitostanol in people with hypercholesterolaemia when supplemented to a diet low in cholesterol. However, evidence has became available in recent years that plant sterols and plant stanols are effective in cholesterol lowering in people with hypercholesterolaemia consuming low-cholesterol or low-fat diets (Cleghorn et al., 2003; Hallikainen et al., 2000; Maki et al., 2001; Shin et al., 2003).

Efficacy of plant sterols and plant stanols may be higher at high blood cholesterol concentrations. Mussner et al. (2002) investigated the cholesterol-lowering effects of 3-week consumption of margarine enriched with plant sterol esters. They demonstrated that the efficacy was more pronounced at higher basal cholesterol absorption. The correlation between initial cholesterol concentration and the degree of cholesterol reduction was demonstrated by our own results (Hendriks et al., 2003). For both total cholesterol and LDL-cholesterol, the cholesterol-lowering after consumption of spread enriched with plant sterol esters tended to be higher when baseline cholesterol concentration was higher. Dividing the study population into quartiles based on baseline cholesterol concentration mean total cholesterol-lowering during the study was 0.2 per cent for the lowest quartile and 5.1 per cent for the highest quartile. Percentage reduction of LDL-cholesterol was 1.5 per cent for the lowest quartile and 7.2 per cent for the highest quartile.

Since the supposed mechanism for cholesterol-lowering is inhibition of cholesterol absorption from the intestine, it may be expected that a constant presence of plant sterols in the intestine may be needed for an optimal effect. It can therefore be hypothesized that for the best effect, plant sterols should be ingested simultaneously with meals containing cholesterol in order to block the absorption of exogenous cholesterol from the intestinal lumen (Pollak and Kritchevsky, 1981). In most trials the total daily intake of plant sterols and stanols is divided over two or three portions per day. In our study (Hendriks et al., 2003) it appeared that after 13 weeks the majority of the volunteers consumed all the spread with breakfast. Since we considered it relevant that the spread enriched with plant sterol esters should be consumed with at least two meals per day, a request was issued half way through the study to eat 10 g of spread with both breakfast and lunch. The request resulted in a clear increase in the proportion of volunteers consuming spread with two meals (from about 40 per cent to about 70 per cent), but the cholesterol-lowering efficacy did not dramatically change. This is in accordance with the results of Plat et al. (2000). They showed that 2.5 g plant stanols taken at lunch produced the same LDL­lowering effect as 2.5 g of plant stanols divided over the three meals. Although both results suggest that eating moment is not essential in the cholesterol-lowering effect of plant sterols and plant stanols, more research is required to draw firm conclusions with respect to this topic.

Plant sterols and stanols are effective not only in people with normal and mild hypercholesterolaemia, but also in those receiving cholesterol-lowering therapies. Maki et al. (2001) studied the effect of daily consumption of low-fat spreads (40 per cent) enriched with 1.1 or 2.2 g plant sterol esters in people with low to mildly hypercholesterolaemia on a cholesterol-reducing diet. After 2 weeks, LDL-cholesterol was reduced with respectively 7.6 and 8.1 per cent compared with control treatment. Also Hallikainen et al. (2000) studied the cholesterol-lowering capacity of margarines enriched with plant sterols and plant stanols in people with hypercholesterolaemia. During 4 weeks, 34 people consumed a control spread, a spread enriched with plant sterols and a spread enriched with plant stanols. Sterols lowered LDL­cholesterol with 10.4 per cent and stanols with 12.7 per cent compared with the control product. Comparable cholesterol-lowering effects were found by Blair et al. (2000) and Vuori et al. (2000). Both groups demonstrated that consumption of plant stanol esters in people with hypercholesterolaemia receiving statin therapy lowered LDL-cholesterol levels by 10 per cent or more compared with consumption of placebo. Neil et al. (2001) investigated the cholesterol-lowering effect of daily consumption of margarine enriched with 2.5 g plant sterols in people with heterozygous familial hyper­cholesterolaemia receiving either statin therapy or no therapy. In both groups, LDL-cholesterol concentrations decreased by 10±15 per cent. There was no difference in response between hypercholesterolaemic patients prescribed statins and those not taking lipid lowering drug.

In people with diabetes, plant stanols have also been shown to be effective in reducing LDL. Gylling and Miettinen (1994) showed a reduction in LDL­cholesterol of 9 per cent in people with diabetes after consumption of plant stanols, which was comparable with the observed effect in those without diabetes. In patients with type 2 diabetes receiving a statin therapy, LDL­cholesterol was lowered by an additional 14 per cent after consumption of stanol esters (Gylling and Miettinen, 1996).

