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Phytosterols and vascular disease
Atherosclerosis 186 (2006) 12–19
Review
Phytosterols and vascular disease Manoj D. Patel, Paul D. Thompson ∗ Section of Preventive Cardiology, Division of Cardiology, Henry Low Heart Center, Hartford Hospital, 80 Seymour Street, Hartford, CT 06102, USA Received 13 October 2005; accepted 13 October 2005 Available online 2 December 2005
Abstract Phytosterols or plant sterols have long been known to lower serum cholesterol concentrations by competing with dietary and biliary cholesterol for intestinal absorption. Food products containing phytosterols and phytostanols are now available to assist in lowering blood cholesterol levels. In contrast to these possibly beneficial effects of plant sterols, a rare genetic condition called sitosterolemia, an autosomal recessive disorder also known as phytosterolemia, is characterized by over absorption of phytosterols and premature coronary artery and aortic valve disease. Phytosterols have also recently been identified in atheromatous plaque obtained from individuals with apparently normal absorption of plant sterols raising the possibility that phytosterols are a novel atherosclerotic risk factor. This article reviews phytosterols and their relationship to vascular disease. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Atherosclerosis; Cholesterol; Coronary disease; Hypercholesterolemia; Risk factors
Contents 1. 2. 3.
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Phytosterols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Absorption of cholesterol and phytosterols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. The therapeutic use of phytosterols to lower LDL-cholesterol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4. Phytosterolemia—clinical features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5. The genetics of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. Phytosterol absorption and turnover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7. Cholesterol absorption and turnover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8. Diagnosis of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9. Treatment of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 10. Plant sterols as a novel CAD risk factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Conflict of interest statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction The introduction of lovastatin in the United States in August 1987 revolutionized the treatment of hypercholes∗
Corresponding author. Tel.: +1 860 545 2899; fax: +1 860 545 2882. E-mail address: [email protected] (P.D. Thompson).
0021-9150/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2005.10.026
terolemia. Since this introduction numerous primary and secondary statin prevention trials, as well as non-statin trials such as the Lipid Research Clinical intervention trial, have confirmed the pivotal role of low-density lipoprotein (LDL) cholesterol (C) in the atherosclerotic disease process [1,2]. Despite these advances in coronary artery disease (CAD) prevention, CAD remains the primary cause of death in Western
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societies. Furthermore, in statin clinical trials, atherosclerotic events continue to occur, albeit less frequently, in statintreated patients. Such observations have led to treatment strategies designed to address other important lipoprotein risk factors, such as high-density lipoprotein (HDL) C, as well as a search for other risk factors and markers of risk [3,4]. Plant sterols or “phytosterols” have long been known to reduce serum LDL-C level by competing with dietary and biliary cholesterol for intestinal absorption [5–7]. Indeed, beta sitosterol was first described as a therapeutic agent for hypercholesterolemia in 1951 [8]. In contrast to these putatively beneficial effects of phytosterols, sitosterolemia or phytosterolemia, is a rare autosomal recessive disorder, in which phytosterols are over absorbed and accumulate in tissue causing tendon xanthoma and premature coronary artery disease [9]. There is recent renewed interest in phytosterols as a possible novel CAD risk factor. This interest has been heightened by the availability of an agent, ezetimibe, which interferes with the absorption of both cholesterol and phytosterols [10,11]. This review examines phytosterols and their possible role in cardiovascular disease and its management. 2. Phytosterols Phytosterols are nonnutritive compounds whose chemical structure differs from that of cholesterol because of the presence of modified side chains at carbon C-24 [12]. Sitosterol, campesterol, and stigasterol are most abundant and on average comprise 65, 30, and 3% of dietary phytosterol intake. Their primary dietary source is fat-rich vegetables and vegetable products, including vegetable oils, fruits and nuts [13,14]. Sitosterol and campesterol have an ethyl and methyl substitute at C-24 (Fig. 1), respectively,
Fig. 1. Structure of cholesterol and common phytosterols (reprinted with permission from Safety and evaluation of phytosterol esters Part-2. Food Chem Toxicol 1999;37:521–32).
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whereas stigasterol is identical to sitosterol except for a double bond at C-22. Stanols are saturated sterols, meaning that they have no double bonds in the sterol ring. Stanols are less abundant in nature [15]. Unabsorbed sterols can undergo bacterial transformation by intestinal microflora to produce metabolites, such as coprosterol and coprostanone [16–19]. Cholesterol and phytosterols are structurally similar but metabolized differently. Mammals do not synthesize phytosterols [20]. The typical western diet contains approximately 200–500 mg of dietary cholesterol and 200–400 mg of non-cholesterol sterols [13,21,22]. Humans absorb and retain 55–60% of dietary cholesterol, but the net absorption of non-cholesterol sterols is <5% [20,23,24]. Most of these non-cholesterol sterols are rapidly excreted by the liver with <1% ultimately retained [20,25]. Absorption of phytosterols depends on the nature of the C-24 side chain. Increasing complexity of the side chain increases hydrophobicity, which reduces absorption [26]. The standard calorimetric and enzymatic techniques for cholesterol measurement identify double bonds between the C5 and C6 carbons and 3-hydroxy groups, respectively. Since phytosterols also contain these bonds, conventional cholesterol measurement methods cannot distinguish between cholesterol and phytosterols. Nevertheless, because phytosterols normally comprise <1% of circulating sterols, they typically have little effect on estimates of plasma cholesterol using these methods. The specific measurement of phytosterols and stanols currently requires the use of gas–liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) [27].
