Cholesterol-Lowering Action of Plant Sterols

Transcription

1 Cholesterol-Lowering Action of Plant Sterols Peter J. H. Jones, PhD Address School of Dietetics and Human Nutrition, Faculty of Agricultural and Environmental Sciences, 21,111 Lakeshore Road, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue, Quebec, H9X 3V9, Canada. Current Atherosclerosis Reports 1999, 1: Current Science Inc. ISSN Copyright 1999 by Current Science Inc. Plant sterols have an extended history of use as cholesterol-lowering agents. Until the 1970s, the principal interest in plant sterols lay in effects of sitosterol, but over the past decade interest has reemerged in using plant sterols in functional foods. Hydrogenated plant sterols hav been shown efficacious in lowering lipid levels, inhibiting cholesterol absorption and regressing plaque in animals. Hydrogenated versus unhydrogenated plant sterol esters have been demonstrated to possess equal efficacy in low-density lipoprotein cholesterol lowering in humans. Unhydrogenated plants sterol esters show consistency in cholesterol-lowering across dosage levels in humans. Unhydrogenated, unesterifed plant sterols yield similar low-density lipoprotein cholesterol lowering efficacy as achieved with hydrogenated sitostanol esters. Solubility and miscibility are likely more important determinants in cholesterol-lowering potential of plant sterols than their specific composition. Introduction Plant sterols for several decades have been noted for their ability to act as cholesterol-lowering agents [1 3]. Numerous animal and humans trials conducted prior to the 1980s demonstrated that oral ingestion of mixtures of sitosterol and other plant sterols possessed efficacy to lower circulating total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) levels in the range of 10% to 15%. Indeed, the plant sterol mixture Cytellin served as a pharmaceutical agent for control of hypercholesterolemia through the 1960s; however, large dosages were required for efficacy. As more powerful cholesterol-lowering agents became available, interest in phytosterols as pharmaceutical agents waned through the 1970s. More recently, the interest in plant sterols as lipid-lowering agents has reemerged in the context of their use in functional foods. Food delivery routes facilitate consumption of higher daily volumes of phytosterols. The resurgence in popularity has also occurred as focus has shifted from unsaturated plant sterols, such as sitosterol, to saturated plant sterols, particularly, sitostanol. Studies have provided evidence that gram for gram sitostanol possesses more potent cholesterol-reducing ability compared with unsaturated analogs [4,5,6,7]. Concurrently, emphasis has shifted to the examination of the cholesterol-lowering efficacy of free plant sterol mixtures versus those which are esterified (Fig. 1). A number of quite recent reports have identified TC and LDL-C lowering actions of both sitostanol esters [8,9 ] and sitosterol esters [10,11 ]. Evidence has been presented to suggest that in esterified form, saturation level of plant sterols does not influence efficacy [10 ]. Moreover, whether esterification is required to demonstrate cholesterol-reducing capacity of free, largely unhydrogenated plant sterols at lower doses has also been questioned [12,13 ]. The purpose of this review is therefore to examine recent literature describing the relative efficacy and safety of various plant stanol mixtures and forms in reducing levels of atherogenic lipoproteins. Recent Studies of Stanol Versus Sterol Efficacy in Lowering Cholesterol Phytosterols influence plasma lipoprotein cholesterol levels in a manner that depends on both composition and dose of the plant sterol mixture. Generally, studies examining the effects of plant sterol mixtures indicate that stanols possess greater cholesterol-lowering efficacy compared with sterols. Using unsaturated plant sterols, daily intake levels of 10 to 15 g/d have been found necessary to appreciably lower lipid levels. However, more recent work has shown that smaller dosages of saturated plant stanols are adequate as lipid-lowering agents. Reductions in circulating LDL-C and TC levels have been observed after consumption of saturated plant sterols or stanol esters in amounts ranging from 1.5 to 4 g/d [4,5,8 10,14 17]. The enhanced efficacy of free phytostanols versus free phytosterols was demonstrated directly by Becker et al. [5], who showed in children that a fourfold greater level of phytosterols was required to evoke a similar cholesterollowering action as 1.5 g/d of unesterified sitostanol. Animals studies conducted more recently indicate that the proportion of a plant sterol mixture given as free stanol to animals receiving an atherogenic diet decreases the frac-

