An Evidence-based Approach to Phytochemicals and Other Dietary Factors

18 Phytosterols

Throughout much of human evolution, it is likely that large amounts of plant foods were consumed.1 In addition to being rich in fiber and plant protein, the diets of our ancestors were also rich in phytosterols—plant-derived sterols that are similar in structure and function to cholesterol. There is increasing evidence that the reintroduction of plant foods providing phytosterols into the modern diet can improve serum lipid (cholesterol) profiles and reduce the risk of cardiovascular disease.²

Although cholesterol is the predominant sterol in animals, including humans, a variety of sterols are found in plants.³ Nutritionists recognize two classes of phytosterols: (1) sterols, which have a double bond in the sterol ring; and (2) stanols, which lack a double bond in the sterol ring (Fig. 18.1). The most abundant sterols in plants and the human diet are sitosterol and campesterol. Stanols are also present in plants, but they comprise only approximately 10% of total dietary phytosterols. Cholesterol in human blood and tissues is derived from the diet as well as endogenous cholesterol synthesis. In contrast, all phytosterols in human blood and tissues are derived from the diet because humans cannot synthesize phytosterols.⁴

Fig. 18.1 Chemical structure of cholesterol compared with plant sterols (sitosterol and campesterol) and plant stanols (sitostanol and campestanol).

Definitions

Phytosterols. A collective term for plant-derived sterols and stanols.
Plant sterols or stanols. Terms generally applied to plant-derived sterols or stanols; these phytochemicals are added to foods or supplements.
Plant sterol or stanol esters. Plant sterols or stanols that have been esterified by creating an ester bond between a fatty acid and the sterol or stanol. Esterification occurs in intestinal cells and is also an industrial process. Esterification makes plant sterols and stanols more fat soluble so they are easily incorporated into fat-containing foods, including margarines and salad dressings. In this chapter, the weights of plant sterol and stanol esters are expressed as the equivalent weights of free (unesterified) sterols and stanols.

Bioavailability and Metabolism

Absorption and Metabolism of Dietary Cholesterol

Dietary cholesterol must be incorporated into mixed micelles to be absorbed by the cells that line the intestine (enterocytes).5 Mixed micelles are mixtures of bile salts, lipids (fats), and sterols formed in the small intestine after a fat-containing meal is consumed. Inside the enterocyte, cholesterol is esterified and incorporated into triglyceride-rich lipoproteins known as chylomicrons, which enter the circulation.⁶ As circulating chylomicrons become depleted of triglycerides, they become chylomicron remnants, which are taken up by the liver. In the liver, cholesterol from chylomicron remnants may be repackaged into other lipoproteins for transport throughout the circulation or, alternatively, secreted into bile, which is released into the small intestine.

Absorption and Metabolism of Dietary Phytosterols

Although varied diets typically contain similar amounts of phytosterols and cholesterol, serum phytosterol concentrations are usually several hundred times lower than serum cholesterol concentrations in humans.7 Less than 10% of dietary phytosterols are systemically absorbed, in contrast to approximately 50%–60% of dietary cholesterol.⁸ Like cholesterol, phytosterols must be incorporated into mixed micelles before they are taken up by enterocytes. Once inside the enterocyte, systemic absorption of phytosterols is inhibited by the activity of efflux transporters, consisting of a pair of ATP-binding cassette (ABC) proteins known as ABCG5 and ABCG8.⁴ ABCG5 and ABCG8 each form one half of a transporter that secretes phytosterols and unesterified cholesterol from the enterocyte into the intestinal lumen. Phytosterols are secreted back into the intestine by ABCG5/G8 transporters at a much greater rate than cholesterol, resulting in much lower intestinal absorption of dietary phytosterols than of cholesterol. Within the enterocyte, phytosterols are not as readily esterified as cholesterol, so they are incorporated into chylomicrons at much lower concentrations. Those phytosterols that are incorporated into chylomicrons enter the circulation and are taken up by the liver. Once inside the liver, phytosterols are rapidly secreted into bile by hepatic ABCG5/G8 transporters. Although cholesterol may also be secreted into bile, the rate of phytosterol secretion into bile is much greater than the rate of cholesterol secretion.⁹ Thus, the low serum concentrations of phytosterols relative to cholesterol can be explained by decreased intestinal absorption and increased excretion of phytosterols into bile.

