A 2019 systematic review and meta-analysis of 32 randomized placebo controlled trials (1700 subjects) using zinc monosupplements or with co-supplements in patients with prediabetes or diabetes, obesity or overweight, and pregnant women with prediabetes or diabetes, found a significant decrease in fasting glucose of 14 mg / dL (0.8 mmol / L) and hemoglobin A1C of 0.55% (10).