3. Absorption of cholesterol and phytosterols Dietary and biliary cholesterol and phytosterols must be solubilized in micellar form for absorption. The sterol-laden micelle interacts with the intestinal brush border thereby facilitating the uptake of sterols by enterocytes. The precise molecular mechanisms for sterol absorption are not well defined but cholesterol and phytosterol absorption both require the Niemann-Pick C1 Like 1 Protein (NPC1L1). NPC1L1-deficient transgenic mice have markedly reduced cholesterol absorption and do not experience a further reduction with the cholesterol absorption inhibitor ezetimibe, indicating that ezetimibe alters an NPC1L1-related sterol absorption pathway. In addition, NPC1L1-deficient transgenic mice have virtually undetectable plasma phytosterol concentrations suggesting that NPC1L1 is the primary pathway for their absorption [28]. Within the enterocyte, cholesterol is esterified by acyl cholesterol acyl transferase (ACAT2), packaged into chylomicrons at the basolateral membrane, and secreted into the lymphatic system for portage to the left subclavian vein. Unesterified cholesterol and phytosterols are transported back into the intestinal lumen by a specific pump called the “sterolin pump” containing the ATP
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Binding Cassette (ABC) proteins, ABCG5 and ABCG8 [29]. Phytosterols are effective in reducing cholesterol absorption because they displace cholesterol from micelles making unesterified cholesterol less available for absorption [30,31]. The majority of phytosterols are excreted, resulting in low net absorption as noted above, but small amounts are absorbed and rapidly secreted into the bile. 3.1. The therapeutic use of phytosterols to lower LDL-cholesterol Phytosterols, and specifically beta-sitosterol, were documented to reduce serum cholesterol concentrations as early as 1951 [8]. This original observation has been confirmed in multiple studies. In most of the studies, doses of phytosterol or phytostanols ranged from 0.8 to 4.0 g daily and reduced LDL-C concentrations 10–15% [32]. A recent review of approximately 40 treatment studies concluded that the placebo-adjusted reduction in LDL-C is 10% with 2 g of phytosterol or stanols daily, with little additional reduction at higher doses [32]. Among 15 studies using 3–4.2 g daily, however, the average reduction in LDL-C was 15.4% [32] suggesting that higher doses may produce greater LDL-C reductions. In 1957, Eli Lilly introduced B-sitosterol as a cholesterollowering agent called “Cytellin,” manufactured as a suspension. Because of its poor water solubility and bioavailability (a dose of up to 18 g/day was required), it was not profitable as a pharmaceutical agent, and consequently abandoned [33]. There has been recent renewed interest in phytosterols with the introduction of these agents as “nutriceuticals” or foods with therapeutic value. Phytosterols and stanols have limited water solubility, a property that can be enhanced by their esterification. This allows them to be used as additives in fatty foods or as a spread, such as Take Control® (Unilever) and Benecol® (McNeil Nutritionals) margarines, which contain stanol and sterol esters, respectively. When added to statin therapy as three servings a day or 5.1 g of phytosterol, these agents can reduce LDL-C 10% more than placebo and statin therapy alone [34]. Phytosterol-induced reductions in blood LDL-C vary among patients [7]. This variation appears to be unrelated to variations in dietary cholesterol intake probably because the biliary excretion of cholesterol via the gut (approximately 2 g daily) is considerably greater than the average dietary cholesterol intake of approximately 500 mg a day [7]. LDL-C reduction with phytosterols is greater in patients with enhanced cholesterol absorption, especially if they have simultaneously reduced hepatic cholesterol production. For example, individuals with the apolipoprotein (Apo) E 4 allele absorb more intestinal cholesterol than Apo E3 subjects. Some studies suggest that phytosterol therapy is more effective at reducing LDL-C in Apo E-4 homozygotes because of their enhanced cholesterol absorption [35], although other reports find lit-
tle or no influence of Apo E on the cholesterol response [36].
4. Phytosterolemia—clinical features Bhattacharyya and Connor in 1974 described two sisters, ages 20 and 22 years, with tendon xanthoma, normal cholesterol levels and plasma sitosterol levels of 27.1 and 17.7 mg/dl, comprising 11 and 16% of their circulating sterols [37]. Normally sitosterol comprises <1% of circulating levels [20,25]. Balance studies demonstrated enhanced absorption of both beta-sitosterol (24 and 28% vs. <5% normally) and cholesterol suggesting a generalized over absorption of sterols [37]. Studies of other patients with sitosterolemia, which is more recently and correctly called phytosterolemia, demonstrated increased phytosterols absorption of between 16 and 63% [16,38]. Phytosterolemia is rare and its true prevalence is unknown. A recent report on the use of ezetimibe in this condition included 37 patients [11]. Patients present at a young age with xanthoma of the Achilles tendon and the extensor tendons of the hand, and in some cases tuberous xanthoma, similar to those of familial hypercholesterolemia (FH). The most common clinical symptoms are produced by premature atherosclerosis, including angina pectoris, myocardial infarction and sudden death [15,39]. Atheromatous coronary ostial narrowing and diffuse CAD leading to sudden death has been reported in patients as young as 5 years [40]. These patients can also develop atheromatous aortic stenosis [9]. The age of subjects at the time of vascular disease diagnosis for all reported cases ranges from 5 to 45 years, with males presenting earlier than females [9]. Non-cardiac manifestations include large abnormally shaped red blood cells, thrombocytopenia, hemolysis and chronic hemolytic anemia presumably related to abnormal sterol content of the red cell membrane. The hemolytic episodes and anemia resolve with successful therapy. Arthritis and arthralgias are also reported in some phytosterolemic patients, as are abnormal liver function tests [9,39,41–44]. There is no evidence that high levels of plant sterols exert hormonal effects in humans, but sitosterol injected into male rats decreases testicular weight and sperm counts, whereas in female rats sitosterol increases uterine weight [45–47]. Phytosterolemia shares several clinical features with FH, such as the development of xanthoma and premature atherosclerosis [43]. In contrast to FH, patients with phytosterolemia usually have normal or only moderately elevated total sterol levels, measured as total blood cholesterol, but a very high ratio of plant sterols to total cholesterol [38]. There may be a transient phase during childhood, however, where phytosterolemic patients have extremely elevated cholesterol in the range of those seen in homozygous FH [44]. The development of extremely premature vascular disease, despite the presence of only normal or only moderately elevated plasma
