Abstract
There have been various clinical studies on the effect of flaxseed-derived products on circulating inflammatory biomarkers, but the findings from these are contradictory. The aim of the present study was to clarify any association. A comprehensive literature search was conducted from inception to May 2018. From the eligible trials, 32 articles describing studies conducted on adults aged 18-70 y were selected for the meta-analysis. Meta-analyses using the random-effects model were performed to investigate the data and results showed significant effects of flaxseed intake on circulating high-sensitivity CRP (hs-CRP) [weighted mean difference (WMD) = -0.75; 95% CI: -1.19, -0.30; P < 0.001] and TNFα (WMD = -0.38; 95% CI: -0.75,-0.01; P = 0.04). However, no significant changes were found in IL6 concentration (WMD = -0.24; 95% CI: -0.70, 0.21; P = 0.28) and C-reactive protein (CRP) (WMD = -0.34; 95% CI:-0.89, 0.20; P = 0.22). Moreover, by eliminating 1 of the studies from the sensitivity analysis, changes in IL6 concentration were significant (WMD = -0.44; 95% CI: -0.81, -0.08). The changes in inflammatory biomarkers were dependent on study design (parallel or crossover), supplement type (flaxseed, flaxseed oil, or lignan), study quality (high or low), and participants’ age and BMI. According to this meta-analysis, flaxseed significantly reduced circulating concentrations of hs-CRP and TNFα, but did not affect IL6 and CRP. Further research is needed to examine the effect of different doses and long-term benefits of flaxseed and its derivatives on inflammatory factors.
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Key Points
Evidence from several studies has shown that some dietary factors such as plant sterols, dietary fiber, isoflavones, phytoestrogens, lignans, and ω-3 PUFA including EPA, DHA, and α-linolenic acid (ALA) have revealed anti-inflammatory properties. ALA can reduce the morbidity and mortality rate of CVD. The mechanism of action is not clear yet. Despite existing claims about the effects of flaxseed or its derivatives on inflammatory markers, the results of these studies remain conflicting. Thus, the present meta-analysis was conducted to summarize the available evidence regarding the effect of flaxseed or its derivatives on inflammatory markers (TNFα, IL6, CRP, and hs-CRP).
The present systematic review and meta-analysis revealed that flaxseed consumption had a beneficial effect on hs- CRP and TNFα, but not on IL6 and CRP. A significant inverse association between flaxseed and IL6 concentration was found. In the current study, flaxseed or its derivatives reduced the hs-CRP concentration significantly. However, a pooled analysis did not reveal any significant change in the serum concentration of CRP after flaxseed supplementation. Flaxseed related active compounds, especially SDG and its metabolites (enterolactone and lignans enterodiol), have shown anti-inflammatory and antioxidant activity, mainly through inhibition of lipid peroxidation. Moreover, some studies have suggested that the flaxseed lignan component can activate the nuclear element (erythroid-derived 2)-like 2, a transcription factor of the antioxidant and detoxifying genes, especially NAD(P)H quinone dehydrogenase 1 and heme oxygenase-1.
The present study found that in subjects with higher BMI (>30), flaxseed resulted in significantly lower TNFα, CRP, and hs-CRP compared to the placebo; however, the changes were not significant in the other subgroups. In addition, flaxseed resulted in a nonsignificant reduction in the IL6 concentration in subjects with higher BMI. Because most of the studies did not report the results separately according to gender, the exact sex specificity effect of flaxseed remains unclear.
In conclusion, the current meta-analysis pooled results from 32 RCTs regarding the effects of consumption of flaxseed or its derivatives on main inflammatory factors. The results of this study showed that flaxseed or its derivatives could have anti-inflammatory effects on the human body.
However, additional trials must be conducted in the future that include adequate durations, well-designed protocols, and larger sample sizes to demonstrate the beneficial effects of flaxseed consumption on inflammation.