Introduction: Glutathione is a major endogenous antioxidant and its deficiency is implicated in the etiology and progression of a number of human diseases. Vitamin D is important for the prevention of osteoporosis, cardiovascular disease, diabetes, autoimmune diseases, and some cancers. Using a monocyte cell model, this study examined the hypothesis that vitamin D upregulates glutamate cysteine ligase (GCLC) and glutathione reductase (GR), which catalyzes GSH biosynthesis. Methods: U937 monocytes were pretreated with and without 1,25 (OH)(2) vitamin D (10-25 nM) for 24 h and then exposed to control and high glucose (HG, 25 mM) for 4 h. Levels of GSH were determined using HPLC; GR activity by oxidation of NADPH; GCLC protein, MCP-1, and IL-8 using ELISA kits. Results: 1,25 (OH)(2) vitamin D supplementation significantly upregulated expression of GCLC and GR, levels of GCLC protein and GR activity, and formation of GSH in control and HG-treated monocytes. 1,25 (OH)(2) vitamin D caused significantly (p < 0.05) lower secretion of IL-8 and MCP-1, and lower ROS levels in monocytes exposed to control and HG-treated monocytes. Conclusions: This study demonstrates a positive link between vitamin D and GSH levels, and that some beneficial effects of vitamin D supplementation may be mediated by an improvement in the cellular GSH levels and a decrease in ROS and pro-inflammatory cytokines. (c) 2013 Elsevier Inc. All rights reserved.