Every 37 seconds, an American dies of heart disease.1 About one in three Americans suffers from some form of CVD1, including strokes. It kills almost 2,400 Americans per day.1 It's no wonder cardiovascular health is on the minds of so many consumers.
Cardiovascular health is directly linked to cholesterol. As circulating levels of low-density lipoprotein cholesterol increase, the risk of cardiovascular events also increases; therefore, strategic lowering of LDL-C is commonly recommended to reduce the risk for cardiovascular disease.2
Plant sterols are touted for naturally reducing total and LDL cholesterol alone3 or in combination with cholesterol-lowering medication; the effect is additive when sterols and cholesterol-lowering medication are combined.4,5
Plant sterols, or phytosterols, are components of plant membranes. They are found naturally in small quantities in many plant-based foods such as legumes, vegetables, fruits, seeds and nuts,6 but most people do not consume enough of these foods to achieve a cholesterol-lowering benefit.7
To make it easier to obtain efficacious sterol doses, manufacturers have created nutritional supplements and functional foods fortified with plant sterols. Sterols are most easily added to foods with a higher fat content, such as butter or margarine-based spreads, but can also be found in low- or no-fat products such as orange juice, milk and yogurt.8,9
Sterols chemically resemble animal cholesterol and carry out similar essential functions in plants, controlling membrane fluidity and permeability along with the activity of membrane-bound enzymes.10 Although it's not clear exactly how sterols work, studies show they may interfere with the uptake of cholesterol from dietary sources as well as the production of cholesterol by the human liver.7,11 Sterols seem to block the human intestine's absorption of cholesterol, and are also poorly absorbed by the human intestine themselves.12
Sterols are used in ester form in functional foods. No difference has been noted in the cholesterol-lowering efficacy of plant sterols when they are converted to their esterified form.13
Numerous studies examine the effects of various foods fortified with sterol esters. In a single-blind, randomized, incomplete crossover study in 58 men and women with moderately elevated total cholesterol (6.2 millimoles per liter), researchers compared the effects of sterol-enriched, low-fat foods (bread, cereal, milk and yogurt); each food's serving size was measured to provide a dose of 1.6 g of sterol esters per day.
There were four total intervention periods of three weeks each across three different study sites located in Australia. Each person ate one food per intervention period. There were no dietary restrictions during the intervention periods. During a control period, none of the foods eaten were enriched with phytosterols.
Both total and LDL cholesterol decreased significantly from the consumption of the sterol-fortified milk (8.7 percent and 15.9 percent respectively) and yogurt (5.6 percent and 8.6 percent respectively). Consumption of sterol-fortified milk was found to be significantly more effective than bread and cereal. However, consumption of both bread and cereal significantly decreased LDLs by 6.5 percent and 5.4 percent respectively. Consumption of sterol-fortified bread and milk led to a 5 percent to 10 percent decrease in lipid-adjusted beta-carotene levels.14
Other studies show that sterol consumption can decrease blood levels of the fat-soluble antioxidants beta carotene,15 alpha carotene and vitamin E, which can lower a person's ability to neutralize free radicals.16 This decrease in plasma levels of antioxidants seems to be greater from the consumption of plant sterol esters in comparison to plant-free sterols.16 However, people can maintain normal plasma-carotenoid levels by increasing their consumption of dietary carotenoids such as lycopene, which is found in fresh and processed tomatoes, during periods when they are consuming effective doses of sterol-fortified functional foods and beverages.17
Sterols have also been incorporated into various other foods such as orange juice and chocolate. To determine whether plant sterols in a reduced-calorie orange juice altered C-reactive protein concentrations and people's lipoprotein profiles, researchers took 72 healthy people and randomly assigned them to receive 240 ml (1 g of plant sterols) of a reduced-calorie, sterol-fortified orange juice or 240 ml of a reduced-calorie, placebo orange juice twice a day with meals for eight weeks.
The sterol-fortified orange juice significantly reduced both total cholesterol and LDL by 5 percent and 9.4 percent respectively. In addition, the sterol-fortified orange juice significantly reduced CRP concentrations from baseline and in comparison to the placebo beverage. CRP concentrations are a measure of inflammation; because arterial damage increases inflammation in the body, high CPR concentrations are an indicator of cardiovascular risk. In this study, no significant changes in vitamin E or carotenoid concentrations were noted.18
Combining sterols with cocoa flavanols (plant compounds that act as antioxidants) may provide cardiovascular benefits. In a double-blind, placebo-controlled, crossover study, researchers evaluated the efficacy of daily consumption of a sterol-fortified, cocoa-flavanol-containing dark chocolate bar on various markers of cardiovascular health in 49 adults with elevated serum cholesterol (serum-total cholesterol concentrations between 5.20 and 7.28 millimoles per liter). Serum lipids and blood pressure were also measured as markers of cardiovascular health.