Biological Activities

Effects on Cholesterol Absorption and Lipoprotein Metabolism

It is well established that high intakes of plant sterols or stanols can lower serum total and low-density lipoprotein (LDL) cholesterol concentrations in humans (see the Cardiovascular Disease section below).10,11 In the intestinal lumen, phytosterols displace cholesterol from mixed micelles and inhibit cholesterol absorption.¹² In humans, the consumption of 1.5–1.8 g/day of plant sterols or stanols reduced cholesterol absorption by 30%–40%.13,14 At higher doses (2.2 g/day of plant sterols), cholesterol absorption was reduced by 60%.¹⁵ In response to decreased cholesterol absorption, tissue LDL-receptor expression was increased, resulting in increased clearance of circulating LDL.¹⁶ Decreased cholesterol absorption is also associated with increased cholesterol synthesis, and increasing phytosterol intake has been found to increase endogenous cholesterol synthesis in humans.¹³ Despite the increase in cholesterol synthesis induced by increasing phytosterol intake, the net result is a reduction in serum LDL cholesterol concentration.

Other Biological Activities

Experiments in cell culture and animal models suggest that phytosterols may have biological activities unrelated to cholesterol lowering. However, their significance in humans is not yet known.

Alterations in Cell Membrane Properties

Cholesterol is an important structural component of mammalian cell membranes.17 Displacement of cholesterol with phytosterols has been found to alter the physical properties of cell membranes in vitro,¹⁸ which could potentially affect signal transduction or membrane-bound enzyme activity.^19,20 Limited evidence from an animal model of hemorrhagic stroke suggested that very high intakes of plant sterols or stanols displaced cholesterol in red blood cell membranes, resulting in decreased deformability and potentially increased fragility.^21,22 However, daily phytosterol supplementation (1 g/1000 kcal) for 4 weeks did not alter red blood cell fragility in humans.²³

Alterations in Testosterone Metabolism

Limited evidence from animal studies suggests that very high phytosterol intakes can alter testosterone metabolism by inhibiting 5-α-reductase, a membrane-bound enzyme that converts testosterone to dihydrotestosterone, a more potent metabolite.^24,25 It is not known whether phytosterol consumption alters testosterone metabolism in humans. No significant changes in free or total serum testosterone concentrations were observed in men who consumed 1.6 g/day of plant sterol esters for 1 year.²⁶

Induction of Apoptosis in Cancer Cells

Unlike normal cells, cancerous cells lose their ability to respond to death signals that initiate apoptosis (programmed cell death). Sitosterol has been found to induce apoptosis when added to cultured human prostate,²⁷ breast,²⁸ and colon cancer cells.²⁹

Anti-inflammatory Effects

Limited data from cell-culture and animal studies suggest that phytosterols may attenuate the inflammatory activity of immune cells, including macrophages and neutrophils.^30,31

Disease Prevention

Cardiovascular Disease

Foods Enriched with Plant Sterols or Stanols

LDL cholesterol. Numerous clinical trials have found that daily consumption of foods enriched with free or esterified forms of plant sterols or stanols lowers concentrations of serum total and LDL cholesterol.10,32–35 A meta-analysis that combined the results of 18 controlled clinical trials found that the consumption of spreads providing an average of 2 g/day of plant sterols or stanols lowered serum LDL cholesterol concentrations by 9%–14%.³⁶ More recently, a meta-analysis that combined the results of 23 controlled clinical trials found that the consumption of plant foods providing an average of 3.4 g/day of plant sterols or stanols decreased LDL cholesterol concentrations by approximately 11%.³⁷ Another meta-analysis examined the results of 23 clinical trials of plant-sterol-enriched foods and 27 clinical trials of plant-stanol-enriched foods, separately.¹¹ At doses of at least 2 g/day, both plant sterols and stanols decreased LDL cholesterol concentrations by approximately 10%. Doses higher than 2 g/day did not substantially improve the cholesterol-lowering effects of plant sterols or stanols. Most recently, a meta-analysis that analyzed the results of 59 randomized controlled trials found that reductions in LDL cholesterol are greater in those with higher baseline levels of LDL cholesterol.³⁸ The results of studies providing lower doses of plant sterols or stanols suggest that 0.8–1.0 g/day is the lowest dose that results in clinically significant LDL cholesterol reductions of at least 5%.39–43 In general, trials that have compared the cholesterol-lowering efficacy of plant sterols with that of stanols have found them to be equivalent.^44–46 Few of these studies lasted longer than 4 weeks, but at least two studies have found that the cholesterol-lowering effects of plant sterols and stanols last for up to 1 year.^26,47 In addition to data from controlled clinical trials, a 5-year study that examined the customary use of phytosterol/stanol-enriched margarines under free-living conditions found beneficial effects on cholesterol levels.⁴⁸ Recently, concerns have been raised that plant sterols are not as effective as stanols in maintaining long-term reductions in LDL cholesterol.^49–51 Long-term trials that directly compare the efficacy of plant sterols and plant stanols are needed to address these concerns.¹¹

