This systematic review and meta-analysis based on the findings from 17 studies involving 11 182 subjects did not yield any evidence of an association between serum concentrations of plant sterols and elevated risk of CVD. This is true for both campesterol and sitosterol, for absolute concentrations and their ratios to total cholesterol, and regardless of whether we considered RR for CVD based on tertiles of plant sterol concentrations or mean concentrations in CVD cases compared with controls.
Interestingly, the studies identified by our systematic literature review reported highly inconsistent findings. Some studies suggested that elevated circulating concentrations of plant sterol concentrations (or their ratios to cholesterol) might increase the risk of CVD, namely the PROspective CArdiovascular Münster study,22
the Framingham Offspring Study,24
the Drugs and Evidence BAsed medicine in The Elderly study,49
a cohort of subjects with aortic stenosis,50
the Pittsburgh Epidemiology of Diabetes Complications Study,45
and two Finish studies in postmenopausal women.23,41
A similar modest trend was also seen in participants of the LUdwigshafen RIsk and Cardiovascular health study in multivariate analysis adjusted for potential confounding variables.46,47
In contrast, other studies reported negative associations of plant sterols with cardiovascular risk, namely the Longitudinal Aging Study Amsterdam,39
the European Prospective Investigation of Cancer Norfolk and Spain cohorts,39,43
and the COronary Risk factors for Atherosclerosis in women study.52
Finally, three studies indicated that plant sterol concentrations are not at all related to CVD, i.e. the Dallas Heart Study,51
the PROspective Study of Pravastatin in the Elderly at Risk trial,42
and the Helsinki Businessmen Study.48
A likely explanation for the inconsistent findings is that authors used different study designs and adjustments for potential confounding variables. Bias is likely if investigators did not appropriately adjust for all potential confounding variables. Most importantly, we have to consider bias due to confounding by lipoprotein concentrations (i.e. LDL-C concentrations, an established important risk factor for CVD). According to our conceptual model, there are common factors affecting intestinal absorption of both cholesterol and plant sterols creating a correlation between sterol and lipoprotein concentrations (Figure
Indeed, cholesterol uptake in the gut resulting in increased total and LDL cholesterol serum concentrations has per se
been suggested to represent an independent cardiovascular risk factor and might therefore account for a potential association of elevated plant sterol concentrations in serum with CVD.46,47,54
Furthermore, absorption in the gut is strongly influenced not only by genetic factors,27
but also by age, gender, diet, metabolic background, and medication (Figure
For example, the metabolic syndrome, insulin resistance, and type 2 diabetes are associated with low-circulating cholesterol absorption.55,58
In contrast, poor nutritional status, an index of frailty in elderly people, is correlated with high plant sterol concentrations. The associations of the metabolic syndrome/type 2 diabetes64,65
with increased cardiovascular risk may thus feign both the inverse and positive relationships of plant sterols with CVD, respectively. Further, confounding due to nutritional factors may occur. Elevated serum plant sterol concentrations reflect enrichment of plant-based food in the diet.12,39
Diets rich in fruits, seeds, vegetable protein, and polyunsaturated fatty acids, such as the Mediterranean diet, are considered healthy, may reduce cardiovascular risk, and thereby underlie the association of subnormal plant sterol concentrations with CVD.
In the meta-analysis based on the RR, we tried to minimize potential bias due to uncontrolled confounders by selecting the best-adjusted estimate according to our conceptual model. Interestingly, there was still substantial heterogeneity among studies; an explanation might be that authors used different statistical approaches and adjusted for different potential confounding variables. Of note, in our meta-analysis of mean differences of sterol concentrations between CVD cases and controls, heterogeneity among studies was even higher. This finding is not surprising since most investigators presented only crude-unadjusted statistics of the exposure distributions, so that our calculated mean differences do not provide unbiased estimates of the effect of sterols on CVD. For example, if there are any CVD risk factors with correlations to sterols (which is likely, e.g. lipoproteins) altered sterol concentrations in CVD cases might only be a proxy for other CVD risk factors.