The apolipoprotein E polymorphism may influence the absorption of cholesterol from the intestine and thus the response of serum cholesterol to diet. Some studies (Miettinen and Vanhanen, 1994; Vanhanen et al., 1993) indicated that subjects with apoE4 genotype responded to sitostanol consumption with a greater reduction in total cholesterol and LDL-cholesterol than individuals with the apoE3 genotype. Others concluded that the serum cholesterol response to plant sterols is not affected by the apolipoprotein E polymorphism in healthy people who consume a low-cholesterol diet (Geelen et al. 2002, Ishiwata et al., 2002).

Subjects suffering from the rare inherited metabolic disease phytosterolaemia (sitosterolaemia) have serum plant sterol concentrations approximately 20±100 times higher than healthy subjects (Bhattacharyya and Connor, 1974; Salen et al., 1985; Ling and Jones, 1995). These elevated concentrations of plant sterols have been implicated as a risk factor for premature atherosclerosis and coronary heart disease. The estimated frequency in the population of the homozygous state is in the order of one in 5 to 10 million. Increased dietary sitosterol absorption and decreased excretion are believed to be responsible for the accumulation of sitosterol in plasma and tissues. Since cholesterol absorption in people with phytosterolaemia seems to be normal, there may be an intestinal defect that discriminates between cholesterol and plant sterols, resulting in excessive plant sterol absorption. Recent literature suggests that intestinal ABCG5 and ABCG8 transporters are involved in the absorption of plant sterols but not cholesterol (Sehayek 2003). Mutations in these genes are suggested to be responsible for at least some forms of phystosterolaemia (Sehayek, 2003). Whether increased plasma concentrations of plant sterols or plant stanols might be a risk factor for coronary heart disease (CHD) in people without phytosterolaemia has not been established.

Intestinal absorption of plant sterols in non-phytosterolaemic subjects is low compared with absorption of cholesterol. Heinemann et al. (1993) reported in a study with human volunteers percentage absorption values for campestanol, campesterol, stigmasterol and sitosterol which were, respectively 12.5 per cent, 9.6 per cent, 4.8 per cent and 4.2 per cent. Sitostanol absorption was negligible, whereas cholesterol absorption in the same study was 31 per cent. Ostlund et al. (2002) reported absorption of plant sterols varying from 0.4 to 3.5 per cent whereas intestinal absorption of plant stanols varied from 0.02 to 0.3 per cent. Sitosterol absorption in people suffering homozygous phytosterolaemia is typically 15±25 per cent (taken from Katan et al., 2003).

Plant sterols and stanols are cleared from the plasma and preferentially taken up by the liver and to a lesser extent distributed to other tissues. Results from rat studies indicate that of the other tissues, the adrenals showed the highest levels on the basis of tissue weight followed by the testes (Subbiah and Kuksis, 1973). No accumulation of plant sterols has been observed in dogs and rats (Shipley et al., 1958). For humans, only a little information is available regarding the distribution of plant sterols in various tissues of the body. Only trace amounts of plant sterols have been determined in the liver and liver microsomes of healthy people (Ling and Jones, 1995). High levels of plant sterols in liver microsomes have been found in people suffering from phytosterolaemia.

Consumption of plant sterols and stanols has been shown to increase plasma levels in healthy people. Weststrate and Meijer (1998) demonstrated that plant sterol esters from soy, which have a high content of both sitosterol and campesterol, raised the concentration of both plant sterols in the plasma. Hendriks et al. (2003) showed that after consumption of spread enriched with plant sterol esters during 1 year, the concentrations of campesterol and 0­ sitosterol in red blood cells were increased as compared with the control. The relative increase in red blood cells was similar to the increase in serum suggesting that plant sterols did not accumulate in cell membranes. Hallikainen et al. (2000) compared the effect of margarines enriched with plant sterol esters and plant stanol esters. Both plant sterol and plant stanol concentrations in plasma increased. Consumption of plant stanols reduced plasma sterol concentrations (Hallikainen et al., 2000). Concentrations of plant sterols in plasma in people who used margarine enriched with sterol esters are within the range of 11±30 µmol/L which is about 20±100 times lower than in people with phytosterolaemia. Whether such an increase in plasma plant sterol concentrations might be a risk factor for coronary heart disease is not known.

