Introduction:
Local inflammation starts in fourth decade of life and increases during aging (1). The enhancement of proinflammatory cytokines causes oxidative stress and results in damage to various intracellular components such as regulatory and structural proteins, DNA, and lipids (2). C-reactive protein (CRP) considers as a hepatically-produced acute phase reactant protein. Its serum concentration is associated with inflammation (3) and increased serum levels of CRP is a predictor of ischemic stroke and first myocardial infarction in humans (4, 5). According to Women’s Health Study, postmenopausal women with the highest CRP concentration at baseline had a 7-fold enhancement in myocardial infarction risk and 5-fold enhancement in any vascular event (6). Thus, it seems that high serum level of CRP can predict cardiovascular events specially among postmenopausal women.
Beside the enhancement of inflammation, endogenous estrogen reduction is also a reason for cardiovascular disease (CVD) among postmenopausal women (7). Hormone replacement therapy (HRT) has favorable effects on endothelial function, antioxidant protection, blood lipids and lipoprotein concentrations, and vascular reactivity (8, 9). Despite the positive effects of estrogen therapy on clinical markers, the benefits of HRT for postmenopausal women remain controversial (10). Thus, lifestyle, cardioprotective nutrients, and selective estrogen receptor modulators, which consider as alternative to HRT, are of interest.
Polyphenolic isoflavones such as genistein and daidzein present in soy and structurally are similar to estradiol (11), therefore; they can bind to estrogen receptor β in vascular wall (12). In vitro and in vivo evidence indicated that genistein, as the predominate soy isoflavones, decreases the oxidative susceptibility of Low-density lipoprotein (LDL), inhibits tyrosine kinase (13, 14), influences vascular tissue metabolism (15), and decreases the proliferation of smooth muscle (16). Animal experiments and cell studies propose that soy isoflavones as well as the genistein or daidzein have anti-inflammatory and lipid lowering effects (17, 18). Scientists suggest soy isoflavones might reduce inflammatory biomarkers through the inhabitation of tyrosine kinase (19, 20). Beside soy isoflavones, soy protein also is recommended as a food for cardiovascular protection through its favorable impact on blood pressure and cholesterol (21, 22). Some studies have indicated that substitution of animal protein with soy protein might reduce circulating levels of inflammatory biomarkers (23, 24).
Previous information regarding soy consumption and inflammatory markers among postmenopausal women are far from conclusive. Some randomized clinical trials support the effectiveness of soy food consumption on proinflammatory cytokine reduction (25-29), while others do not (30-36). A meta-analysis in 2011 with 14 trials examined soy isoflavones effect on serum concentration of CRP among postmenopausal women and their results proposed non-significant effect of soy isoflavones on CRP (37). In meta-analysis by Dong J-Y et al., the effect of soy isoflavones and soy protein did not report separately and authors performed meta-analysis on 14 trials without considering whether participants were taking soy protein, soy isoflavones or both. Meanwhile, several new randomized clinical trials (RCTs) are published and higher dose of soy protein or soy isoflavones were used with longer intervention duration. Whether new RCTs after 2011 could change previous meta-analysis result is unknown.
Since they are new trials after year 2011, we performed an update systematic review to report a summary of RCTs exploring the effect of soy isoflavones and soy isoflavones plus soy protein on CRP and if possible performing a meta-analysis.