Background: Literature increasingly supports the inverse relationship of vitamin D (VitD) level and type 2 diabetes (T2DM). Proposed protective mechanisms of VitD include its anti-inflammatory effects, increased insulin secretion via pancreatic β-cell stimulation, and downregulation of parathyroid hormone levels. Interventional studies show mixed results of VitD therapy in prediabetic patients with VitD deficiency or diabetic patients with normal VitD levels.
Study question: Does high-dose VitD replacement improve glycemic control and microalbuminuria (MAU) in uncontrolled T2DM and concurrent VitD deficiency?
Study design: This placebo-controlled, double-blinded study randomized 30 subjects aged 30-65 years with an elevated HbA1c level of 7.5%-10% and a low total 25-hydroxyvitamin-D value of <20 ng/mL to either placebo (n = 16) or ergocalciferol 50,000 IU (n = 14) once weekly for 8 weeks then once monthly for 4 months.
Measures and outcomes: Primary outcome was difference in HbA1c from baseline to month 6 between the VitD-intervention group and the placebo-controlled group. Secondary end points were differences in total 25-hydroxyvitamin-D and MAU. Paired t tests and linear mixed-effects models were used for statistical analysis.
Results: No significant differences were seen in HbA1c or MAU between baseline versus postintervention visits within the placebo group (HbA1c: 8.4% ± 0.2 vs. 8.1% ± 0.3, P = 0.088; MAU: 94.1 mg/g ± 43.9 vs. 45.9 mg/g ± 20.2, P = 0.152) and the intervention group (HbA1c: 8.8% ± 0.3 vs. 8.7% ± 0.4, P = 0.692; MAU: 167.8 mg/g ± 70.1 vs. 108.5 mg/g ± 39.9, P = 0.356). The difference between placebo-slope and intervention-slope was nonsignificant for MAU (β = -0.1 mg/g ± 0.4, P = 0.835) but was significant for total 25-hydroxyvitamin-D (β = 11.7 ng/mL ± 2.5, P ≤ 0.001). Greater HbA1c reduction occurred unexpectedly in the placebo group ((Equation is included in full-text article.)= -0.4% ± 0.2) than in the intervention group ((Equation is included in full-text article.)= -0.2% ± 0.4), although the difference in slopes was not significant (β = 0.2% ± 0.4, P = 0.640).
Conclusions: Our proof-of-concept study found no benefit of high-dose VitD therapy in glycemic control and MAU in uncontrolled T2DM and VitD deficiency. Post hoc analyses raise concerns for high-dose VitD therapy to delay glycemic improvement. Large-scale interventional trials are much needed in this patient population to substantiate our findings and elucidate VitD's mechanisms on glucose metabolism.