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=0.001). No significant effect on HbA1c (WMD, -0.045%; 95%CI,-0.16 to-0.07; P=0.468) was found. In subgroup analysis, a significant reduction in blood 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.