August 1, 2006
Thanks to the success of products fortified with phytosterols (the collective term for plant sterols and stanols), such as Benecol® spread, consumers have come to recognize and seek out these powerful cholesterol-lowering ingredients. This comes as no surprise, since an estimated 50% of American households manage their cholesterol levels in order to help stave off the number one killer in America: coronary heart disease (CHD). In a 2003 Prepared Foods survey, 45% of manufacturers listed cardiovascular health and cholesterol reduction as ingredient characteristics or benefits with the greatest opportunities for product development. The news about phytosterols has spread, earning these hard-working compounds an FDA-approved health claim, an American Heart Association (AHA) recommendation for lowering LDL cholesterol, and a spot in the National Cholesterol Education Programs ATP III Therapeutic Lifestyle Change recommendations for reducing the risk of CHD.
So how did phytosterols earn such acclaim in the fight against CHD? Recent evidence shows that a dietary program that includes phytosterols is as effective at lowering cholesterol as cholesterol- lowering drugs. A study published in the March issue of American Journal of Clinical Nutrition (2006; 83 (3):582-91) showed that a portfolio program of cholesterol-lowering foods, including plant sterols (1 gram/1,000 kcal), soy protein, viscous fibers and almonds resulted in more than 30% of study participants lowering their LDL cholesterol by more than 20%, not significantly different from their response to a first-generation statin. Studies also show that phytosterols may further decrease blood cholesterol levels in patients already on statin-drug therapy. During a parallel clinical trial published in The American Journal of Cardiology (2006; 97 (3):376-9), study participants who followed the AHA Heart Healthy Diet, were on long-term statin therapy and received 1.8 grams per day of soy stanols experienced a 9.1% reduction in LDL cholesterol.
Both sterols and stanols are recognized as having equivalent cholesterol- lowering properties, says Brent Flickinger, Ph.D., senior research manager, nutritional science, ADM, Decatur, IL. Phytosterols are also being studied regarding their potential benefits for immune health, urinary tract symptoms, benign prostatic hyperplasia and prevention of cancer, but much more research is needed to clarify findings. Studies have shown that higher intakes of plant foods containing phytosterols are associated with decreased cancer risk, but it is not clear whether phytosterols or other compounds are the protective factors. Foods rich in phytosterols also contain healthy substances like fiber, essential fatty acids and antioxidants.
The science of phytosterols
Both plant stanols and sterols are essential components of plant cell membranes and, structurally, they resemble cholesterol. Phytosterols decrease serum levels of cholesterol by blocking the absorption of dietary cholesterol into the bloodstream and inhibiting cholesterol reabsorption from bile acids in digestion.
Many phytosterols are found in plants, with beta-sitosterol the most abundant. Plant sterols are found naturally in small quantities in fruits, vegetables, nuts, seeds, cereals, legumes, vegetables, grains and oils. Plant stanols occur in smaller quantities in many of the same sources. Although people may consume phytosterols every day in food, the amount may not be significant enough to have cholesterol-lowering benefits. The early human diet was probably rich in phytosterols, perhaps providing up to 1 gram per day, but the modern Western diet is relatively low in phytosterols, containing about 150 to 450 mg per day.
By modifying plant stanols and sterols structurally into stanol and sterol esters, they are easily incorporated into fat-containing foods without losing their effectiveness. In the ester form and in appropriate quantities, dietary plant stanols and sterols inhibit the absorption of cholesterol in the small intestine by up to 50%, which in turn can lower LDL blood cholesterol by up to 14%.
Potential downside of phytosterols
One possible side effect of phytosterol intake seems to be a reduction in carotenoid plasma levels. Some studies have shown that the absorption of beta carotene is affected by phytosterol intake, but not outside of the normal range for blood levels. This slight reduction might be compensated for by a daily serving of carotenoid-rich fruit or vegetable. It appears that plant sterol and stanol-enriched foods at doses of 1.5 grams per day or more have not been found to have adverse effects on fat-soluble vitamin status in well-nourished populations. Other concerns have arisen regarding very high phytosterol intake and altered testosterone metabolism, although no significant changes in testosterone concentrations were observed in men who consumed 1.6 grams per day of plant sterol esters for one year. A rare hereditary condition known as sitosterolemia (also known as phytosterolemia) results in dramatically elevated serum phytosterol concentrations, due to increased intestinal absorption and decreased excretion of phytosterols. People with this condition should avoid foods or supplements with added plant sterols.
Because sufficient studies have not yet established safety for pregnant and lactating women, experts advise against supplements or foods fortified with plant sterols or stanols for this group. Formal guidelines for phytosterol use in children awaits further research. One randomized, controlled trial in children showed that 20 grams per day of plant sterol-containing margarine lowered LDL cholesterol 8%. The researchers concluded that these products may be used for children to reduce elevated cholesterol levels, but caution is recommended due to the potential for decreased absorption of fat-soluble vitamins and beta carotene. Health experts have expressed fear that as a larger variety of food products become enriched with phytosterols, the likelihood of rare, adverse effects will increase.
How much is enough?
Studies have established that at least 1 gram of plant stanol and sterol in ester form can result in a statistically significant reduction in cholesterol, but maximum benefits come with doses of 2 to 3 grams per day. Intakes greater than this do not seem to yield any additional benefits. Most of the clinical trails have demonstrated the cholesterol-lowering effect of phytosterols by dividing the dose among two or three meals per day, although a few trials have found a daily dose in a single meal to be effective.
In 2000, FDA authorized the use of a health claim for products containing plant sterol and stanol esters in reducing the risk of CHD. In order to qualify for this health claim, a food must contain at least 0.65 grams of plant sterol esters per serving, or at least 1.7 grams of plant stanol esters per serving. Further, the claim must specify that plant sterol esters and plant stanol esters be consumed in two servings, eaten at different times of the day with other foods.
Today, plant sterols and stanols are finding themselves mixed into many products, including supplements, lozenges, margarine, salad dressing, mayonnaise, vegetable oil, milk, soymilk, yogurt, orange juice, snack bars, meat, cheese, cereal and muffins. While eating foods naturally rich in phytosterols is known to reduce high cholesterol levels, numerous studies have found that daily consumption of foods enriched with plant sterols or stanols in free or esterified forms also lowers serum cholesterol concentrations. A meta-analysis combining the results of 18 controlled clinical trails found the consumption of spread providing an average of 2 grams per day of plant sterols or stanols lowered LDL cholesterol by 9% to 14%.
To fully realize the potential of phytosterols, scientists suggest finding alternative sources of high phytosterol content; minimizing loss of phytosterols caused by industrial processes; including phytosterols on food composition tables; determining the potential of the different types of phytosterols; clarifying the genetic bases of their action; studying synergistic effects with other compounds; minimizing side effects; and defining the effects of long-term treatment.
Sharon Palmer, R.D., is a freelance food and nutrition writer located in Southern California.
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