Vitamin D Insufficiency is Associated with Reduced Parasympathetic Nerve Fiber Function in Type 2 Diabetes

Vitamin D insufficiency is prevalent in subjects with type 2 diabetes mellitus (T2DM) and is associated with peripheral neuropathy. However, there are little data regarding vitamin D status in patients with cardiovascular autonomic neuropathy. Our objective was to evaluate the association of cardiovascular autonomic function, 25-hydroxyvitamin D (25[OH]D) insufficiency (i.e., levels <30 ng/mL), and multiple metabolic parameters in subjects with T2DM.

We examined 50 individuals with T2DM. Cardiovascular autonomic function (i.e., parasympathetic function) was assessed by RR-variation during deep breathing (i.e., mean circular resultant [MCR] and expiration/inspiration [E/I] ratio). Metabolic parameters included measures of adiposity, glycemic control, insulin resistance, calcium metabolism, and 25(OH)D.

Participants with 25(OH)D insufficiency (n = 26) were younger (66 ± 9 vs. 60 ± 10 years, P<.05), more insulin resistant, had a higher body mass index (BMI) and lower adiponectin levels. The MCR (39.5 ± 26.3 vs. 27.6 ± 17.2, P<.01) and E/I ratio (1.21 ± 0.17 vs. 1.15 ± 0.09, P<.01) were lower for those with 25(OH)D insufficiency after controlling for age. A stepwise selection procedure regressing MCR and E/I ratio on a number of metabolic parameters resulted in a model identifying age and 25(OH)D insufficiency as significant determinants for both measures. The interaction of age x 25(OH)D insufficiency was also included (MCR model, R2 = 0.491, P<.001; E/I ratio, R2 = 0.455, P<.001). Neither glycemic control nor other metabolic parameters were selected.

Our results suggest that 25(OH)D insufficiency is associated with reduced parasympathetic function, with a stronger association in younger persons with T2DM. Studies are needed to determine if vitamin D supplementation into the sufficient range could prevent or delay the onset of cardiovascular autonomic dysfunction.