Flaxseed supplementation on glucose control and insulin sensitivity: a systematic review and meta-analysis of 25 randomized, placebo-controlled trials

Context: The results of human clinical trials investigating the effects of flaxseed on glucose control and insulin sensitivity are inconsistent. Objective: The present study aimed to systematically review and analyze randomized controlled trials assessing the effects of flaxseed consumption on glycemic control. Data Sources: PubMed, Medline via Ovid, SCOPUS, EMBASE, and ISI Web of Sciences databases were searched up to November 2016. Study Selection: Clinical trials in which flaxseed or its products were administered as an intervention were included. Data Extraction: The outcomes were fasting blood glucose, insulin concentration, insulin resistance (HOMA-IR), insulin sensitivity (QUIKI), and hemoglobin A1c (HbA1c). Results: A total of 25 randomized clinical trials (30 treatment arms) were included. Meta-analysis suggested a significant association between flaxseed supplementation and a reduction in blood glucose (weighted mean difference [WMD], -2.94mg/dL; 95% CI, -5.31 to -0.56; P = 0.015), insulin levels (WMD, -7.32pmol/L; 95% CI, -11.66 to -2.97; P = 0.001), and HOMA-IR index (WMD, -0.49; 95% CI,: -0.78 to -0.20; P = 0.001) and an increase in QUIKI index (WMD, 0.019; 95% CI, 0.008-0.031; P = od glucose, insulin, and HOMA-IR and a significant increase in QUIKI were found only in studies using whole flaxseed but not flaxseed oil and lignan extract. Furthermore, a significant reduction was observed in insulin levels and insulin sensitivity indexes only in the subset of trials lasting >= 12 weeks. Conclusions: Whole flaxseed, but not flaxseed oil and lignan extract, has significant effects on improving glycemic control. Further studies are needed to determine the benefits of flaxseed on glycemic parameters. The protocol for this systematic review was registered in the international prospective register of systematic reviews (PROSPERO) database (http://www.crd.york.ac.uk/PROSPERO;registration no: CRD42016043500).