Numerous studies on toxicity of plant sterols and plant stanols have been performed. The acute toxicity of plant sterols and plant stanols is low; an abstract by Robinson et al. (1998) reported an acute oral LD50 for plant stanols of >5000 mg/kg body weight. There are no reports in literature of allergic reactions associated with plant sterols and plant stanols. Test substances for sub­acute and semichronic toxicity of various plant sterols or plant stanols in general did not reveal any adverse toxicological effects. No observed adverse effect levels (NOAEL) for plant sterols and plant stanols were established by Turnbull et al. (1999a) and Hepburn et al. (1999). Turnbull et al. (1999a) conducted a 13­ week oral toxicity test with two stanol esters in rats consuming dietary sterol concentrations from 0 up to 50 g/kg food. They concluded that the mid-dose level (10 g/kg food, equal to 0.5 g total stanols/kg bw/day) was the NOAEL for both stanol ester preparations. Hepburn et al. (1999) reported a 90-day oral toxicity study with plant sterol esters in rats. Plant sterols were obtained from a variety of common edible vegetable oil distillates (mainly soybean) and then re­esterified with fatty acids from sunflower oil. It was concluded by the authors that the highest dose level (81 g/kg food, equal to 6.6 g/kg bw/d and equivalent to a plant sterol concentration of 50 g/kg food, equal to 4.1 g/kg bw/day) was the NOAEL. Plant sterols and stanols are not genotoxic (Turnbull et al., 1999b) or teratogenic (Slesinski et al., 1999). It has been suggested that plant sterols or stanols have oestrogenic activity. However, in vitro and in vivo assays did not reveal any oestrogenic or uterotrophic activity of either stanol esters, free stanol fatty acids (Turnbull et al. (1999c) or plant sterols sourced from a variety of edible vegetable oil distillates (Baker et al., 1999).

Clinical safety

As described in this post many clinical studies with plant sterols and stanols, with both sexes, in both children and adults, with both healthy volunteers and those with hypercholesterolaemia have been performed. The aim of most of these studies was mainly to demonstrate the reduction of serum cholesterol levels by the addition of plant sterols in the diet. In general, the occurrence of adverse events associated with the use of plant sterols and plant stanols is rare and the adverse events reported are mild and not considered to be treatment-related. Studies investigating effects of plant sterols or plant stanols on blood chemistry and haematology parameters did not show adverse effects in these parameters after short-term consumption (Weststrate and Meijer, 1998; Plat and Mensink, 1998; Plat et al., 1999). Two studies were conducted to evaluate long term effects of consumption of plant stanol esters and plant sterol esters. Miettinen et al. (1995) studied in a randomized double-blind placebo-controlled parallel trial the effects of daily consumption of a sitostanol-enriched margarine (1.8 g sitostanol/day) in mildly hypercholesterolaemic people for 1 year. No adverse effects on body weight were observed and no adverse clinical signs were reported. Hendriks et al. (2003) evaluated the clinical and safety parameters after daily consumption of a spread enriched with plant sterol esters (1.6 g/day) during one year in a randomized double-blind, parallel placebo-controlled study in healthy volunteers. No adverse side effects, defined as reported adverse events or undesirable changes in clinical chemical parameters, haematological parameters and urinalysis, were observed. In addition, hormone levels in males and females were unaffected.

Nutritional safety

Several studies indicate that plant sterol enrichment may interfere with the uptake of fat-soluble vitamins and nutrients, primarily carotenoids, from the intestine. Hendriks et al. (1999) observed a decrease (3±19 per cent) in plasma ?-carotene, lycopene and ?-tocopherol concentration after consumption of spread enriched with plant sterols (0.83±3.24 g/day). Correction for the reductions in the total plasma lipids, however, showed only plasma (? + ?)-carotene concentrations to be reduced by about 8 and 15 per cent after consumption of the low (0.83 g/day) and the high dose (3.24 g/day) of plant sterols, respectively. We concluded that these doses affect plasma carotenoid concentrations to a limited extent.

In another study (Weststrate and Meijer, 1998), plasma (? + ?)-carotene and lycopene concentrations were both reduced by 22 per cent after an intake of 3 g plant sterols per day. Expressed per plasma lipid concentration, however, lycopene concentrations were not affected, but (? + ?)-carotene concentration was decreased (19 per cent). Sierksma et al. (1999) found that plasma lipid-standardised concentrations of ? + ?-carotene were not statistically signifi­cantly affected by the soybean-oil sterol spread (0.8 g/d), in contrast to lipid-standardised plasma lycopene levels which showed a statistically significant decrease (9.5 per cent). Hallikainen and Uusitupa (1999) did not observe a significant effect of lipid-standardized ?-carotene levels from plant sterol ester consumption. However, Hallikainen et al. (1999) found that serum ?-carotene concentration did not change significantly when subjects were fed with either low-fat wood stanol ester (2.34 g/day) or vegetable oil stanol ester (2.20 g/day). Decreases in ? + ?-carotene concentrations were significantly greater in both experimental groups than in the control group, although change in ?-, ? or (? + ?)-carotene/cholesterol ratio did not differ significantly among the groups. No significant changes were found in serum lycopene or lycopene/total cholesterol ratios in both experimental groups.