Coronary heart disease risk. The effect of long-term use of foods enriched with plant sterols or stanols on coronary heart disease (CHD) risk is not known. The results of numerous intervention trials suggest that a 10% reduction in LDL cholesterol induced by medication or diet modification could decrease the risk of CHD by as much as 20%.⁵² The National Cholesterol Education Program (NCEP) Adult Treatment Panel III has included the use of plant sterol or stanol esters (2 g/day) as a component of maximal dietary therapy for elevated LDL cholesterol.⁵³ The addition of plant-sterol- or stanol-enriched foods to a heart-healthy diet that is low in saturated fat and rich in fruits and vegetables, whole grains, and fiber offers the potential for additive effects in CHD risk reduction. For example, following a diet that substituted monounsaturated and polyunsaturated fats for saturated fat resulted in a 9% reduction in serum LDL cholesterol after 30 days, but the addition of 1.7 g/day of plant sterols to the same diet resulted in a 24% reduction.⁵⁴ More recently, 1-month adherence to a diet providing a portfolio of cholesterol-lowering foods, including plant sterols (1 g/1000 kcal), soy protein, almonds, and viscous fibers lowered serum LDL cholesterol concentrations by an average of 30%—a decrease that was not significantly different from that induced by statin therapy (drugs that inhibit the enzyme, 3-hydroxy-3-methyl-glutaryl-coenzyme A [HMG-CoA] reductase).⁵⁵ However, analysis of individuals on such a cholesterol-lowering diet for 1 year found that the average LDL cholesterol reduction was only 13%, but almost one-third of the participants experienced LDL cholesterol reductions that were greater than 20%.⁵⁶ Plant sterols are the major component in this diet responsible for the observed reductions in cholesterol concentrations.⁵⁷ The United States Food and Drug Administration (FDA) has authorized the use of health claims on food labels indicating that regular consumption of foods enriched with plant sterol or stanol esters may reduce the risk of heart disease.⁵⁸

Dietary Phytosterols

Clinical trials finding daily consumption of foods enriched with plant sterols or stanols can significantly lower LDL cholesterol concentrations do not account for naturally occurring phytosterols in the diet.⁵⁹ Relatively few studies have considered the effects of dietary phytosterol intakes on serum LDL cholesterol concentrations. Dietary phytosterol intakes have been estimated to range from approximately 150 mg/day to 450 mg/day in various populations.⁶⁰ Limited evidence suggests that dietary phytosterols may play an important role in decreasing cholesterol absorption. A cross-sectional study in the United Kingdom found that dietary phytosterol intakes were inversely related to serum total and LDL cholesterol concentrations, even after adjusting for saturated fat and fiber intake.⁶¹ Similarly, an analysis in a Swedish population found that dietary intake of phytosterols was inversely associated with total cholesterol in both men and women and with LDL cholesterol in women.⁶² In single-meal tests, removal of 150 mg of phytosterols from corn oil increased cholesterol absorption by 38%,⁶³ and removal of 328 mg of phytosterols from wheat germ increased cholesterol absorption by 43%.⁶⁴ Although more research is needed, these findings suggest that dietary intakes of phytosterols from plant foods could have an important impact on cardiovascular health.

