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Abstract
BackgroundVitamin D deficiency is associated with indicators of pre-diabetes including, insulin resistance, β-cell dysfunction and elevated plasma glucose with controversial findings from current trials. This study aims to investigate the long-term effect of vitamin D on glucose metabolism and insulin sensitivity in pre-diabetic and highly vitamin-deficient subjects.MethodsOne hundred thirty-two participants were randomized to 30,000 IU vitamin D weekly for 6 months. Participants underwent oral glucose tolerance test (OGTT) at 3-month intervals to determine the change in plasma glucose concentration at 2 h after 75 g OGTT (2hPCG). Secondary measurements included glycated hemoglobin, fasting plasma glucose and insulin, post-prandial insulin, indices of insulin sensitivity (HOMA-IR, Matsuda Index), β-cell function (HOMA-β, glucose and insulin area under the curve (AUC), disposition and insulinogenic indices), and lipid profile.ResultsA total of 57 (vitamin D) and 75 (placebo) subjects completed the study. Mean baseline serum 25(OH) D levels were 17.0 ng/ml and 14.9 ng/ml for placebo and vitamin D group, respectively. No significant differences were observed for 2hPC glucose or insulin sensitivity indices between groups. HOMA-β significantly decreased in the vitamin D group, while area under curve for glucose and insulin showed a significant reduction in β-cell function in both groups. Additionally, HOMA-β was found to be significantly different between control and treatment group and significance persisted after adjusting for confounding factors.ConclusionVitamin D supplementation in a pre-diabetic and severely vitamin-deficient population had no effect on glucose tolerance or insulin sensitivity. The observed reduction in β-cell function in both placebo and vitamin D groups could be attributed to factors other than supplementation.Trial registrationNCT02098980, 28/03/2014 (www.clinicaltrials.gov).
Highlights
Vitamin D deficiency is associated with indicators of pre-diabetes including, insulin resistance, β-cell dysfunction and elevated plasma glucose with controversial findings from current trials
Several possible mechanisms have been proposed to mediate the protective role of vitamin D against diabetes risk including alterations in the pancreatic β-cell function [10], insulin sensitivity [11] and systemic inflammation [12]
Recent systematic reviews and meta-analyses of clinical trials have found no association between vitamin D and glycemic indices and insulin resistance in patients with Type 2 diabetes (T2DM) apart from a modest reduction on fasting glucose in some cases [13,14,15]
Summary
Vitamin D deficiency is associated with indicators of pre-diabetes including, insulin resistance, β-cell dysfunction and elevated plasma glucose with controversial findings from current trials. Several possible mechanisms have been proposed to mediate the protective role of vitamin D against diabetes risk including alterations in the pancreatic β-cell function [10], insulin sensitivity [11] and systemic inflammation [12]. Recent systematic reviews and meta-analyses of clinical trials have found no association between vitamin D and glycemic indices and insulin resistance in patients with T2DM apart from a modest reduction on fasting glucose in some cases [13,14,15]. Other have reported a small but positive effect of vitamin D supplementation on glycemic control, insulin resistance and glucose tolerance [5, 16, 17]. The disparity in the reported results could be attributed to various types (pills, drops, and vitamin D fortified foods) and doses of vitamin D supplements, participants’ vitamin D status (deficient/insufficient), other comorbid conditions, small study samples and possibly restricted time frame of supplementation
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