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total sterol levels, suggests that the phytosterols are especially injurious to vascular tissue. Also, phytosterols comprised <17% dry weight of the tendon xanthoma removed from one of the two original phytosterolemia patients with the remaining sterol being free and esterified cholesterol, consistent with the hypothesis that phytosterols may facilitate the entry of cholesterol into tissue [37]. 5. The genetics of phytosterolemia Phytosterolemia is an autosomal recessive disease that occurs 1 in 5 million people due to defects in either the ABCG5 or ABCG8 genes. The protein products, now called ABCG5 and ABCG8, were previously labeled sterolin 1 and sterolin 2, respectively [48]. These genes are located on human chromosome 2p21 and span ∼60 kb [49]. Both genes contain 13 exons with translational start sites only 374 base pairs (bp) apart [49]. Most ABC proteins contain 12 domains, but ABCG5 and ABCG8 each encode for only six domains, half of the required ABC transporter. Phytosterolemic patients invariably have two mutant alleles of ABCG5 or two mutant alleles of ABCG8. Interestingly, no patient with phytosterolemia and mutations in both ABCG5 and ABCG8 has been described suggesting that absence of either functional ABCG5 or ABCG6 is required to produce phytosterolemia. There are no clinical differences between patients homozygous for the ABCG5 or ABCG8 mutations. The disease had been described in Amish-Mennonite, Indian, and Japanese subjects. Most Japanese probands have demonstrated defects in ABCG5, whereas most, but not all, Caucasian families have defects in ABCG8 [50,51]. Mutation is more common in ABCG8 and most of these are located in exons 4 and 11. Obligate heterozygotes for functional defects in ABCG5 or ABCG8 do not manifest clinical symptoms of disease, although these subjects are reported to have modestly increased plant sterol absorption, with normal or accelerated excretion [16,41]. There appear to be many polymorphisms of these genes, but their clinical significance and effects on sterol absorption are not known.
6. Phytosterol absorption and turnover ABCG5 and G8 not only regulate sterol absorption, but are also responsible for biliary excretion of phytosterols. Consequently, the total body phytosterol pool in phytosterolemia is increased because of both increased absorption and decreased excretion. In a study of one control and two patients, calculated total sitosterol pool size in the control subject was 290 mg and linearly related to a low rate of absorption. In the phytosterolemic patients the sitosterol pools were 13 (3500 mg) and 17 (4800 mg) times higher than the control due to both increased absorption and reduced biliary excretion of sitosterol [52]. As noted above, heterozygotes have higher absorption rates than controls, but
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normal body pool sizes due to the ability of heterozygotes to excrete sitosterol normally [53].
7. Cholesterol absorption and turnover The augmented phytosterol absorption characteristic of phytosterolemia does not appear to interfere with cholesterol absorption. However, sitosterolemia is associated with a 50–80% reduction in cholesterol synthesis associated with a marked reduction in HMG-CoA reductase activity in liver samples from phytosterolemic patients [54]. HMG-CoA reductase activity is also reduced in intestinal mucosa and in monocytes from affected patients [54,55]. Low-density lipoprotein receptor activity is normal or enhanced in phytosterolemic patients possibly to compensate for reduced cholesterol production [54,56]. Phytosterolemia also inhibits CYP7A and hepatic sterol 27-hydroxylase, the rate-limiting enzymes in bile acid metabolism [57,58], and this may increase serum cholesterol levels in some phytosterolemic patients. In phytosterolemic patients with normal cholesterol levels, very high levels of cholestanol, the saturated derivative of cholesterol, have been reported. The normal diet contains very little cholestanol and most is derived from endogenous cholesterol metabolism presumably from reduced clearance of endogenous cholesterol [15]. The mechanism for, and the significance of, increased cholestanol levels in sitosterolemic patients are not clear.
8. Diagnosis of phytosterolemia The diagnosis of phytosterolemia should be considered in childhood or adolescent patients with tendon xanthoma and no family history of hypercholesterolemia despite markedly elevated “cholesterol” levels in the child. These elevated “cholesterol” are usually due to the inability of standard cholesterol measurement techniques to differentiate phytosterols and cholesterol, although children may go through a transient stage of markedly elevations in the real cholesterol concentration [44]. The diagnosis of phytosterolemia should also be considered if there is a precipitous fall in serum cholesterol with a low cholesterol diet (10–20% in normal subjects versus >45% in phytosterolemic subjects) and a less than expected response to HMG CoA reductase inhibitors [44,59,60] Gas liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) is required to differentiate elevated phytosterols from other sterols including cholesterol [27].
9. Treatment of phytosterolemia Historically, treatment of patients with phytosterolemia consisted of dietary restriction of phytosterol intake and the use of bile sequestrant resins [61,62]. Recalcitrant cases
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were treated with ileal bypass surgery or plasma apheresis. More recently, ezetimibe has emerged as the treatment of choice for phytosterolemia, because it is effective, does not require surgery or recurrent apheresis, and has very few side effects [10]. Ezetimibe is active at the intestinal brush border and inhibits the absorption of sterols including cholesterol and phytosterols. Ezetimibe lowers LDL-C on average ∼20% in typical hypercholesterolemic patients [63], while sitosterol and campesterol have been reported to be reduced by 41 and 48%, respectively, after 12 weeks of therapy [10]. Ezetimibe in phytosterolemic patients can be combined with bile sequestrant resins if necessary. Phytosterolemic patients generally do not respond to statin therapy. Indeed, statin therapy may increase phytosterol plasma levels, possibly as a result of reduced biliary secretion and enhanced absorption of sterols [60,64].