Cancer

Limited data from animal studies suggest that very high intakes of phytosterols, particularly sitosterol, may inhibit the growth of breast and prostate cancer.65–67 Only a few epidemiological studies have examined associations between dietary phytosterol intakes and cancer risk in humans because databases providing information on the phytosterol content of commonly consumed foods have only recently been developed. A series of case–control studies in Uruguay found that dietary phytosterol intakes were lower in people diagnosed with stomach, lung, or breast cancer than in control groups of individuals who were cancer free.68–70 Case–control studies in the United States found that women diagnosed with breast or endometrial (uterine) cancer had lower dietary phytosterol intakes than women who did not have cancer.^71,72 In contrast, another case–control study in the United States found that men diagnosed with prostate cancer had higher dietary campesterol intakes than men who did not have cancer, but total phytosterol consumption was not associated with prostate cancer risk.⁷³ Although some epidemiological studies have found that higher intakes of plant foods containing phytosterols are associated with decreased cancer risk, it is not clear whether the protective factors are phytosterols or other compounds in plant foods.

Disease Treatment

Benign Prostatic Hyperplasia

Benign prostatic hyperplasia (BPH) is the term used to describe a noncancerous enlargement of the prostate. The enlarged prostate may exert pressure on the urethra, resulting in difficulty urinating. Plant extracts that provide a mixture of phytosterols (marketed as β-sitosterol) are often included in herbal therapies for urinary symptoms related to BPH. However, relatively few controlled studies have examined the efficacy of phytosterol supplements in men with symptomatic BPH. In a 6-month study of 200 men with symptomatic BPH, 60 mg/day of a β-sitosterol preparation improved symptom scores, increased peak urinary flow, and decreased post-void residual urine volume compared with placebo.74 A follow-up study reported that these improvements were maintained for up to 18 months in the 38 participants who continued β-sitosterol treatment.⁷⁵ Similarly, in a 6-month study of 177 men with symptomatic BPH, 130 mg/day of a different β-sitosterol preparation improved urinary symptom scores, increased peak urinary flow, and decreased post-void residual urine volume compared with placebo.⁷⁶ A systematic review that combined the results of these and two other controlled clinical trials found that β-sitosterol extracts increased peak urinary flow by an average of 3.9 mL/s and decreased post-void residual volume by an average of 29 mL.⁷⁷ Although the results of a few clinical trials suggest that relatively low doses of phytosterols can improve lower urinary tract symptoms related to BPH, further research is needed to confirm these findings.⁷⁸

Sources

Food Sources

Unlike the typical diet in most developed countries today, the diets of our ancestors were rich in phytosterols, likely providing as much as 1000 mg/day.1 Present-day dietary phytosterol intakes have been estimated to vary from 150 to 450 mg/day in different populations.³ Vegetarians, particularly vegans, generally have the highest intakes of dietary phytosterols.⁷⁹ Phytosterols are found in all plant foods, but the highest concentrations are found in unrefined plant oils, including vegetable, nut, and olive oils.³ Nuts, seeds, whole grains, and legumes are also good dietary sources of phytosterols.⁵ The phytosterol contents of selected foods are presented in Table 18.1.

Foods Enriched with Plant Sterols and Plant Stanols

The majority of clinical trials that demonstrated a cholesterol-lowering effect used plant sterol or stanol esters solubilized in fat-containing foods, such as margarine or mayonnaise.11 More recent studies indicate that low-fat or even nonfat foods can effectively deliver plant sterols or stanols if they are adequately solubilized.^10,59 Plant sterols or stanols added to low-fat yogurt,^43,84–86 low-fat milk,^87–89 low-fat cheese,⁹⁰ dark chocolate,⁹¹ and orange juice^92,93 have been reported to lower LDL cholesterol in controlled clinical trials. A variety of foods containing added plant sterols or stanols, including margarines, mayonnaises, vegetable oils, salad dressings, yogurt, milk, soy milk, orange juice, snack bars, and meats, are available in the United States, Europe, Asia, Australia, and New Zealand.¹⁰ A recent meta-analysis found that plant sterols/stanols added to fat spreads, mayonnaise, salad dressings, milk, or yogurt were more effective in reducing LDL cholesterol levels compared with plant sterols/stanols incorporated into other products, such as chocolate, orange juice, cheese, meats, and cereal bars.³⁸ Available research indicates that the maximum effective dose for lowering LDL cholesterol is approximately 2 g/day¹¹ and the minimum effective dose is 0.8–1.0 g/day.¹⁰ In the majority of clinical trials that demonstrated a cholesterol-lowering effect, the daily dose of plant sterols or stanols was divided among two or three meals, which may be more effective in lowering LDL cholesterol.³⁸ However, consumption of the daily dose of plant sterols or stanols with a single meal has been found to lower LDL cholesterol in a few clinical trials.^{43,85,86,94,95}