The authors concluded that low-fat stanol ester margarine appeared to have little effect on serum concentrations of ?- , ?- or ?+ ?-carotene, or lycopene. Gylling et al. (1999) investigated whether sitostanol ester margarine affects the serum levels of vitamin D, retinol, ?-tocopherol and ?- and ?-carotenes during 1-year treatment in subjects and controls with hypercholesterolaemia. Vitamin D and retinol concentrations and the ratio of ?-tocopherol to cholesterol were unchanged by sitostanol ester. Serum ?-carotene and ?-carotene concentrations, but not their proportion, were significantly reduced in the sitostanol group from baseline and in relation to controls. The authors concluded that sitostanol ester did not affect vitamin D and retinol concentrations and the ?-tocopherol/cholesterol proportion, but reduced serum ?-carotene levels (25 per cent). Hendriks et al. (2003) showed that consumption of a plant sterol esters enriched spread during 1 year reduced lipid adjusted a and ?-carotene concentrations by 15±25 per cent compared with the control. Furthermore, lipid-adjusted fat-soluble vitamin concentrations were not affected by plant sterol intake.

In the meta-analysis of Katan et al. (2003), 18 trials testing doses of 1.5 g/day or more reported plasma concentrations of fat soluble vitamins. Mean reductions across the trials were 9 per cent for ?-carotene, 28 per cent for ?-carotene, and 7 per cent for lycopene. When adjusted for the change in cholesterol only ?- carotene was significantly reduced by plant stanols and plant sterols by 12.1 per cent (6.8±17.4 per cent). The reduction in ?-tocopherol was explained by the reduction in cholesterol (Katan et al., 2003). The meta-analysis of Katan et al. (2003) did not indicate that consumption of plant sterols and plant stanols would affect vitamin A, D or K status.

Carotenoids are not essential nutrients, but both ?-carotene and ?-carotene have provitamin A activity. They may be of importance in situations where vitamin A requirements are greater than normal, as in pregnancy, lactation or infancy. The consequences of a persistent decrease of blood concentrations of carotenoids, specifically ?-carotene, as observed after consumption of plant sterols and plant stanols on human health are unknown. Dietary advice to consume an additional daily serving of a high-carotenoid vegetable or fruit when consuming spreads containing sterol or stanol esters may be effective to maintain plasma carotenoid concentrations (Noakes et al., 2002).

Products enriched with plant sterols and plant stanols can be considered as functional foods because functional foods are defined as foods with additional benefits on top of the nutritive value. It has been shown that consumption of plant sterols and plant stanols is effective in lowering total and LDL-cholesterol concentrations. As shown in the present post, reductions in total and LDL­cholesterol varying from 0.3 to 6 per cent can be obtained depending on the dosage. The most common dosage given in different trials is about 1.5±2.5 g/ day, resulting in a mean decrease of LDL-cholesterol of about 10 per cent. Such a decrease may, on a population basis, substantially contribute to the prevention of coronary heart disease (Law et al., 1994). Analysis of cohort studies by Law et al. (1994) indicated that the longer-term risk reduction would be about 20 per cent. Plant sterols and stanols were not only effective in healthy volunteers, but also in people with hypercholesterolaemia receiving statin therapy. Besides the cholesterol-lowering potential, animal studies have shown that plant sterols and plant stanols may reduce development of atherosclerotic lesions (de Jong et al., 2003). The potency of reduction of development and progression of atherosclerotic lesions ideally should be confirmed in very large intervention trials. Smaller and shorter-term trials with surrogate endpoints, such as flow-mediated dilatation, pulse wave velocity or intima media thickness, might help in proving the efficacy of plant sterols and stanols in reducing cardiovascular disease. In addition to the effects on CHD risk, effects of plant sterols and plant stanols on risk on colon cancer and prostate cancer are suggested.