2 Cholesterol-Lowering Action of Plant Sterols Jones 231 Figure 1. Plant sterols and sterol esters used as cholesterol-lowering agents tional absorption of cholesterol [6 ], circulating lipid levels [7], and the amount of arterial plaque build up [18 ]. These findings have led to the opinion that in the case of free phytosterols, the extent of hydrogenation is associated directly with the extent of cholesterol-lowering of the mixture. This assertion is not with opposition; Denke [19] observed no action on circulating cholesterol levels of 3 g/ d of free stanols in men. On the basis of these findings, it generally has been assumed that stanols possess improved TC and LDL-C lowering ability over sterols in hypercholesterolemic individuals. In this regard, popularity has shifted towards use of free and esterified stanols in place of other phytosterols as cholesterol-lowering agents. Particularly, Miettinen et al. [8] demonstrated in 153 hypercholesterolemic individuals that either 1.8 or 2.6 g/d of sitostanol ester resulted in a decline in TC and LDL-C levels of 10.2% and 14.1%, respectively, over 12 months. This level of LDL-C reduction is much greater than would be expected with dosages of unsaturated free cholesterol, although, one report suggests that levels of free unsaturated plant sterols were almost equally effective when given at less than 1 g/d [20]. Thus, through the 1990s, the increased efficacy of free stanols and stanol esters over sterols became apparent. Most recently, Hallikainen and Uusitupa [9 ] examined the relative efficacy of margarines containing stanol esters derived either from wood or vegetable oil on circulating lipid levels in 55 subjects consuming low-fat diets. The study was a parallel, double-blind trial. Subjects consumed either a margarine devoid of stanol esters per day, one containing 2.3 g of wood-derived stanol esters per day or one containing 2.2 g of plant-derived stanol esters per day. During the run-in phase, subjects were instructed to consume 36% to 38% of calories as fat, then, during the 8- week margarine phases to target fat intake to attain 28% to 30% of energy as fat. Subjects participated in the study as outpatients, receiving instructions on precise amounts and types of foods to be consumed on each of the three diets. After 8 weeks, reductions in serum TC levels were 10.6% and 8.1% for the wood- and vegetable oil-derived stanol esters, respectively, relative to the control group. For LDL- C, levels were reduced by 13.7% and 8.6%, respectively, with respect to the control group. Overall, circulatory LDL- C level declined by 23.6%, 18.4%, and 9.9%, respectively, in the wood-derived stanol ester, vegetable oil-derived stanol ester, and control low-fat diets, respectively (Fig. 2). This appreciable decline in the combined low-fat, sitostanol ester margarine diet underscores the important potential in combined therapy of diet and phytosterols to reduce cholesterol levels to the same level as cholesterollowering drugs do. The fact that a level of efficacy in lipidlowering not dissimilar to that seen in the 12-month study using sitostanol ester [8] was observed in a low-fat, lowcholesterol diet indicates that suppression of absorption of biliary cholesterol must be occurring with sitostanol esters. In this report [9 ], the decrease in serum -carotene levels at week 8 from baseline were greater in the groups receiving sitostanol ester, compared with the group receiving the control diet. Declines in seru -tocopherol and calcidiol over the trial also were observed in groups consuming plant sterols; however, values also fell in the group consuming the control diet.