Table 18.1 Total phytosterol content of selected foods 80–83

Food	Serving	Phytosterols (mg)
Wheat germ	½ cup	197
Rice bran oil	1 tbs	162
Sesame oil	1 tbs	118
Corn oil	1 tbs	102
Canola oil	1 tbs	92
Peanuts	1 oz	62
Wheat bran	½ cup	58
Almonds	1 oz	39
Brussels sprouts	½ cup	34
Rye bread	2 slices	33
Macadamia nuts	1 oz	33
Olive oil	1 tbs	22
Take Control^a spread	1 tbs	1650 mg plant sterol esters (1000 mg free sterols)
Benecol spread	1 tbs	850 mg plant stanol esters (500 mg free stanols)
^a Known as Flora in Europe.

Supplements

Phytosterol supplements marketed as β-sitosterol are available without a prescription in the United States. Doses of 60–130 mg/day of β-sitosterol have been found to alleviate the symptoms of BPH in a few clinical trials (see Benign Prostatic Hyperplasia section above). Soft gel chews providing 0.5 g of plant stanols are being marketed for cholesterol lowering at a recommended dose of 2 g/day. Phytosterol supplements should be taken with meals that contain fat.

Safety

In the United States, plant sterols and stanols added to a variety of food products are generally recognized as safe (GRAS) by the FDA.96 Additionally, the Scientific Committee on Foods of the European Union concluded that plant sterols and stanols added to various food products are safe for human use.⁹⁷ However, this committee recommended that intakes of plant sterols and stanols from food products should not exceed 3 g/day because there is no evidence of health benefits at higher intakes and there might be undesirable effects at high intakes.

Adverse Effects

Few adverse effects have been associated with regular consumption of plant sterols or stanols for up to 1 year. People who consumed a plant-sterol-enriched spread providing 1.6 g/day did not report any more adverse effects than those consuming a control spread for up to 1 year,26 and people consuming a plant stanol-enriched spread providing 1.8–2.6 g/day for 1 year did not report any adverse effects.⁴⁷ Consumption of up to 8.6 g/day of phytosterols in margarine for 3–4 weeks was well tolerated by healthy men and women and did not adversely affect intestinal bacteria or female hormone levels.⁹⁸ Although phytosterols are usually well tolerated, nausea, indigestion, diarrhea, and constipation have occasionally been reported.^74,76

Sitosterolemia (Phytosterolemia)

Sitosterolemia, also known as phytosterolemia, is a very rare hereditary disease that results from inheriting a mutation in both copies of the ABCG5 or ABCG8 gene.99 Individuals who are homozygous for a mutation in either transporter protein have dramatically elevated serum phytosterol concentrations due to increased intestinal absorption and decreased biliary excretion of phytosterols. Although serum cholesterol concentrations may be normal or only mildly elevated, individuals with sitosterolemia are at high risk for premature atherosclerosis. People with sitosterolemia should avoid foods or supplements with added plant sterols.10 Two studies have examined the effect of plant sterol consumption in heterozygous carriers of sitosterolemia, a more common condition. Consumption of 3 g/day of plant sterols for 4 weeks by two heterozygous carriers,¹⁰⁰ and consumption of 2.2 g/day of plant sterols for 6–12 weeks by 12 heterozygous carriers did not result in abnormally elevated serum phytosterols.¹⁰¹

Pregnancy and Lactation

Plant sterols or stanols added to foods or supplements are not recommended for pregnant or breast-feeding women because their safety has not been studied.10 At present, there is no evidence that high dietary intakes of naturally occurring phytosterols, such as those consumed by vegetarian women, adversely affect pregnancy or lactation.