10. Plant sterols as a novel CAD risk factor The observation that over absorption of phytosterols can produce premature CAD and aortic stenosis in individuals with defects in ABCG5/8 raises the possibility that increased serum levels of phytosterols can contribute to CAD in the general population. Recently, Berge et al. concluded that variation in the plasma concentration of plant sterols is highly heritable and that genetic polymorphisms in ABCG8 contribute to this variability [65]. These variations in phytosterol absorption could affect individual CAD risk. Very few studies have addressed phytosterols as CAD risk factors. Glueck et al. in 1991 examined plant sterol levels in 595 patients with hypercholesterolemia and concluded that phytosterols were a risk factor for premature coronary artery disease, independent of cholesterol, in subjects <55 years of age [66]. Sudhop et al. in 2002 measured sitosterol and campesterol levels in 53 patients not on lipid-lowering medications who were admitted for elective coronary artery bypass surgery. Patients with a family history of CAD had similar LDL-C levels but significantly higher serum levels of both sitosterol and campesterol, suggesting that differences in plant sterol metabolism contribute to the risk associated with family history [67]. Furthermore, a recent preliminary report observed that even modest increases in sitosterol levels in hypercholesterolemic patients doubles the risk of CAD events [68]. Phytosterols have also recently been detected in mature atheromatous plaques obtained by atherectomy from otherwise normal adults documenting that phytosterols could be involved in the atherosclerotic process even in the absence of markedly abnormal phytosterol metabolism [69]. All of these studies support the possibility that elevated phytosterols could contribute to the development of premature coronary artery diseases in certain families. Phytosterols not only could contribute to atherosclerotic bulk, but might also accelerate the atherosclerotic inflammatory process. This possibility is supported by the observations, mentioned earlier, that premature vascular disease
occurs in patients with sitosterolemia despite the absence of markedly elevated sterol levels and that the tendon xanthoma in these patients are composed largely of cholesterol suggesting that plant sterols can somehow accelerate cholesterol deposition [9]. Oxidized LDL is recognized as an inflammatory component of the atherosclerotic process [70,71]. Interestingly, plant sterols are more readily oxidized than cholesterol, which could contribute to their atherosclerotic risk [72]. The possibility that phytosterols are a CAD risk factor is speculative, but deserves further consideration for several reasons. First, plant sterols and their esters are used in food products to reduce LDL-C and putatively reduce the risk of CAD. Esterification of phytosterols in food preparations further reduces, but does not eliminate, their absorption, and any increase in their serum concentration could be problematic if they increase CAD risk. Since phytosterols are measured as cholesterol, by commercial tests, and since phytosterols and stanol esters reduce serum cholesterol, it is assumed that the LDL-C lowering benefit of plant sterols outweighs any cardiovascular risk. Several groups monitor the safety of these food additives, but long-term outcome studies on atherosclerotic events are not presently available for subjects taking such preparations [32]. Currently 2500 users of stanol margarines and matched controls are being monitored to detect possible side effects of these compounds, although this study lacks sufficient power to exclude an increase in cardiovascular events in the stanol user group [73]. Second, phytosterol concentrations have been reported to increase during therapy with statins, and this effect could theoretically undermine some of the beneficial effects of statin therapy [60]. Third, identification of phytosterols as a CAD risk factor may permit the identification of individuals who over absorb these compounds because of genetic variation, and thereby permit earlier aggressive therapy. Studies examining the relationship of phytosterol levels to CAD events are required to evaluate any contribution of natural or therapeutic plant sterols to CAD risk. Such trials would be demanding because of the number of subjects required for a definitive conclusion, the confounding effects of individual dietary intake, and the cost. Trials using surrogate markers of atherosclerosis, such as carotid intima medial thickening, would be useful, but are not to our knowledge presently available. Nevertheless, the available data on the CAD risk of phytosterols are reassuring. The Dallas Heart Study measured coronary artery calcification using electron beam computer tomography in 2542 subjects aged 30–67 years. Plasma cholesterol levels, but not sitosterol or campesterol, were significantly higher in subjects with positive EBCT scans [74]. Similarly, ABCG5/ABCG8 knockout mice, used as an animal model of phytosterolemia, have similar cholesterol levels to control mice, markedly higher plasma sterol levels (12% versus 0.2% of total sterols), but no difference in atherosclerotic aortic lesion area [74]. Such results suggest that phytosterol levels are not a risk factor for heart disease, but definitive data are lacking.
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11. Summary Phytosterols and their esters are used clinically to reduce LDL-C and atherosclerotic risk. In contrast, extremely rare genetic defects in the ABCG5 and ABCG8 genes, which regulate sterol absorption, can produce hyper absorption of phytosterols resulting in “sitosterolemia” or “phytosterolemia,” with premature CAD, atherosclerotic aortic valve disease, and a variety of other clinical manifestations. Treatment of phytosterolemia includes bile sequestrant resins, plasma and LDL apheresis, intestinal ileal bypass and more recently, ezetimibe, which inhibits phytosterol absorption and can markedly lower plasma phytosterols. In addition to major defects in ABCG5 and ABCG8, there are multiple genetic polymorphisms, which may produce increased phytosterol absorption in some patients. There is conflicting data as to whether or not phytosterols are a risk factor for CAD. This issue is not settled, but should receive increased attention because of the potential importance of the problem and the present availability of an agent that inhibits intestinal phytosterol absorption.
Conflict of interest statement Dr. Thompson has received grants and speaking honoraria from Schering Plough and Merck who manufacture and market ezetimibe.
