Alvin Berger1 , Peter JH Jones2 and Suhad S Abumweis2
Abstract
Plant sterols are naturally occurring molecules that humanity has evolved with. Herein, we have critically evaluated recent literature pertaining to the myriad of factors affecting efficacy and safety of plant sterols in free and esterified forms. We conclude that properly solubilized 4-desmetyl plant sterols, in ester or free form, in reasonable doses (0.8–1.0 g of equivalents per day) and in various vehicles including natural sources, and as part of a healthy diet and lifestyle, are important dietary components for lowering low density lipoprotein (LDL) cholesterol and maintaining good heart health. In addition to their cholesterol lowering properties, plant sterols possess anti-cancer, anti-inflammatory, anti-atherogenicity, and anti-oxidation activities, and should thus be of clinical importance, even for those individuals without elevated LDL cholesterol. The carotenoid lowering effect of plant sterols should be corrected by increasing intake of food that is rich in carotenoids. In pregnant and lactating women and children, further study is needed to verify the dose required to decrease blood cholesterol without affecting fat-soluble vitamins and carotenoid status.
Abbrevations: ABC, ATP-binding cassette; apo, apolipoprotein; LDL, Low density lipoprotein; PLD, phospholipase D; PUFA, polyunsaturated fatty acids.
Background
Plant sterols are plant compounds with similar chemical structure and biological functions as cholesterol [1]. Plant sterols contain an extra methyl, ethyl group or double bond. The most abundant plant sterols are sitosterol, campesterol and stigmasterol [2]. The daily dietary intake of plant sterol is 160–400 mg among different populations [3-9]. However, in the earlier stages of human evolution, some 5–7 million years ago, plant sterol intake in Myocene diets would have been considerably higher, up to 1 g/d [10]. Dietary sources include vegetable oils (especially unrefined oils), nuts, seeds and grains [1]. Absorption efficiency for plant sterols in humans is considerably less than that of cholesterol. Percent absorption of the former is 2–5% [11] versus 60% for the latter [12]. Consequently, blood levels of plant sterols in humans are only 0.1–0.14% of cholesterol levels [13]. Due to their structural similarity to cholesterol, plant sterols were first and foremost studied for their cholesterol absorption inhibition properties. In addition to their cholesterol lowering effect, plant sterols may possess anti-cancer [14], anti-atherosclerosis [15,16], anti-inflammation [17] and anti-oxidation activities [18]. The objective of the present review is to assess the evidence supporting the various physiological effects of plant sterols with emphasis on recent advances in knowledge.
Physiological effects of plant sterols
Cholesterol lowering actions
The cholesterol lowering effect of plant sterols is well documented in the literature. It is now accepted, after much earlier scientific debate and study, that 4-desmethyl plant sterols or stanols, either in their free or esterified form, decrease blood levels of total cholesterol and LDL-cholesterol through reduction of cholesterol absorption. Generally speaking, properly solubilized free sterols and esterified sterols possess similar cholesterol lowering ability [19,20]. In some studies such comparisons have been flawed because the free sterols were not properly solubilized [21]. Ostlund et al. [22] showed that emulsions of sitostanol, mixed with lecithin containing 0.7 g of sterol, reduced cholesterol absorption considerably, whereas less effect was seen with sitosterol in crystalline form.
This review will focus on the effects of 4-desmethyl sterols, stanols, and esterified forms. Methylated sterols (4α-monomethyl and 4,4-dimethyl) in sources such as shea and M. alpina fungi for example, and those sterols esterified to non-fatty acids such as ferulate (such as the sterols in rice bran oil), may not be equivalent in cholesterol lowering ability compared with the forms present in tall and soybean oils [19,20,23-26].
Important issues that remain to be verified regarding the cholesterol lowering effect of plant sterols includes (i) efficacy of low dose of plant sterols, (ii) the effect diet background on plant sterol efficacy, (iii) the efficacy of plant sterols when incorporated in food other than fat spread (iv) the optimal number of plant sterols servings and (v) the relative efficacy of plant sterols among different populations.