Drug Interactions

The LDL-cholesterol-lowering effects of plant sterols or stanols may be additive to those of HMG-CoA reductase inhibitors (statins).102,103 The results of controlled clinical trials suggest that consumption of 2–3 g/day of plant sterols or stanols by individuals on statin therapy may result in an additional 7%–11% reduction in LDL cholesterol, an effect comparable to doubling the statin dose.^50,104–106 Consumption of 4.5 g/day of stanol esters for 8 weeks did not affect prothrombin times (international normalized ratios—INRs) in patients on warfarin (Coumadin) for anticoagulation.¹⁰⁷

Nutrient Interactions

Fat-Soluble Vitamins (Vitamins A, D, E, and K)

Because plant sterols and stanols decrease cholesterol absorption and serum LDL cholesterol concentrations, their effects on fat-soluble vitamin status have also been studied in clinical trials. Plasma vitamin A (retinol) concentrations were not affected by plant stanol or sterol ester consumption for up to 1 year.11,26 Although the majority of studies found no changes in plasma vitamin D (25-hydroxyvitamin D₃) concentrations, one placebo-controlled study in individuals consuming 1.6 g/day of sterol esters for 1 year observed a small (7%) but statistically significant decrease in plasma 25-hydroxyvitamin D₃ concentrations.²⁶ There is little evidence that plant sterol or stanol consumption adversely affects vitamin K status. Consumption of 1.6 g/day of sterol esters for 6 months was associated with a nonsignificant 14% decrease in plasma vitamin K₁ concentrations, but carboxylated osteocalcin, a functional indicator of vitamin K status, was unaffected.²⁶ In other studies of shorter duration, consumption of plant sterol and stanol esters did not significantly change plasma concentrations of vitamin K₁^108,109 or vitamin-K-dependent clotting factors.¹¹⁰ Consumption of plant sterol or stanol-enriched foods has been found to decrease plasma vitamin E (α-tocopherol) concentrations in several studies.^11,109 However, those decreases generally do not persist when plasma α-tocopherol concentrations are standardized to LDL cholesterol concentrations. This suggests that observed reductions in plasma α-tocopherol are due in part to reductions in its carrier lipoprotein, LDL. In general, consumption of plant-sterol- and stanol-enriched foods at doses of 1.5 g/day or more have not been found to have adverse effects on fat-soluble vitamin status in well-nourished populations.

Carotenoids

Dietary carotenoids are fat-soluble phytochemicals that circulate in lipoproteins. Several studies have observed 10%–20% reductions in plasma carotenoids after short-term and long-term consumption of plant-sterol- or stanol-enriched foods.¹¹ Even when standardized to serum total or LDL cholesterol concentrations, decreases in α-carotene, β-carotene, and lycopene may persist, suggesting that phytosterols can inhibit the absorption of these carotenoids.¹¹¹ It is not clear whether reductions in plasma carotenoid concentrations confer any health risks, but several studies have found that increasing intakes of carotenoid-rich fruits and vegetables can prevent phytosterol-induced decreases in plasma carotenoids.¹¹² In one case, advice to consume five daily servings of fruits and vegetables, including one serving of carotenoid-rich vegetables, was enough to maintain plasma carotenoid levels in people consuming 2.5 g/day of plant sterol or stanol esters.¹¹³

Summary

• Phytosterols are plant-derived compounds that are similar in structure and function to cholesterol.

• Early human diets were rich in phytosterols, providing as much as 1 g/day; however, the typical Western diet today is relatively low in phytosterols.

• Phytosterols inhibit the intestinal absorption of cholesterol.

• Numerous clinical trials have demonstrated that daily consumption of foods enriched with at least 0.8 g of plant sterols or stanols lowers serum LDL cholesterol.

• Although some epidemiological studies have found that higher intakes of plant foods containing phytosterols are associated with decreased cancer risk, it is not clear whether phytosterols or other compounds in plant foods are the protective factors.

• The results of a few clinical trials suggest that phytosterol supplementation at relatively low doses can improve urinary tract symptoms related to benign prostatic hyperplasia, but further research is needed to confirm these findings.

• Foods rich in phytosterols include unrefined vegetable oils, whole grains, nuts, and legumes.

• Foods and beverages with added plant sterols or stanols are now available in many countries throughout the world, and many countries now allow health claims for such commercial products.