Plant sterol and stanols--comparison and contrasts. Sterols versus stanols in cholesterol-lowering: is there a difference?

Abstract

Sterols are ubiquitous in eukaryotic cells but absent from prokaryotes. Both animals and plants produce sterols, essential components of cell membranes. The characteristic sterol ring structure is common to cholesterol, a mainly animal product, and to all plant sterols (Fig. 1). Plant sterols (phytosterols) are naturally found in both free and esterified forms [1]. Although structurally very similar to cholesterol, they are differentiated by their degree of saturation and by their side chain configuration at the C24 position. Saturated plant sterol derivatives (termed plant stanols) are produced by the hydrogenation of sterols and are not abundant in nature. Saturation of phytosterols by commercial hydrogenation processes, including the saturation of sitosterol and campesterol at the 5 a-ring position, results in phytostanol compounds such as sitostanol and campestanol. The terms plant sterol and phytosterol are sometimes used as generic terms to include both unsaturated sterols and saturated stanols, but they are used here to refer specifically to the unsaturated compounds. While over 40 plant sterols from seven different plant classes have been identified [2], campesterol, stigmasterol, and especially b-sitosterol are the most abundant. Ingestion of free phytosterols, especially b-sitosterol, has been shown to reduce plasma cholesterol in both animals [1 /3] and humans [4 /7]. The esterified forms of phytosterols have also been used as cholesterol-lowering agents [8]. Plant sterols were first esterified and tested by Mattson [9,10]. For years research has focused on the esterified forms of plant stanols, i.e. plant stanol esters [11 /20]. A recent development has been the synthetic conversion of plant sterols to their corresponding stanols followed by esterification to more fat-soluble forms. The first advantage is that the hydrogenation of sterols to their corresponding stanols makes them almost unabsorbable in the gut. Secondly, these plant stanol esters are ring-saturated analogues of common dietary sterols that have been trans-esterified with fatty acids from vegetable oils (e.g. canola) and butter. In contrast to free sterols and stanols, which are crystalline and largely insoluble, esters of the same sterols and stanols are easily dissolved in different fat-containing foodstuffs. Thus, they may be readily consumed in dietary fat, which has already been shown to be the most effective vehicle for delivering plant stanols and sterols to the small intestine [10]. The commercial esterification of plant sterols and stanols with fatty acids from vegetable oil has made it possible to produce spreads and other foods containing the desired esters.