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.