Plant sterols are an essential component of the membranes of all eukaryotic organisms. They are either synthesised de novo or taken up from the environment. Their function appears to be to control membrane fluidity and permeability, although some plant sterols have a specific function in signal transduction. The phytosterols are products of the isoprenoid pathway. The dedicated pathway to sterol synthesis in photosynthetic plants occurs at the squalene stage through the activity of squalene synthetase. Although the activity of 3-hydroxymethyl-3-glutaryl coenzyme A (HGMR) is rate-limiting in the synthesis of cholesterol, this does not appear to be the case with the plant sterols. Up-regulation of HGMR appears to increase the biosynthesis of cycloartenol but not the Δ5-sterols. A decline in sterol synthesis is associated with a suppression of squalene synthetase activity, which is probably a critical point in controlling carbon flow and end-product formation. The major post-squalene biosynthetic pathway is regulated by critical rate-limiting steps such as the methylation of cycloartenol into cycloeucalenol. Little is known about the factors controlling the biosynthesis of the end-point sterol esters or stanols. The commonly consumed plant sterols are sitosterol, stigmasterol and campesterol which are predominantly supplied by vegetable oils. The oils are a rich source of the steryl esters. Less important sources of sterols are cereals, nuts and vegetables. The nutritional interest derives from the fact that the sterols have a similar structure to cholesterol, and have the capacity to lower plasma cholesterol and LDL cholesterol. Since the morbidity and mortality from cardiovascular disease have been dramatically reduced using cholesterol-lowering drugs (statins), the interest in plant sterols lies in their potential to act as a natural preventive dietary product. Stanols (saturated at C-5) occur in low amounts in the diet and are equally effective in lowering plasma cholesterol and do not cause an increase in plasma levels, unlike the sterols which can be detected in plasma.

© 2000 Society of Chemical Industry

Plant sterols: biosynthesis, biological function and their importance to human nutrition.

We studied the metabolic effects of stearic acid (18:0) on plasma lipoprotein levels in 11 subjects during three dietary periods of three weeks each. The three liquid-formula diets, which were used in random order, were high in palmitic acid (16:0), stearic acid, and oleic acid (18:1), respectively. Caloric intakes were the same during the three periods. As compared with the values observed when the subjects were on the high-palmitic-acid diet, plasma total cholesterol decreased by an average of 14 percent during consumption of the high-stearic-acid diet (P less than 0.005) and by 10 percent during consumption of the high-oleic-acid diet (P less than 0.02). Low-density lipoprotein cholesterol levels fell by 21 percent in subjects on the high-stearic-acid diet (P less than 0.005) and by 15 percent in subjects on the high-oleic-acid diet (P less than 0.005). No significant differences were observed in the plasma levels of triglycerides or high-density lipoprotein cholesterol among the three diets. Measurements of the intestinal absorption of palmitic, stearic, and oleic acids revealed essentially complete absorption of each during the three dietary periods. The oleic acid content of plasma triglycerides and cholesteryl esters increased significantly during the high-stearic-acid period, suggesting that stearic acid is rapidly converted to oleic acid. We conclude that stearic acid appears to be as effective as oleic acid in lowering plasma cholesterol levels when either replaces palmitic acid in the diet.

Effect of Dietary Stearic Acid on Plasma Cholesterol and Lipoprotein Levels

Quantitative isolation and gas–liquid chromatographic analysis of total fecal bile acids

To examine the effects of dietary fatty acids and carbohydrate on plasma lipids and lipoproteins, 11 patients with a mean plasma total cholesterol level of 251 +/- 10 mg per deciliter were studied on a metabolic ward during three dietary periods, each lasting four weeks. A liquid diet rich in monounsaturated fatty acids ("High-Mono") and a diet low in fat ("Low-Fat") were compared with a diet high in saturated fatty acids ("High-Sat"). The High-Sat and High-Mono diets contained 40 percent of their total calories as fat and 43 percent as carbohydrate; the Low-Fat diet had 20 percent fat and 63 percent carbohydrate. Body weight was kept constant by adjusting total caloric intake. As compared with the High-Sat diet, both the High-Mono and Low-Fat diets lowered plasma total cholesterol (by 13 percent and 8 percent, respectively) and low-density lipoprotein cholesterol (by 21 percent and 15 percent, respectively). As compared with the High-Sat diet, the Low-Fat diet raised triglyceride levels and significantly reduced plasma high-density lipoprotein cholesterol. In contrast, the High-Mono diet had no effect on levels of triglycerides or high-density lipoprotein cholesterol. The ratio of low-density to high-density lipoprotein cholesterol was also significantly lower when the High-Mono diet rather than the Low-Fat diet was followed. Therefore, in short-term studies in which liquid diets are used and body weight is kept constant, a diet rich in monounsaturated fatty acids appears to be at least as effective in lowering plasma cholesterol as a diet low in fat and high in carbohydrate.

Tatu A. Miettinen

Papers

Quantitative isolation and gas–liquid chromatographic analysis of total fecal bile acids

Quantitative isolation and gas--liquid chromatographic analysis of total dietary and fecal neutral steroids.