The effects of oral vitamin D on insulin resistance in pre-diabetic patients

BACKGROUND: Some epidemiological and interventional studies have shown the role of vitamin D on insulin secretion and resistance. A previous study in our center showed that intramuscular vitamin D decreases insulin sensitivity in pre-diabetic patients. We investigated the role of oral vitamin D on the insulin sensitivity index and insulin resistance in pre-diabetic patients. MATERIALS AND METHODS: In a randomized clinical trial, we divided 45 people with pre-diabetes aged 47.4 ± 6.6 (range 33-61) years into three groups: group A subjects treated with 50,000 IU oral vitamin D and 500 mg calcium carbonate (n = 21), group B subjects treated with a single 300,000 IU intramuscular vitamin D and 500 mg calcium carbonate (n = 9), and group C subjects treated with 500 mg calcium carbonate alone (n = 15). Serum 25-hydroxyvitamin D [25(OH) D] was measured at baseline. If it was less than 75 nmol/l, 50,000 IU vitamin D was given weekly, and if serum 25(OH) D was more than that, vitamin D was administered every 2 weeks. Before and after 12 weeks of treatment, a 75-g glucose tolerance test was performed. We used paired t-test and analysis of variance (ANOVA) to analyze the data. P values less than 0.05 were considered significant. RESULTS: Mean (SD) of serum vitamin D increased from 77.5 ± 39.2 to 118.8 ± 56.3 nmol/l (P = 0.009) in group A and from 80 ± 36 to 102.8 ± 43.3 nmol/l (P = 0.053) in group B, and decreased from 44.8 ± 18.3 to 34.6 ± 13.9 nmol/l (P = 0.06) in group C. Insulin sensitivity index (Matsuda) decreased from 11.4 ± 3 to 9.9 ± 3.2 (P = 0.046) in group A, but in comparison with other groups, it was not significant. CONCLUSION: Oral vitamin D had no effect on insulin sensitivity in pre-diabetes patients in 12 weeks treatment. A randomized double-blind study with a longer duration of treatment is suggested to investigate the effect of vitamin D on insulin resistance.