Abstract: We identified three RCTs with a total of 50 participants. Oral resveratrol not combined with other plant polyphenols was administered at 10 mg, 150 mg, or 1000 mg daily for a period ranging from four weeks to five weeks. The comparator intervention was placebo. Overall, all three included studies had low risk of bias. None of the three included studies reported long-term, patient-relevant outcomes such as all-cause mortality, diabetes-related complications, diabetes-related mortality, health-related quality of life, or socioeconomic effects. All three included studies reported that no adverse events were observed, indicating that no deaths occurred (very low-quality evidence for adverse events, all-cause mortality, and diabetes-related mortality). Resveratrol versus placebo showed neutral effects for glycosylated haemoglobin A1c (HbA1c) levels (mean difference (MD) 0.1%, 95% confidence interval (CI) -0.02 to 0.2; P = 0.09; 2 studies; 31 participants; very low-certainty evidence). Due to the short follow-up period, HbA1c results have to be interpreted cautiously. Similarly, resveratrol versus placebo showed neutral effects for fasting blood glucose levels (MD 2 mg/dL, 95% CI -2 to 7; P = 0.29; 2 studies; 31 participants), and resveratrol versus placebo showed neutral effects for insulin resistance (MD -0.35, 95% CI -0.99 to 0.28; P = 0.27; 2 studies; 36 participants). We found eight ongoing RCTs with approximately 800 participants and two studies awaiting assessment, which, when published, could contribute to the findings of this review. Currently, research is insufficient for review authors to evaluate the safety and efficacy of resveratrol supplementation for treatment of adults with T2DM. The limited available research does not provide sufficient evidence to support any effect, beneficial or adverse, of four to five weeks of 10 mg to 1000 mg of resveratrol in adults with T2DM. Adequately powered RCTs reporting patient-relevant outcomes with long-term follow-up periods are needed to further evaluate the efficacy and safety of resveratrol supplementation in the treatment of T2DM.

Plain-language summary: We identified three randomised controlled trials (clinical studies where people are randomly put into one of two or more treatment groups) with a total of 50 participants with type 2 diabetes. Among the included studies, the duration of resveratrol supplementation ranged from four to five weeks. Resveratrol as a capsule or Softgel was taken at 10 mg, 150 mg, or 1000 mg daily and was compared to placebo. This evidence is up-to-date as of December 2018. None of the included studies reported on important long-term, patient-relevant outcomes such as death from any cause, diabetes-related death, diabetes-related complications, health-related quality of life, or impact on treatment costs. However, no side effects and no deaths were observed in these short-term studies. No clear changes were observed for indicators of glucose management. We found eight ongoing studies with approximately 800 participants and two studies awaiting assessment, which, when published, could contribute to our findings. The overall certainty of evidence from the included studies was very low, mainly because the number of participants and the number of studies reporting the outcomes were small . Also, the duration of the studies was very short.