Plant sterols have been ascribed atherogenic effects, mainly because atherosclerosis has been encountered in patients with phytosterolaemia which is accompanied by extremely high concentrations of plant sterols. Equally, however, serum phytosterol concentrations have been demonstrated to correlate with atherosclerotic plaque burden in individuals not affected by this disease.34,35
Our meta-analysis, however, argues against an association between moderately elevated concentrations of plant sterols and CVD.
In the studies recruited for this meta-analysis, plant sterol concentrations vary at least by threefold, as indicated by a comparison of the midpoints of the lower and upper tertiles of sitosterol and campesterol distributions (Table ). This range should be wide enough to detect relevant changes of cardiovascular risk due to moderate and commonly occurring variations in phytosterol levels.
Average serum concentrations of plant sterols in the first and third tertiles of their distributions in the studies included in the meta-analysis
There is evidence that the tissue content of plant sterols relates to their concentration in the plasma.67
Tissue levels were higher in individuals consuming sterol-enriched foods than in individuals who did not. Tissue deposition of plant sterols is most likely a passive process driven by the respective plasma concentration. A similar correlation may also exist between plasma and tissue cholesterol, but this was unfortunately not examined in that study. A mere correlation between minute amounts of plant sterols in the plasma and in tissue would, in addition, not prove a specific causal role of plant sterols in atherosclerosis, at least as long as it has not been shown that plant sterols would biologically be much more active than cholesterol on a equimolar basis.
Our systematic review and meta-analysis have several limitations. First, it was based only on few observational studies and is therefore prone to bias due to uncontrolled confounding. In particular, our inability to properly adjust for all potential confounders might confound the association estimates. We have implemented an analytical strategy to minimize this potential bias due to uncontrolled confounding. First, we defined a conceptual model specifying the set of potential confounding variables. Secondly, we extracted from each study the best-adjusted estimates according to this model. Unfortunately, not all authors provided fully adjusted estimates so that we must assume some residual bias. Thus, we also considered the possibility of a sensitivity analysis, i.e. repeating the analysis by excluding all studies without appropriate adjustment for potential confounding variables. However, the number of studies was too small to effectively implement this approach. Further, the second meta-analysis based on SMD between CVD cases and controls (MA 2) is likely to be biased since most studies only provided crude-unadjusted statistics of the sterol distributions. An analysis based on multivariate predicted means of sterol concentrations adjusted for potential confounding variables, such as provided in the LUdwigshafen RIsk and Cardiovascular Health study,44
would be a preferable analytical approach to obtain further evidence for any causal relationship between sterol serum concentrations and CVD. Secondly, since only a few studies were eligible, we pooled results from studies involving different designs and dissimilar definitions of CVD.
Thus, as a direct consequence, it was necessary to define a composite cardiovascular endpoint, including not only coronary but also cerebrovascular and peripheral vascular events which might increase noise and reduce the power of our meta-analysis. Finally, we also had to consider publication bias as indicated by a lack of small studies published resulting in a slight overestimation of the true effect of plant sterols on CVD risk.
Even though our meta-analysis has several limitations, it represents the first systematic attempt to critically appraise the evidence surrounding the relationship of plant sterols and their ratios to cholesterol with CVD. We are convinced that even by pooling heterogeneous studies, one can substantially contribute to new insights if the basic assumption holds that there was no systematic methodological bias present. Study heterogeneity might increase noise and reduce power, but, if none of the investigators tried to force/mask the true effect by systematically selecting a particular research design, statistical approach, and/or strategy to adjust for potential confounding variables, then a meta-analysis should be able to obtain consistent summary estimates. However, based on the present meta-analysis of observational studies, evidence is still limited and further research will be necessary to assess whether plant sterols are causally involved in atherogenesis. The next level of evidence might be the conduct of an individual patient data meta-analysis retrieving the original data of the studies considered here. A centralized reanalysis of the data in each study in a consistent way would afford a decrease in heterogeneity and bias in summary estimates by enhancing correction for potential confounding variables.
In conclusion, our systematic review and meta-analysis do not provide any evidence that there is an association between moderate fluctuations of serum concentrations of plant sterols and CVD risk. However, our meta-analysis does not address the vascular effects or safety of long-term consumption of plant sterol-enriched foods.