3 232 Nutrition Figure 2. Total (solid lines) and low-density lipoproteincholesterol (hatched lines) level response to consumption of wood-derived stanol esters (closed circles), vegetable oil-derived stanol esters (closed triangles) and control closed squares) low-fat diets, respectively, for 8 weeks. (Adapted from Hallikainen MA et al. [9 ].) One consideration of the study design [9 ] was that subjects lost over 1 kg of body weight over all phases, a likely result of the lower energy dietary energy density. It has been demonstrated that even a modest weight loss of 6% of initial body weight over 12 weeks results in significant reductions in TC and LDL-C levels, as well as in biosynthesis [21]. Thus, the subjects weight loss may have accounted for some of the decline in lipids levels observed. Direct Comparison of Stanol Esters Versus Sterol Esters The assertion that stanol esters are the most potent of cholesterol-lowering plant sterols has been challenged in recent studies. Particularly, data have raised questions over the improved cholesterol-lowering ability of saturated plant sterol esters versus both unsaturated esters [10,11 ] and free unsaturated mixtures [12,13 ]. The results of these new reports begin to suggest that solubility and dispersion may be more important determinants of efficacy than plant sterol composition itself. In a cross-comparison of efficacy of sitostanol esters versus sitosterol esters recently, 100 healthy subjects each completed four treatments of a five-treatment randomized, placebo-controlled, balanced, double-blind trial [10 ]. During four consecutive periods, each of 3.5 weeks duration, margarines were provided either as in control form, containing no plant sterols, or with one of four different plant sterol mixtures, including plant sterol concentrates derived from soybean oil, ricebran oil, sheanut oil, or Benecol margarine (Raisio, Finland). The soy-derived plant sterols were composed largely of sitosterol and campesterol esterified to fatty acids, mostly from sunflower seed oil. Ricebran and sheanut oils contained 4,4'-dimethyl sterols esterified mostly to ferulic, cinnamic, and acetic acids, whereas Benecol contained sitostanol and campestanol, Figure 3. Total and low-density lipoprotein cholesterol level response to consumption of plant sterols and stanols derived from soybean oil (closed circles), ricebran oil (closed triangles), sheanut oil (closed squares) or Benecol margarine (Raisio Inc, Finland) (closed crosses) for 3.5 weeks. The soy-derived plant sterols were composed largely of sitosterol and campesterol esterified to fatty acids, mostly from sunflower seed oil. Ricebran and sheanut oils contained 4,4'-dimethyl sterols esterified mostly to ferulic, cinnamic and acetic acids, while Benecol contained sitostanol and campestanol, esterified to fatty acids derived from rapeseed oil, respectively. Plant sterol and stanol dosages ranged from 1.7 to 3.2 g/d across treatments. (Adapted from Weststrate JA et al. [10 ].) esterified to fatty acids derived from rapeseed oil. Levels of plasma TC, LDL-C, and high-density lipoprotein cholesterol (HDL-C) were assessed in subjects after 2.5 and 3.5 weeks on each treatment. Subjects reported excellent compliance consuming approximately the intended targets of 30 g/d of each margarine. Mean body weights did not differ across diets. Total plant sterol intakes ranged from 1.7 to 3.2 g/d of soybean or sheanut margarines. Energy intakes did not differ across treatments. Plasma lipid data showed that consumption of both the Benecol and soybean plant sterol-containing spreads resulted in significantly lower TC and LDL-C concentrations, compared with consumption of the control margarine, which did not contain plant sterols (Fig. 3). Margarines containing sterols derived from ricebran and sheanut oils did not significantly reduce TC or LDL-C. No treatment effect on HDLC was observed. Plasma - and -carotene as well as lycopene levels were reduced with almost all plant sterol-fortified margarines. Treatment effects on - and -carotene levels exhibited gender-specificity. Benecol margarine produced a greater reduction of plasma carotenoid concentrations in males, whereas soybean-containing margarine demonstrated a greater carotenoid suppression in females.

