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OBJECTIVE Low vitamin D status has been associated with impaired glycemic control in patients with type 2 diabetes. The purpose of our study was to evaluate the effect of vitamin D supplementation on glycemic control in patients with type 2 diabetes. RESEARCH DESIGN AND METHODS This randomized, double-blind, placebo-controlled trial was conducted in 275 adult patients with type 2 diabetes without insulin treatment. Patients were randomly assigned to receive either vitamin D3 (50,000 IU/month) or placebo for 6 months. To assess the primary outcome of the study, change in HbA1c, we performed a linear regression analysis. RESULTS Mean baseline serum 25-hydroxyvitamin D [25(OH)D] increased from 60.6 ± 23.3 to 101.4 ± 27.6 nmol/L and 59.1 ± 23.2 to 59.8 ± 23.2 nmol/L in the vitamin D and placebo group, respectively. Mean baseline HbA1c was 6.8 ± 0.5% (51 ± 6 mmol/mol) in both groups. After 6 months, no effect was seen on HbA1c (mean difference: β = 0.4 [95% CI -0.6 to 1.5]; P=0.42) and other indicators of glycemic control (HOMA of insulin resistance, fasting insulin, and glucose) in the entire study population. Subgroup analysis in patients with a serum 25(OH)D <50 nmol/L or an HbA1c level >7% (53 mmol/mol) did not differ the results. CONCLUSIONS In a well-controlled group of patients with type 2 diabetes, intermittent high-dose vitamin D supplementation did not improve glycemic control. [ABSTRACT FROM AUTHOR]