Effect of vitamin K supplementation on insulin sensitivity: A meta-analysis

Objective: To perform a systematic review and meta-analysis of randomized, placebo-controlled trials to assess the effect of vitamin K supplementation on insulin sensitivity. Data sources: MEDLINE, the Cochrane Library, CINAHL, Web of Science, Scopus, clinicaltrials. gov, and clinicaltrialresults.org were searched up to January 2017. Reference lists of related papers were also scanned. Study selection: Randomized controlled trials were selected if they compared vitamin K supplementation with placebo or no treatment and reported homeostasis model assessment of insulin resistance, fasting plasma glucose, fasting plasma insulin, C-reactive protein, adiponectin, leptin, or interleukin-6 levels. Data extraction: Data extraction and study quality assessment were performed independently by two investigators using a standardized data extraction form. Any inconsistencies were resolved by a third reviewer. Effect estimates were pooled using inverse-variance weighted method. Heterogeneity was assessed by the I2 and Q statistic. Results: A total of eight trials involving 1,077 participants met the inclusion criteria. A wide variety of participants were enrolled, including older men, postmenopausal women, prediabetic premenopausal women, and participants with a history of diabetes, hypertension, or vascular disease. Vitamin K1 and vitamin K2 (MK-4 and MK-7 subtypes) were assessed. Supplementation period ranged from 4 weeks to 3 years. Vitamin K supplementation did not affect insulin sensitivity as measured by homeostasis model assessment of insulin resistance, fasting plasma glucose, fasting plasma insulin, C-reactive protein, adiponectin, leptin, and interleukin-6 levels. Conclusion: Our analysis suggests no effect of vitamin K supplementation on insulin sensitivity.