4 Cholesterol-Lowering Action of Plant Sterols Jones 233 Plasma plant sterol levels were lowest with Benecol compared with other treatments. The main finding of this report [10 ] was that when esterified to fatty acids, unsaturated plant sterols are equally as effective as saturated plant sterols in lowering circulating TC and LDL-C levels. The lower plant sterol concentrations in the ricebran margarine and the differen plant sterol types in sheanut margarine may be reasons contributing to their absence of action. Ricebran and sheanut margarines also differed in fatty acid composition from the other spreads. Here, the novel aspect is the similarity of response of unsaturated and saturated plant sterol forms when esterified, contrary to results from other studies, which have compared the relative efficacy of action of plant sterols versus stanols and concluded that the latter is over twofold more potent as a cholesterol-lowering agent [5,6,7,18 ]. The parallel action of these unsaturated and saturated plant sterols may exist as an extension of their esterified state, perhaps due to enhanced solubility. Equally as remarkable as the finding of comparable action of sitosterol esters on LDL-C lowering is the recent observation that variations in daily dosage level exerts very little effect on the cholesterol-lowering ability of sterol esters [11 ]. In this dose ranging trial, 100 subjects were tested using a randomized, double blind, placebo-controlled trial where they received for 3.5 weeks either butter, a commercially available margarine-based spread, or three similar spreads fortified with 0.83, 1.61, and 3.24 g/d of sitosterol esters [11 ]. All three sterol ester intakes significantly reduced TC and LDL-C levels by 4.9% to 6.8% and 6.7% to 9.9%, respectively (Fig. 4). No differences in extent of the choles terol-reducing effect could be identified between dosages. These findings of efficacy in cholesterol modulation using low dosage of sterol esters return us almost full circle to perceptions held during the pre-1980s era when use of nonsaturated sterols was advocated [1 3]. During that period, however, the importance of suspension into foods and dis tribution of intake over the day perhaps was not fully appreciated. In the study by Hendriks et al. [11 ] plant sterols were suspended in the spreads at concentrations of 3.4% to 13% (wt/wt) and consumed as divided between lunch an dinner meals. Thus, formulation may be a key consideration. In this work, lipid-standardized plasma carotenoid levels were decreased 11% and 19% in the groups consum ing 0.83 and 3.24 g/d of sterol ester, respectively. Importance of Hydrogenation of Plant Sterols as a Determinant of Efficacy In addition to the question of hydrogenation, more recently the issue of the need for esterification of plant sterol mixtures to ensure effective LDL-C lowering remains in question. Recent evidence suggests that esterification of plants sterol mixtures is not essential for efficacy in lipid Figure 4. Total cholesterol (solid line) and low-density lipoprotein cholesterol (hatched line) level response to 0.83 closed circles), 1.6 (closed triangles), and 3.2 (closed squares) g/d of sitosterol esters for 3.5 weeks. (Adapted from Hendriks HFJ et al. [11 ].) modification. Jones et al. [12], using a randomized crossover study design showed that 1.5 g/d of free sterols containing mostly unhydrogenated sterols with about 20% sitostanol given to 22 subjects resulted in reductions in TC and LDL-C levels of 6% and 10%, respectively, compared with a control period [12]. In this study, phytosterols were mixed into the oil within the prepared meals of the diet. Similarly, a parallel controlled feeding study design was employed to examine the effects of a mixture of free sterols containing mostly unhydrogenated sterols with about 20% sitostanol on plasma lipid and plant sterol concentrations and cholesterol biosynthesis rates over 30 days [13 ]. Subjects received either a prudent 35% energy as fat fixed-foods diet or the same diet with 1.7 g/d of plant sterols, consuming all meals from a metabolic kitchen during the study. Mean body weight of subjects remained stable across the experimental periods. Compared with the control diet, TC levels were lower at day 30 in subjects consuming the sitosterol-containing free plant sterol mixture. Circulating LDL-C levels decreased by 8.95% and 24.4% with the control and plant sterol dietary treatments, respectively (Fig. 5); whereas, HDL-C and triglyceride concentrations did not differ across groups. Neither plant sterol levels, nor cholesterol biosynthesis rates were altered by treatment. This study [13 ] concluded that addition of freeblended plant sterols to a prudent North American diet most notably reduced LDL-C levels by mechanisms that did not result in a change in biosynthesis rates. Of particular interest, was the finding that the combined influence of diet and the free plant sterol mixture provided the same LDL-C-lowering impact over 30 days as did the low-fat sitostanol ester mixtures fed to subjects for 8 weeks at doses of more than 2 g/d [9 ], which reduced LDL-C by 23.6%. Fat-soluble vitamin levels in plasma were not examined; however, blood and urine chemistry remained normal in subjects throughout the study.

