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], -2.94 mg/dL; 95%CI,  -5.31 to - 0.56; P = 0.015), insulin levels (WMD,  -7.32 pmol/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.