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Review
Phytosterols and vascular disease Manoj D. Patel, Paul D. Thompson ∗ Section of Preventive Cardiology, Division of Cardiology, Henry Low Heart Center, Hartford Hospital, 80 Seymour Street, Hartford, CT 06102, USA Received 13 October 2005; accepted 13 October 2005 Available online 2 December 2005
Abstract Phytosterols or plant sterols have long been known to lower serum cholesterol concentrations by competing with dietary and biliary cholesterol for intestinal absorption. Food products containing phytosterols and phytostanols are now available to assist in lowering blood cholesterol levels. In contrast to these possibly beneficial effects of plant sterols, a rare genetic condition called sitosterolemia, an autosomal recessive disorder also known as phytosterolemia, is characterized by over absorption of phytosterols and premature coronary artery and aortic valve disease. Phytosterols have also recently been identified in atheromatous plaque obtained from individuals with apparently normal absorption of plant sterols raising the possibility that phytosterols are a novel atherosclerotic risk factor. This article reviews phytosterols and their relationship to vascular disease. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Atherosclerosis; Cholesterol; Coronary disease; Hypercholesterolemia; Risk factors
Contents 1. 2. 3.
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Phytosterols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Absorption of cholesterol and phytosterols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. The therapeutic use of phytosterols to lower LDL-cholesterol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4. Phytosterolemia—clinical features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5. The genetics of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6. Phytosterol absorption and turnover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7. Cholesterol absorption and turnover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8. Diagnosis of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9. Treatment of phytosterolemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 10. Plant sterols as a novel CAD risk factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Conflict of interest statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction The introduction of lovastatin in the United States in August 1987 revolutionized the treatment of hypercholes∗
Corresponding author. Tel.: +1 860 545 2899; fax: +1 860 545 2882. E-mail address: [email protected] (P.D. Thompson).
0021-9150/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2005.10.026
terolemia. Since this introduction numerous primary and secondary statin prevention trials, as well as non-statin trials such as the Lipid Research Clinical intervention trial, have confirmed the pivotal role of low-density lipoprotein (LDL) cholesterol (C) in the atherosclerotic disease process [1,2]. Despite these advances in coronary artery disease (CAD) prevention, CAD remains the primary cause of death in Western
M.D. Patel, P.D. Thompson / Atherosclerosis 186 (2006) 12–19
societies. Furthermore, in statin clinical trials, atherosclerotic events continue to occur, albeit less frequently, in statintreated patients. Such observations have led to treatment strategies designed to address other important lipoprotein risk factors, such as high-density lipoprotein (HDL) C, as well as a search for other risk factors and markers of risk [3,4]. Plant sterols or “phytosterols” have long been known to reduce serum LDL-C level by competing with dietary and biliary cholesterol for intestinal absorption [5–7]. Indeed, beta sitosterol was first described as a therapeutic agent for hypercholesterolemia in 1951 [8]. In contrast to these putatively beneficial effects of phytosterols, sitosterolemia or phytosterolemia, is a rare autosomal recessive disorder, in which phytosterols are over absorbed and accumulate in tissue causing tendon xanthoma and premature coronary artery disease [9]. There is recent renewed interest in phytosterols as a possible novel CAD risk factor. This interest has been heightened by the availability of an agent, ezetimibe, which interferes with the absorption of both cholesterol and phytosterols [10,11]. This review examines phytosterols and their possible role in cardiovascular disease and its management. 2. Phytosterols Phytosterols are nonnutritive compounds whose chemical structure differs from that of cholesterol because of the presence of modified side chains at carbon C-24 [12]. Sitosterol, campesterol, and stigasterol are most abundant and on average comprise 65, 30, and 3% of dietary phytosterol intake. Their primary dietary source is fat-rich vegetables and vegetable products, including vegetable oils, fruits and nuts [13,14]. Sitosterol and campesterol have an ethyl and methyl substitute at C-24 (Fig. 1), respectively,
Fig. 1. Structure of cholesterol and common phytosterols (reprinted with permission from Safety and evaluation of phytosterol esters Part-2. Food Chem Toxicol 1999;37:521–32).
13
whereas stigasterol is identical to sitosterol except for a double bond at C-22. Stanols are saturated sterols, meaning that they have no double bonds in the sterol ring. Stanols are less abundant in nature [15]. Unabsorbed sterols can undergo bacterial transformation by intestinal microflora to produce metabolites, such as coprosterol and coprostanone [16–19]. Cholesterol and phytosterols are structurally similar but metabolized differently. Mammals do not synthesize phytosterols [20]. The typical western diet contains approximately 200–500 mg of dietary cholesterol and 200–400 mg of non-cholesterol sterols [13,21,22]. Humans absorb and retain 55–60% of dietary cholesterol, but the net absorption of non-cholesterol sterols is <5% [20,23,24]. Most of these non-cholesterol sterols are rapidly excreted by the liver with <1% ultimately retained [20,25]. Absorption of phytosterols depends on the nature of the C-24 side chain. Increasing complexity of the side chain increases hydrophobicity, which reduces absorption [26]. The standard calorimetric and enzymatic techniques for cholesterol measurement identify double bonds between the C5 and C6 carbons and 3-hydroxy groups, respectively. Since phytosterols also contain these bonds, conventional cholesterol measurement methods cannot distinguish between cholesterol and phytosterols. Nevertheless, because phytosterols normally comprise <1% of circulating sterols, they typically have little effect on estimates of plasma cholesterol using these methods. The specific measurement of phytosterols and stanols currently requires the use of gas–liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) [27].