Subjects took either the functional-food bar (1.1 g of sterol esters and about 180 mg of cocoa flavanols per bar) or an isocaloric, macronutrient-matched placebo bar twice a day for four weeks. The groups then switched treatments; those receiving the functional-food bar received the placebo and vice versa. Consumption of the functional-food bar resulted in statistically significant decreases in total cholesterol and LDL, 2 percent and 5.3 percent respectively.19
Due to the plethora of research indicating that sterols can lower blood cholesterol and possibly reduce biomarkers of oxidative stress and inflammation, such as C-reactive protein concentrations, the National Institutes of Health advocates the consumption of plant stanols/sterols as a first-line therapy for reducing LDL. Plant stanols and sterols are both phytosterols and are chemically similar, but stanols are rare, comprising 10 percent of dietary phytosterols. The NIH's National Heart, Lung and Blood Institute's Adult Treatment Panel (ATP III) of the National Cholesterol Education Program indicates that 30 percent to 40 percent reductions in LDL can be attained by combining statins (cholesterol-lowering drugs) with other drugs or products, including plant stanols/sterols. In addition, the report indicates that "maximizing of dietary therapy (including the use of plant stanols/sterols) combined with a standard dose of statin may be sufficient to attain the ATP III goal in some patients."4
In addition to the NIH's stance on sterols, the Food and Drug Administration approves the following health claim for products containing plant sterols: "Foods containing at least 0.65 g per serving of plant sterol esters, eaten twice a day with meals for a daily total intake of at least 1.3 g, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease."20 All products carrying this claim must be low in saturated fat (1 g or less per serving) and cholesterol (20 mg or less per serving), and contain no more than 13 g of total fat per serving of 50 g. Additionally, with the exception of salad dressing and dietary supplements, the food must also contain at least 10 percent of the Reference Daily Intake or Daily Reference Value for vitamin A, vitamin C, iron, calcium, protein or fiber. Spreads and salad dressings are not required to meet the limit for total fat per 50 g if the label directs consumers to look at the Nutrition Facts panel for more information about the product's fat content.
Along with the endorsements by the NIH and FDA, the American Heart Association noted in its 2006 diet and lifestyle recommendations paper that sterols/stanols can reduce LDL by up to 15 percent, and maximum effects are achieved at a dose of 2 g per day.21
Despite the positive research indicating that plant sterols may offer a natural approach for lowering LDL, like many medications, sterols may also come with harmful side effects.
A recent study examined whether plant sterols accumulate in aortic-valve lesions in individuals with aortic-valve stenosis (AS), an abnormal narrowing of the aortic valve that is often characterized by the accumulation of cholesterol in the diseased valve. In this study, researchers collected serum samples from 12 control subjects and 82 patients with severe AS; they also collected aortic valves from 21 AS patients undergoing valve surgery, and 10 controls. They found that the higher absorption of cholesterol in the blood was associated with higher plant-sterol content in the stenoic aortic valves. This suggests that the accumulation of both plant sterols and LDL-C may be partly responsible for the pathogenesis of aortic-valve disease.
However, the scientists indicated that more research needs to be conducted to verify their findings.22 Patients with severe AS in this study may have sitosterolemia, a rare, inherited lipid-storage disease. People with this disease have an increased intestinal absorption of and decreased biliary elimination of all sterols, including cholesterol and plant sterols, leading to a 50- to 200-fold increase in plasma plant-sterol concentrations.23
Conclusion Studies indicate that plant-sterol-fortified functional foods and beverages in doses of at least 1.3 g per day 20 have favorable effects on cholesterol. Individuals who would like to add sterol-rich foods to their diets should first consult their physicians and also consider correspondingly increasing their consumption of produce rich in alpha- and beta-carotenes. In addition, consuming plant-sterol-fortified foods in smaller doses throughout the day may be more effective than consuming them in one dose.24
Marie Spano, R.D., is a food industry consultant and vice president of the International Society of Sports Nutrition.
Natural Foods Merchandiser volume XXVI/number 8/p. 36