New margarines containing plant sterols and stanols claim to lower serum cholesterol concentrations. However, these products cost three to four times more than conventional margarines or butter. Law reviewed the results of randomized trials to assess the ability of margarine or butter containing plant stanols or sterols to lower serum cholesterol concentrations.

Sterols are an important component of cell membranes and are produced by plants and animals, while cholesterol is exclusively an animal product. Plant sterols reduce serum concentrations of cholesterol by lowering cholesterol absorption from the gut by competing at the mucosal cell level for the limited space in the mixed micelles that transport lipids across gastrointestinal membranes. Considerably less plant sterol is absorbed than cholesterol; thus, serum cholesterol concentrations decrease despite compensatory increase in cholesterol synthesis occurring in the liver and other tissues. Although marketing of plant sterols as lipid-lowering drugs has been limited, it is understood that as natural occurring substances, they could be used as additives in fat-containing foods. Because fats are necessary to solubilize sterols, margarines are an ideal vehicle. Sterols have also been used in yogurt, cheese products and salad dressings.

The results from 14 randomized double-blind trials were compared to assess the efficacy of sterol-containing margarines versus polyunsaturated margarines, with and without the added plant sterols, in reducing serum concentrations of cholesterol. Trial participants generally included middle-aged persons with average to moderately elevated serum cholesterol concentrations. The trials involved relatively small numbers (a range of eight to 80 participants), and all but one trial lasted less than nine weeks. The average daily sterol dose ranged from 0.8 to 4.0 g. The results demonstrated a dose-related (about 2.0 g per day) reduction in low-density lipid (LDL) cholesterol levels; however, no further reduction occurred above that level, confirming results from earlier nonrandomized studies that a plateau effect is evident. The reduction was greater in older participants than in younger participants.

Results from observational studies and randomized trials indicate that persons 50 to 59 years of age would realize a 25 percent reduction in heart disease risk after two years of using the new margarines; this expected reduction is based on the decrease in cholesterol levels. The benefit is likely to be less in younger adults. Because plant sterols reduce the absorption of some fat-soluble vitamins, researchers remain concerned about the results of randomized trials revealing that plant sterols and stanols lower the blood concentration levels of beta-carotene by about 25 percent, alpha-carotene by about 10 percent and vitamin E by about 8 percent. Blood concentration levels of the vitamins were adjusted in the trials for the lower concentrations of LDL cholesterol, resulting in no decrease in vitamin E concentrations, but concentrations of beta-carotene were reduced between 8 and 19 percent. Eating more fruits and vegetables could counter the decrease in vitamin absorption. Stanol margarines have been on the market in Finland for three years with no evidence of adverse effects or adverse alteration in the taste of the margarine. The excess cost is estimated to be about 35 cents (based on the British price of sterol margarines).

The authors conclude that adding plant sterols and stanols to food could be an important public health policy if costs could be lowered and if new technology could improve the extraction process. These margarines may become an important mechanism in the primary prevention of ischemic heart disease.