3. Absorption of cholesterol and phytosterols Dietary and biliary cholesterol and phytosterols must be solubilized in micellar form for absorption. The sterol-laden micelle interacts with the intestinal brush border thereby facilitating the uptake of sterols by enterocytes. The precise molecular mechanisms for sterol absorption are not well defined but cholesterol and phytosterol absorption both require the Niemann-Pick C1 Like 1 Protein (NPC1L1). NPC1L1-deficient transgenic mice have markedly reduced cholesterol absorption and do not experience a further reduction with the cholesterol absorption inhibitor ezetimibe, indicating that ezetimibe alters an NPC1L1-related sterol absorption pathway. In addition, NPC1L1-deficient transgenic mice have virtually undetectable plasma phytosterol concentrations suggesting that NPC1L1 is the primary pathway for their absorption [28]. Within the enterocyte, cholesterol is esterified by acyl cholesterol acyl transferase (ACAT2), packaged into chylomicrons at the basolateral membrane, and secreted into the lymphatic system for portage to the left subclavian vein. Unesterified cholesterol and phytosterols are transported back into the intestinal lumen by a specific pump called the “sterolin pump” containing the ATP
14
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Binding Cassette (ABC) proteins, ABCG5 and ABCG8 [29]. Phytosterols are effective in reducing cholesterol absorption because they displace cholesterol from micelles making unesterified cholesterol less available for absorption [30,31]. The majority of phytosterols are excreted, resulting in low net absorption as noted above, but small amounts are absorbed and rapidly secreted into the bile. 3.1. The therapeutic use of phytosterols to lower LDL-cholesterol Phytosterols, and specifically beta-sitosterol, were documented to reduce serum cholesterol concentrations as early as 1951 [8]. This original observation has been confirmed in multiple studies. In most of the studies, doses of phytosterol or phytostanols ranged from 0.8 to 4.0 g daily and reduced LDL-C concentrations 10–15% [32]. A recent review of approximately 40 treatment studies concluded that the placebo-adjusted reduction in LDL-C is 10% with 2 g of phytosterol or stanols daily, with little additional reduction at higher doses [32]. Among 15 studies using 3–4.2 g daily, however, the average reduction in LDL-C was 15.4% [32] suggesting that higher doses may produce greater LDL-C reductions. In 1957, Eli Lilly introduced B-sitosterol as a cholesterollowering agent called “Cytellin,” manufactured as a suspension. Because of its poor water solubility and bioavailability (a dose of up to 18 g/day was required), it was not profitable as a pharmaceutical agent, and consequently abandoned [33]. There has been recent renewed interest in phytosterols with the introduction of these agents as “nutriceuticals” or foods with therapeutic value. Phytosterols and stanols have limited water solubility, a property that can be enhanced by their esterification. This allows them to be used as additives in fatty foods or as a spread, such as Take Control® (Unilever) and Benecol® (McNeil Nutritionals) margarines, which contain stanol and sterol esters, respectively. When added to statin therapy as three servings a day or 5.1 g of phytosterol, these agents can reduce LDL-C 10% more than placebo and statin therapy alone [34]. Phytosterol-induced reductions in blood LDL-C vary among patients [7]. This variation appears to be unrelated to variations in dietary cholesterol intake probably because the biliary excretion of cholesterol via the gut (approximately 2 g daily) is considerably greater than the average dietary cholesterol intake of approximately 500 mg a day [7]. LDL-C reduction with phytosterols is greater in patients with enhanced cholesterol absorption, especially if they have simultaneously reduced hepatic cholesterol production. For example, individuals with the apolipoprotein (Apo) E 4 allele absorb more intestinal cholesterol than Apo E3 subjects. Some studies suggest that phytosterol therapy is more effective at reducing LDL-C in Apo E-4 homozygotes because of their enhanced cholesterol absorption [35], although other reports find lit-
tle or no influence of Apo E on the cholesterol response [36].
4. Phytosterolemia—clinical features Bhattacharyya and Connor in 1974 described two sisters, ages 20 and 22 years, with tendon xanthoma, normal cholesterol levels and plasma sitosterol levels of 27.1 and 17.7 mg/dl, comprising 11 and 16% of their circulating sterols [37]. Normally sitosterol comprises <1% of circulating levels [20,25]. Balance studies demonstrated enhanced absorption of both beta-sitosterol (24 and 28% vs. <5% normally) and cholesterol suggesting a generalized over absorption of sterols [37]. Studies of other patients with sitosterolemia, which is more recently and correctly called phytosterolemia, demonstrated increased phytosterols absorption of between 16 and 63% [16,38]. Phytosterolemia is rare and its true prevalence is unknown. A recent report on the use of ezetimibe in this condition included 37 patients [11]. Patients present at a young age with xanthoma of the Achilles tendon and the extensor tendons of the hand, and in some cases tuberous xanthoma, similar to those of familial hypercholesterolemia (FH). The most common clinical symptoms are produced by premature atherosclerosis, including angina pectoris, myocardial infarction and sudden death [15,39]. Atheromatous coronary ostial narrowing and diffuse CAD leading to sudden death has been reported in patients as young as 5 years [40]. These patients can also develop atheromatous aortic stenosis [9]. The age of subjects at the time of vascular disease diagnosis for all reported cases ranges from 5 to 45 years, with males presenting earlier than females [9]. Non-cardiac manifestations include large abnormally shaped red blood cells, thrombocytopenia, hemolysis and chronic hemolytic anemia presumably related to abnormal sterol content of the red cell membrane. The hemolytic episodes and anemia resolve with successful therapy. Arthritis and arthralgias are also reported in some phytosterolemic patients, as are abnormal liver function tests [9,39,41–44]. There is no evidence that high levels of plant sterols exert hormonal effects in humans, but sitosterol injected into male rats decreases testicular weight and sperm counts, whereas in female rats sitosterol increases uterine weight [45–47]. Phytosterolemia shares several clinical features with FH, such as the development of xanthoma and premature atherosclerosis [43]. In contrast to FH, patients with phytosterolemia usually have normal or only moderately elevated total sterol levels, measured as total blood cholesterol, but a very high ratio of plant sterols to total cholesterol [38]. There may be a transient phase during childhood, however, where phytosterolemic patients have extremely elevated cholesterol in the range of those seen in homozygous FH [44]. The development of extremely premature vascular disease, despite the presence of only normal or only moderately elevated plasma
