Abstract
Introduction: Many experimental and clinical trials have suggested that flaxseed might be a potent antihypertensive, but the evidence concerning the effects of flaxseed supplements on plasma C-reactive protein (CRP) concentrations has not been fully conclusive. We assessed the impact of the effects of flaxseed supplementation on plasma CRP concentrations through a systematic review of literature and meta-analysis of available randomised controlled trials (RCTs). Material and methods: The literature search included EMBASE, ProQuest, CINAHL, and PUBMED databases up to 1st February 2016 to identify RCTs investigating the effect of flaxseed supplements on plasma CRP concentrations. Meta-analysis was performed using a random-effects model, and effect size was expressed as weighed mean difference (WMD) and 95% confidence interval (CI). Results: Meta-analysis of 17 selected RCTs with 1256 individuals did not suggest a significant change in plasma CRP concentrations following supplementation with flaxseed-containing products (WMD: -0.25 mg/l, 95% CI: -0.53, 0.02, p = 0.074). The effect size was robust in the leave-one-out sensitivity analysis. Subgroup analysis did not suggest any significant difference in terms of changing plasma CRP concentrations among different types of flaxseed supplements used in the included studies, i.e. flaxseed oil (WMD: -0.67 mg/l, 95% CI: -2.00, 0.65, p = 0.320), lignan extract (WMD: -0.32 mg/l, 95% CI: -0.71, 0.06, p = 0.103) and ground powder (WMD: -0.18 mg/l, 95% CI: -0.42, 0.06, p = 0.142). Conclusions: The meta-analysis of RCTs did not show a significant change in plasma CRP concentrations following supplementation with various flaxseed products. Large, well-designed studies should be still performed to validate the current results.
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Key Points
A number of factors such as tumour necrosis factor (TNF), pro-inflammatory cytokines, and interleukins (IL) are responsible for increased levels of C-reactive protein (CRP), an important marker of systemic inflammation. C-recative protein is also considered as a strong predictor of CV risk in comparison to several other inflammatory markers. Some studies suggest that this acute-phase protein marker, synthesised by the adipose tissue or by the liver, might be significantly influenced by the administration or consumption of different formulations of flaxseed, like flaxseed oil, flaxseed lignan, or flaxseed supplementation. Therefore, the aim of the present study is to review available randomised clinical trials (RCTs) involving the use of different forms of flaxseed to evaluate their effectiveness on the CRP plasma concentration. After final assessment, 17 eligible trials achieved the inclusion criteria and were preferred for the final meta-analysis. The meta-analysis has some limitations. There were only a few eligible RCTs, and most of them had a small number of participants with suitable short time of supplementation, and they were heterogeneous regarding the characteristics of patients and study design. Many characteristics that vary within studies, such as the type of flaxseed products, the background of the patients included, the control groups, or the quality of the studies, could have been factors of between-study heterogeneity. The results of the meta-analysis of available randomised controlled trials does not suggest any significant benefit of flaxseed product supplementation in decreasing plasma CRP concentrations.