5 234 Nutrition References and Recommended Reading Papers of particular interest, published recently, have been highlighted as: Of importance Of major importance Figure 5. Total cholesterol (solid line) and low-density lipoprotein cholesterol (hatched line) level response to 1.7 g per day to all oilderived free phytosterols containing mostly unhydrogenated sterols (closed circles) and control (closed triangles) for 30 days. (Adapted from Jones PJH et al. [13 ].) Conclusions Recent work in the area of phytosterols as cholesterolmodifying agents have brought us practically full circle to the state-of-the-art of two decades ago [1]. Emphasis has shifted from sterols to stanols; from stanols to stanol esters and finally from stanol esters to sterol esters and free sterol/stanol mixtures. The disparity across studies reflects both varied experimental design and differential responses of animal models compared with human subjects. The likely primary determinant of efficacy of phytosterols, whether hydrogenated or not, whether esterified or not, is solubility. The enhanced solubility of sterol or stanol esters in margarine might increase their miscibility in bile salt micelles, compared with sterols given as powder or crystal forms. Such enhancement in the biliary dissolution of plant sterols may more effectively decrease the partitioning of cholesterol into micelles, and hence lower cholesterol absorption. The finding of similar efficacy across plant sterol matrices is of substantive importance, as hydrogenation and subsequent processing of plant sterols is a labor-intensive process that adds cost to the manufacturing of mixtures for functional food usage. Concentrating on the means of improving solubility within foods should enable cholesterol-lowering margarines to become less expensive and more widely available to the general population. The consequence of this shift hopefully will be a move towards lowering lipid levels consistent with a significant reduction in incidence of cardiovascular disease. 1. Pollack OJ, Kritchevsky D: Monographs on Atherosclerosis. New York: Karger; Ling WH, Jones PJH: Dietary phytosterols: a review of metabolism, benefits and side effects. Life Sci : Jones PJH, MacDougall DE, Ntanios F, Vanstone CA: Dietary phytosterols as cholesterol-lowering agents in humans. Can J Physiol Pharmacol 1997, 75: Heinemann T, Axtmann G, von Bergmann K: Comparison of intestinal absorption of cholesterol with different plant sterols in man. Euro J Clin Invest 1993, 23: Becker M, Staab D, von Bergmann K: Treatment of severe familial hypercholesterolemia in childhood with sitosterol and sitostanol. J Pediatr 1993, 122: Ntanios FY, Jones PJH: Dietary sitostanol reciprocally influences cholesterol absorption and biosynthesis in hamsters and rabbits. Atherosclerosis 1999, 143: Demonstration in animals that inhibition of cholesterol absorption increases and circulatory lipid levels decrease with the extent of hydrogenation of plant sterols. 