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total sterol levels, suggests that the phytosterols are especially injurious to vascular tissue. Also, phytosterols comprised <17% dry weight of the tendon xanthoma removed from one of the two original phytosterolemia patients with the remaining sterol being free and esterified cholesterol, consistent with the hypothesis that phytosterols may facilitate the entry of cholesterol into tissue [37]. 5. The genetics of phytosterolemia Phytosterolemia is an autosomal recessive disease that occurs 1 in 5 million people due to defects in either the ABCG5 or ABCG8 genes. The protein products, now called ABCG5 and ABCG8, were previously labeled sterolin 1 and sterolin 2, respectively [48]. These genes are located on human chromosome 2p21 and span ∼60 kb [49]. Both genes contain 13 exons with translational start sites only 374 base pairs (bp) apart [49]. Most ABC proteins contain 12 domains, but ABCG5 and ABCG8 each encode for only six domains, half of the required ABC transporter. Phytosterolemic patients invariably have two mutant alleles of ABCG5 or two mutant alleles of ABCG8. Interestingly, no patient with phytosterolemia and mutations in both ABCG5 and ABCG8 has been described suggesting that absence of either functional ABCG5 or ABCG6 is required to produce phytosterolemia. There are no clinical differences between patients homozygous for the ABCG5 or ABCG8 mutations. The disease had been described in Amish-Mennonite, Indian, and Japanese subjects. Most Japanese probands have demonstrated defects in ABCG5, whereas most, but not all, Caucasian families have defects in ABCG8 [50,51]. Mutation is more common in ABCG8 and most of these are located in exons 4 and 11. Obligate heterozygotes for functional defects in ABCG5 or ABCG8 do not manifest clinical symptoms of disease, although these subjects are reported to have modestly increased plant sterol absorption, with normal or accelerated excretion [16,41]. There appear to be many polymorphisms of these genes, but their clinical significance and effects on sterol absorption are not known.
6. Phytosterol absorption and turnover ABCG5 and G8 not only regulate sterol absorption, but are also responsible for biliary excretion of phytosterols. Consequently, the total body phytosterol pool in phytosterolemia is increased because of both increased absorption and decreased excretion. In a study of one control and two patients, calculated total sitosterol pool size in the control subject was 290 mg and linearly related to a low rate of absorption. In the phytosterolemic patients the sitosterol pools were 13 (3500 mg) and 17 (4800 mg) times higher than the control due to both increased absorption and reduced biliary excretion of sitosterol [52]. As noted above, heterozygotes have higher absorption rates than controls, but
15
normal body pool sizes due to the ability of heterozygotes to excrete sitosterol normally [53].
7. Cholesterol absorption and turnover The augmented phytosterol absorption characteristic of phytosterolemia does not appear to interfere with cholesterol absorption. However, sitosterolemia is associated with a 50–80% reduction in cholesterol synthesis associated with a marked reduction in HMG-CoA reductase activity in liver samples from phytosterolemic patients [54]. HMG-CoA reductase activity is also reduced in intestinal mucosa and in monocytes from affected patients [54,55]. Low-density lipoprotein receptor activity is normal or enhanced in phytosterolemic patients possibly to compensate for reduced cholesterol production [54,56]. Phytosterolemia also inhibits CYP7A and hepatic sterol 27-hydroxylase, the rate-limiting enzymes in bile acid metabolism [57,58], and this may increase serum cholesterol levels in some phytosterolemic patients. In phytosterolemic patients with normal cholesterol levels, very high levels of cholestanol, the saturated derivative of cholesterol, have been reported. The normal diet contains very little cholestanol and most is derived from endogenous cholesterol metabolism presumably from reduced clearance of endogenous cholesterol [15]. The mechanism for, and the significance of, increased cholestanol levels in sitosterolemic patients are not clear.
8. Diagnosis of phytosterolemia The diagnosis of phytosterolemia should be considered in childhood or adolescent patients with tendon xanthoma and no family history of hypercholesterolemia despite markedly elevated “cholesterol” levels in the child. These elevated “cholesterol” are usually due to the inability of standard cholesterol measurement techniques to differentiate phytosterols and cholesterol, although children may go through a transient stage of markedly elevations in the real cholesterol concentration [44]. The diagnosis of phytosterolemia should also be considered if there is a precipitous fall in serum cholesterol with a low cholesterol diet (10–20% in normal subjects versus >45% in phytosterolemic subjects) and a less than expected response to HMG CoA reductase inhibitors [44,59,60] Gas liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) is required to differentiate elevated phytosterols from other sterols including cholesterol [27].
9. Treatment of phytosterolemia Historically, treatment of patients with phytosterolemia consisted of dietary restriction of phytosterol intake and the use of bile sequestrant resins [61,62]. Recalcitrant cases
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were treated with ileal bypass surgery or plasma apheresis. More recently, ezetimibe has emerged as the treatment of choice for phytosterolemia, because it is effective, does not require surgery or recurrent apheresis, and has very few side effects [10]. Ezetimibe is active at the intestinal brush border and inhibits the absorption of sterols including cholesterol and phytosterols. Ezetimibe lowers LDL-C on average ∼20% in typical hypercholesterolemic patients [63], while sitosterol and campesterol have been reported to be reduced by 41 and 48%, respectively, after 12 weeks of therapy [10]. Ezetimibe in phytosterolemic patients can be combined with bile sequestrant resins if necessary. Phytosterolemic patients generally do not respond to statin therapy. Indeed, statin therapy may increase phytosterol plasma levels, possibly as a result of reduced biliary secretion and enhanced absorption of sterols [60,64].
