The main finding of the present study is the presence of a significant correlation between serum 25(OH)D levels and RHI in T2D patients; in particular, low levels of serum 25(OH)D (less than 16.5 ng/mL) were associated with more than 4-fold increase in the risk of vascular endothelial dysfunction. Given the high incidence of cardiovascular events in T2D patients, we advocate evaluation of vascular endothelial function in this population. Compared to FMD measurement, which is conventionally used for the assessment of vascular endothelial function in clinical practice, RHI, which was used in this study, is advantageous in that it is a simple test that does not require high level of skills. Moreover, one of the major conclusions of the Framingham Heart Study was that the EndoPAT-based RHI measurement is a useful measure of peripheral vascular function . To this effect, both RHI and FMD are useful measures of vascular endothelial function and can predict cardiovascular events, but they do not correlate with each other and have been shown to be associated with different risk factors . With the exception of one study that examined the beneficial effects of vitamin D supplementation on RHI , there is little or no information on the association between vitamin D levels and RHI in T2D patients. Although our study was retrospective in nature, it was the first to demonstrate the association of low 25(OH)D levels and RHI in T2D patients.
What are the cellular mechanisms behind the effects of vitamin D on vascular endothelial function? While our study did not directly examine these mechanisms, we postulate the following scenarios. First, calcitriol, an active form of vitamin D, enhances angiogenic responses, such as endothelial repair by promoting the differentiation of monocytes into myeloid angiogenic cells. Second, calcitriol augments endothelial function by increasing endothelial nitric oxide (NO) synthase expression . Third, vitamin D can also improve vascular endothelial function through the regulation of vascular smooth muscle cell proliferation and chronic inflammation .
In this study, vitamin D deficiency, defined by a 25(OH)D level < 20 ng/mL, was observed in 66% of T2D patients. Using the same definition, a previous 5-year observational study of T2D patients identified hypovitaminosis D to be associated with increased risks of microvasculopathies as well as macrovasculopathies . Moreover, high hazard ratios of severe vitamin D deficiency were identified for all-cause mortality (2.03) and cardiovascular mortality (1.90) in T2D patients . These findings highlight the clinical importance of hypovitaminosis D in T2D patients. It is noteworthy that our study showed that serum vitamin D level had no significant effect on the incidence of microvasculopathies or macrovasculopathies. The discrepancy in this finding between our study and the above previous studies is probably due to the differences in the sample size, racial composition, and study design. In contrast, a significant correlation was observed between RHI and serum 25(OH)D levels, suggesting that vascular endothelial dysfunction due to vitamin D deficiency can occur in relatively early stages of T2D. The ROC curve analysis used in the present study found that the cut-off level of 25(OH)D for predicting an RHI < 1.67 was 16.5 ng/mL (OR 4.598, 95% CI 1.961–10.783, p < 0.001; Table 4). Moreover, serum 25(OH)D level of < 20 ng/mL was also identified by the multivariate logistic regression analysis to be an independent predictor of RHI < 1.67 (OR 2.574, 95% CI 1.043–6.355, P = 0.040), suggesting that vitamin D deficiency defined as 25(OH)D levels < 20 ng/mL is associated with vascular endothelial dysfunction.
It is important to determine whether vitamin D supplementation can ameliorate vascular endothelial dysfunction in vitamin D deficient T2D patients. Among the limited number of studies using RHI, one study showed that treatment of vitamin D deficiency with 2,000 or 4,000 IU of cholecalciferol for 16 weeks resulted in only a borderline increase in RHI (p = 0.07) in T2D patients . Inconsistent results have also been obtained using the FMD, with some studies demonstrating improvement  and others reported no changes [28, 29]. In a recent meta-analysis study investigating the effects of supplemental vitamin D on endothelial function, a sub-analysis involving T2D patients with vitamin D supplementation showed no significant improvement in FMD . Meanwhile, in the D2d Study, which evaluated the effects of vitamin D supplementation in individuals with prediabetes, although the risk of T2D was not reduced in the entire population, a 62% reduction in diabetes risk was observed in a subgroup of patients with a 25(OH)D level < 12 ng/mL . While that study did not evaluate vascular endothelial function, the results do not rule out the possibility that vitamin D supplementation improves vascular endothelial function in vitamin D-deficient T2D patients, thus warranting further research on this topic.
This study has several limitations. First, it did not include a control group of non-diabetics. However, it has already been documented that serum 25(OH)D levels are lower in diabetics than in non-diabetics. Moreover, our retrospective study only included inpatients and could not include other non-diabetic inpatients. Other limitations include a small sample size due to the retrospective design and the lack of consideration of seasonal variations in 25(OH)D measurements. Finally, this study did not consider whether antidiabetic treatment could by itself improved both vascular endothelial function and serum vitamin D levels of diabetic patients, which should be addressed in future studies.