7. Ntanios FY, Jones PJH: Effects of variable dietary sitostanol concentrations on plasma lipid profile and phytosterol metabolism in hamsters. Bioch Biophys Acta 1998, 1390: Miettinen TA, Puska P, Gylling H, et al.: Reduction of serum cholesterol with sitostanol-ester margarine in a mildly hypercholesterolemic population. New Engl J Med 1995, 333: Hallikainen MA, Uusitupa MIJ: Effects of 2 low-fat stanol ester- containing margarines on serum cholesterol concentrations as part of a low-fat diet in hypercholesterolemic subjects. Am J Clin Nutr 1999, 69: A 23.6% reduction in LDL cholesterol levels was achieved after 8 weeks of consumption of 2.2 g per day saturated, esterified plant sterols in hyperlipidemic in combination with a low fat diet. 10. Weststrate JA, Meijer GW: Plant sterol-enriched margarines and reduction of plasma total- and LDL-cholesterol concentrations in normocholesterolaemic and mildly hypercholesterolaemic subjects. Eur J Clin Nutr 1998, 52: Sitosterol ester and sitostanol ester containing margarines fed at 1.7 to 3.2 g per day for 3.5 weeks reduced total cholesterol levels by 13% and 13%, and LDL cholesterol levels by 8.3% and 7.3%, respectively. 11. Hendriks HFJ, Westrate JA, van Vliet T, Meijer GW: Spreads enriched with three different levels of vegetable oil sterols and the degree of cholesterol lowering in normocholesterolaemic and mildly hypercholesterolaemic subjects. Eur J Clin Nutr 1999, 83: Sitosterol ester containing margarines show similar cholesterollowering abilities across a range of daily dosage levels when given over 3.5 weeks. 12. Jones PJH, Howell T, MacDougall DE, et al.: Short-term administration of tall oil phytosterols improves plasma lipid profiles in subjects with different cholesterol levels. Metabolism 1998, 47:

6 Cholesterol-Lowering Action of Plant Sterols Jones Jones PJH, Ntanios FY, Raeini-Sarchaz M, Vanstone C: Cholesterol-lowering efficacy of a sitostanol-containing phytosterol mixture with a prudent diet in hyperlipidemic men. Am J Clin Nutr 1999, 69: A 24.4% reduction in LDL cholesterol levels was achieved after 30 days of consumption of 1.7 g per day of free partially esterified plant sterols in hyperlipidemic subjects in combination with a medium fat diet. 14 Heinemann T, Leis O, von Bergmann K: Effect of low-dose/ sitostanol on serum cholesterol in patients with hypercholesterolemia. Atherosclerosis 1986, 61: Vanhanen HT, Kajander J, Lehtovirta H, Miettinen TA: Serum levels, absorption efficiency, faecal elimination and synthesis of cholesterol during increasing doses of dietary sitostanol esters in hypercholesterolemic subjects. Clin Sci 1994, 87: Vanhanen HT, Blomqvist S, Ehnholm C, et al.: Serum cholesterol, cholesterol precursors, and plant sterols in hypercholesterolemic subjects with different apoe phenotypes during dietary sitostanol ester treatment. J Lipid Res 1993, 34: Gylling H, Miettinen TA: Serum cholesterol and cholesterol and lipoprotein metabolism in hypercholesterolemic NIDDM patients before and during sitostanol estermargarine treatment. Diabetologia 1994, 37: Ntanios FY, Jones PJH, Frohlich JJ: Dietary sitostanol reduces plaque formation but not lecithin cholesterol acyl transferease activity in rabbits. Atherosclerosis 1998, 138: Demonstration in animals that plaque reduction increases with the extent of hydrogenation of plant sterols. 19. Denke MA: Lack of efficacy of low-dose sitostanol therapy as an adjunct to a cholesterol-lowering diet in men with moderate hypercholesterolemia. Am J Clin Nutr 1995, 61: Pelletier X, Belbraouet S, Mirabel D, et al.: A diet moderately enriched in phytosterols lowers plasma cholesterol concentrations in normocholesterolemic humans. Ann Nutr Metab 1995, 39: Di Buono N, Hannah JS, Katzel LI, Jones PJH: Weight loss due to energy restriction suppresses cholesterol biosynthesis in overweight, mildly hypercholesterolemic men. J Nutr 1999, in press.