10. Plant sterols as a novel CAD risk factor The observation that over absorption of phytosterols can produce premature CAD and aortic stenosis in individuals with defects in ABCG5/8 raises the possibility that increased serum levels of phytosterols can contribute to CAD in the general population. Recently, Berge et al. concluded that variation in the plasma concentration of plant sterols is highly heritable and that genetic polymorphisms in ABCG8 contribute to this variability [65]. These variations in phytosterol absorption could affect individual CAD risk. Very few studies have addressed phytosterols as CAD risk factors. Glueck et al. in 1991 examined plant sterol levels in 595 patients with hypercholesterolemia and concluded that phytosterols were a risk factor for premature coronary artery disease, independent of cholesterol, in subjects <55 years of age [66]. Sudhop et al. in 2002 measured sitosterol and campesterol levels in 53 patients not on lipid-lowering medications who were admitted for elective coronary artery bypass surgery. Patients with a family history of CAD had similar LDL-C levels but significantly higher serum levels of both sitosterol and campesterol, suggesting that differences in plant sterol metabolism contribute to the risk associated with family history [67]. Furthermore, a recent preliminary report observed that even modest increases in sitosterol levels in hypercholesterolemic patients doubles the risk of CAD events [68]. Phytosterols have also recently been detected in mature atheromatous plaques obtained by atherectomy from otherwise normal adults documenting that phytosterols could be involved in the atherosclerotic process even in the absence of markedly abnormal phytosterol metabolism [69]. All of these studies support the possibility that elevated phytosterols could contribute to the development of premature coronary artery diseases in certain families. Phytosterols not only could contribute to atherosclerotic bulk, but might also accelerate the atherosclerotic inflammatory process. This possibility is supported by the observations, mentioned earlier, that premature vascular disease
occurs in patients with sitosterolemia despite the absence of markedly elevated sterol levels and that the tendon xanthoma in these patients are composed largely of cholesterol suggesting that plant sterols can somehow accelerate cholesterol deposition [9]. Oxidized LDL is recognized as an inflammatory component of the atherosclerotic process [70,71]. Interestingly, plant sterols are more readily oxidized than cholesterol, which could contribute to their atherosclerotic risk [72]. The possibility that phytosterols are a CAD risk factor is speculative, but deserves further consideration for several reasons. First, plant sterols and their esters are used in food products to reduce LDL-C and putatively reduce the risk of CAD. Esterification of phytosterols in food preparations further reduces, but does not eliminate, their absorption, and any increase in their serum concentration could be problematic if they increase CAD risk. Since phytosterols are measured as cholesterol, by commercial tests, and since phytosterols and stanol esters reduce serum cholesterol, it is assumed that the LDL-C lowering benefit of plant sterols outweighs any cardiovascular risk. Several groups monitor the safety of these food additives, but long-term outcome studies on atherosclerotic events are not presently available for subjects taking such preparations [32]. Currently 2500 users of stanol margarines and matched controls are being monitored to detect possible side effects of these compounds, although this study lacks sufficient power to exclude an increase in cardiovascular events in the stanol user group [73]. Second, phytosterol concentrations have been reported to increase during therapy with statins, and this effect could theoretically undermine some of the beneficial effects of statin therapy [60]. Third, identification of phytosterols as a CAD risk factor may permit the identification of individuals who over absorb these compounds because of genetic variation, and thereby permit earlier aggressive therapy. Studies examining the relationship of phytosterol levels to CAD events are required to evaluate any contribution of natural or therapeutic plant sterols to CAD risk. Such trials would be demanding because of the number of subjects required for a definitive conclusion, the confounding effects of individual dietary intake, and the cost. Trials using surrogate markers of atherosclerosis, such as carotid intima medial thickening, would be useful, but are not to our knowledge presently available. Nevertheless, the available data on the CAD risk of phytosterols are reassuring. The Dallas Heart Study measured coronary artery calcification using electron beam computer tomography in 2542 subjects aged 30–67 years. Plasma cholesterol levels, but not sitosterol or campesterol, were significantly higher in subjects with positive EBCT scans [74]. Similarly, ABCG5/ABCG8 knockout mice, used as an animal model of phytosterolemia, have similar cholesterol levels to control mice, markedly higher plasma sterol levels (12% versus 0.2% of total sterols), but no difference in atherosclerotic aortic lesion area [74]. Such results suggest that phytosterol levels are not a risk factor for heart disease, but definitive data are lacking.
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11. Summary Phytosterols and their esters are used clinically to reduce LDL-C and atherosclerotic risk. In contrast, extremely rare genetic defects in the ABCG5 and ABCG8 genes, which regulate sterol absorption, can produce hyper absorption of phytosterols resulting in “sitosterolemia” or “phytosterolemia,” with premature CAD, atherosclerotic aortic valve disease, and a variety of other clinical manifestations. Treatment of phytosterolemia includes bile sequestrant resins, plasma and LDL apheresis, intestinal ileal bypass and more recently, ezetimibe, which inhibits phytosterol absorption and can markedly lower plasma phytosterols. In addition to major defects in ABCG5 and ABCG8, there are multiple genetic polymorphisms, which may produce increased phytosterol absorption in some patients. There is conflicting data as to whether or not phytosterols are a risk factor for CAD. This issue is not settled, but should receive increased attention because of the potential importance of the problem and the present availability of an agent that inhibits intestinal phytosterol absorption.
Conflict of interest statement Dr. Thompson has received grants and speaking honoraria from Schering Plough and Merck who manufacture and market ezetimibe.
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