Relationship between total vitamin D metabolites and complications in patients with type 2 diabetes

In our previous study, it was shown that endogenous vitamin D₃ and its metabolites are associated with diabetic microvascular complications and cardiovascular risk factors. The aim of the present study was to determine if the relationship between total vitamin D (vitamin D₂ supplements plus endogenous vitamin D₃) was a better predictor of complications in type 2 diabetes (T2DM). A total of 460 patients with T2DM participated in the present cross-sectional study. Plasma levels of total vitamin D and its metabolites (1,25-dihydroxyvitamin D (1,25(OH)D), 25-hydroxyvitamin D (25(OH)D) and 24,25-dihydroxyvitamin D (24,25(OH)D) were measured by isotope-dilution liquid chromatography tandem mass spectrometry analysis. 1,25-dihydroxyvitamin D₃ and 25-hydroxyvitamin D₃ were associated with diabetic retinopathy and coronary artery disease, but total 1,25-dihydroxyvitamin D and total 25-hydroxyvitamin D levels were not statistically associated with any complications. Total 1,25-dihydroxyvitamin D showed the same positive association as 1,25-dihydroxyvitamin D₃ for hypertension and dyslipidemia, and total 25-hydroxyvitamin D showed the same positive association as 25-hydroxyvitamin D₃ for dyslipidemia. Total 24,25-dihydroxyvitamin D showed the same positive association only with dyslipidemia as did 24,25-dihydroxyvitamin D₃. However, total 25-hydroxyvitamin D was associated with hypertension, whereas 25-hydroxyvitamin D₃ was not. Vitamin D₃ metabolites were associated with diabetic retinopathy, whereas total vitamin D levels were not, suggesting that endogenous vitamin D₃ metabolites are a better measure of diabetic microvascular complications. However, both total vitamin D and vitamin D₃ metabolites were associated with cardiovascular risk factors in patients with type 2 diabetes.