Compared with the general population, Seventh-day Adventists have lower rates of cancer at many sites, including colorectal cancer, and higher dietary intakes of plant sterols (Nair et al., 1984). Since bile acids are reported to be tumour promoters in colon cancer (Cohen and Raicht, 1981), a proposed mechanism is that plant sterol consumption reduce bile acid excretion and thus the risk on colon cancer. On the other hand, increased consumption of plant sterols and plant stanols results in increased presence of cholesterol in the large intestine, which in turn can be converted into mutagenic metabolites (4­ cholesten-3-one). Wolfreys and Hepburn (2002) investigated the mutagenic potential of plants sterols and plant sterol esters as well as 4-cholesten-3-one in a bacterial mutation assay and an in vitro chromosome aberration assay. None of the components showed any evidence of mutagenic activity in any of these assays. Moreover, studies with mice and rats showed a decreased mucosal cell proliferation after consumption of different dosages of 3-sitosterol (Janezic and Rao, 1992; Awad et al., 1997). However, results of a prospective cohort study by NormeÂn et al. (2001) indicated that high dietary intake of plant sterols was not associated with a lower risk of colon and rectal cancers in the Netherlands Cohort Study on Diet and Cancer.

It has also been suggested that plant sterols may beneficially affect prostate cancer. Dietary supplementation of sitosterol has been shown to improve the clinical symptoms of prostatic hyperplasia in humans (Berges et al., 1995). Awad et al. (1998) showed that, among other things, plant sterol feeding in rats reduced serum testosterone concentrations. They concluded that dietary plant sterols may reduce risk of prostate cancer by stimulating the activities of the enzymes of testosterone metabolism. However, Hendriks et al. (2003) could not demonstrate an effect of 1-year consumption of spread enriched with plant sterol esters on serum testosterone concentrations in healthy men.

The introduction of new products containing plant sterols and plant stanols is proceeding. Margarines containing either plant stanol or plant sterol esters have been marketed in the United Stated and several countries in Europe for more than 2 years and in Finland for over 5 years. More recently other formulations, including yoghurt, cream cheese spreads and cereal bars have been introduced in some countries, and cereals containing free (unesterified) plant sterols and stanols are being marketed in the United States. With the growing number of products enriched with plant sterols and stanols, some consumers may reach high intake levels when consuming different enriched products. Safety of plant sterols and plant stanols has been extensively tested. Several GRAS (generally regarded as safe) notification dossiers are present. The Scientific Committee on Food concluded that the use of plant sterols in yellow fat spread at a maximum level corresponding to 8 per cent free plant sterols is safe for human use (SCF, 2000). More recently the Scientific Committee on Food (SCF, 2002) concluded that `the available data do not provide a basis for setting a numerical upper level of daily intake of phytosterols’. However, the committee indicated that `it is prudent to avoid plant sterol intakes exceeding a range of 1±3 g/day’. Based on results of animal studies discussed in this post this still leaves a large margin of safety.

With the growing number of enriched foods the Scientific Committee on Foods suggests that `additional management measures may be needed to avoid excessive intakes’ (SCF, 2002). Although there is no major concern on adverse health effects, the lack of very long-term effects leaves a possibility of unforeseen effects. Therefore follow-up of samples from the general population eating these foods by post-marketing surveillance techniques is important to monitor both the beneficial as well as the adverse effects on the very long term. This is underscored by the Scientific Committee on Food (SCF, 2003) in its most recent opinion in which `the committee encourages the Commission to initiate a programme monitoring the total intake of phytosterols-enriched products’.

Products enriched with plant sterols and plant stanols either in its esterified or unesterified form have demonstrated to be effective in cholesterol lowering. Dosages of 1.5±2.5 g per day reduce total cholesterol up to 10 per cent and LDL­cholesterol up to 15 per cent. The mean reduction in LDL-cholesterol is about 10 per cent which on a population basis may result in a 20 per cent risk reduction for coronary heart disease. Sterols and stanols seem to be equally effective. Plant sterols and stanols are not only effective in people with normal and mild hypercholesterolaemia, but also in people with hypercholesterolaemia receiving cholesterol-lowering therapies. No effects are observed on HDL-cholesterol and triacylglycerol level. Plant sterols and stanols are poorly absorbed and plasma concentrations are low, except in subjects with a rare inherited disorder phytosterolaemia. Safety of plant sterols and stanols has been extensively tested and no adverse effects seem to be present, although there is a lack of chronic exposure data. Consumption of plant sterols and stanols results in a substantial reduction of carotenoid levels. The implication of this reduction for human health in the long term is not clear. With the growing number of enriched products entering the market the consumption of plant sterols and stanols should be monitored. An intake not exceeding 1±3 g per day is advised.

A follow-up of samples from the general population eating plant sterol and stanol enriched foods by post-marketing surveillance techniques is important to evaluate both the beneficial as well as the adverse effects in the very long term.