Vitamin D functions as a hormone to regulate bone metabolism via calcium homeostasis. The recent renewed interest in vitamin D results from a worsening trend of worldwide deficiency as well as novel insights regarding its effects on glucose metabolism [1,2], endothelial function [3,4], and the cardiovascular system [5,6,7,8].

Nearly half of Koreans are vitamin D-deficient [9], which is cyclically aggravated in seasons with low physical activity, such as winter and spring. Obesity, including a higher body mass index (BMI), waist circumference, total fat mass, and percentage of fat mass, is also a major risk factor for vitamin D deficiency [10]. In patients with type 2 diabetes, the prevalence of vitamin D deficiency is almost twice that in nondiabetic individuals [11].

Vitamin D levels are negatively related to insulin resistance (IR) [12] and arterial stiffness [13,14]. Diabetes combined with a vitamin D-deficient state doubles the relative risk of developing cardiovascular disease and mortality compared with diabetes with normal vitamin D levels [15,16]. IR might link vitamin D deficiency and arterial stiffness in type 2 diabetes. Several interventional trials were performed to evaluate the effect of vitamin D supplementation on IR [17,18,19], endothelial function [20,21,22], and arterial stiffness [23] in nondiabetic, insulin-resistant subjects. One interventional study evaluated the effect of vitamin D on endothelial function in patients with type 2 diabetes [24]. However, no interventional trials have evaluated the effect of vitamin D on both IR and arterial stiffness in type 2 diabetes.

Therefore, we investigated the effect of high-dose vitamin D3 (cholecalciferol) supplementation on IR and arterial stiffness in vitamin D-deficient patients with type 2 diabetes.

Fig. 1 shows a flow diagram of the trial. Of the 90 participants who were screened, 81 were randomized into the placebo (n = 41) and vitamin D groups (n = 40). There were no differences between the groups in terms of age (56.7 ± 7.9 years vs. 54.5 ± 7.4 years, p = 0.437), diabetes duration (7.3 ± 6.1 years vs. 8.3 ± 5.7 years, p = 0.437), medications (antidiabetic, antihypertensive, antihyperlipidemic, and antiplatelet agents), smoking status, and daytime outdoor physical activity level. In addition, the glycemic status (HbA1c, 7.54% ± 0.50% vs. 7.50% ± 0.49%, p = 0.989), 25(OH)D levels (10.7 ± 2.6 ng/mL vs. 12.3 ± 3.0 ng/mL, p = 0.188), HOMA-IR, lipid profiles, and hsCRP were similar between the groups. Only one participant had PTH levels > 65 ng/mL. Baseline cSBP, baPWV, and AIx were similar between the groups (placebo group, n = 33; vitamin D group, n = 38) (Table 1).

The data of 62 participants (30 and 32 from the placebo and vitamin D groups, respectively) were analyzed because some patients withdrew, changed their medications (antidiabetic, antihypertensive, antilipid, and antiplatelet drugs), or had poor compliance (< 80% intake of the study medication). At the end of the study, 25(OH)D levels reached 35.4 ± 8.5 and 18.4 ± 7.3 ng/mL in the vitamin D and placebo groups, respectively (p < 0.001). There were also significant differences in the adequacy of the vitamin D level (≥ 30 ng/mL). In the placebo group, 10% (n = 3) had adequate levels of vitamin D, compared with 68.8% (n = 22) in the vitamin D group (p < 0.001). There were no differences in changes in HbA1c, HOMA-IR, lipid profiles, hsCRP, and PTH between groups. There were also no differences in AIx (ΔAIx, -4.3% ± 7.8% vs. -2.2% ± 9.4%, p = 0.399), baPWV (ΔbaPWV, -60 ± 176 cm/sec vs. -16 ± 137 cm/sec, p = 0.348), and cSBP between the placebo (n = 21) and vitamin D (n = 24) groups (Table 2). Similarly, there were no significant changes in baPWV or AIx after adjusting for age, DM duration, and pulse pressure before and after the study (Fig. 2). During the trial, no adverse effects related to calcium metabolism or renal and hepatic function occurred.

In this study, we evaluated the effect of 24-week, high-dose (2,000 IU/day) vitamin D supplementation in 25(OH)D-deficient type 2 diabetes patients. High-dose vitamin D supplementation was very safe and effective at elevating 25(OH)D levels. Nearly 70% of the participants reached the target levels of 25(OH)D in the vitamin D-treated group, compared with only 10% in the placebo group during summer. Nevertheless, there was no beneficial effect of high-dose vitamin D supplementation on cardiovascular risk factors-including glycemic control, HOMA-IR, lipid profiles, hsCRP, cSBP, baPWV, AIx, or PTH levels-in this 24-week trial.

Vitamin D deficiency is more prevalent and severe in patients with type 2 diabetes compared with the normal population [11]. Epidemiological studies revealed that vitamin D deficiency accompanied by type 2 diabetes is associated with an increased risk of all-cause and cardiovascular mortality [15,16]. Vitamin D receptors are found in several tissue types throughout the body, including vascular smooth muscle [30], the endothelium [31], and cardiomyocytes [32]. In many observational studies, vitamin D deficiency was strongly associated with cardiometabolic risk factors such as endothelial dysfunction [3] and diabetes [1,2]. Vitamin D was suggested to negatively regulate the renin-angiotensin system [33,34] and inhibit smooth muscle cell proliferation and macrophage activation [35,36].

IR is associated with arterial stiffness and cardiovascular disease independent of glucose tolerance status and hypertension [37,38]. Arterial stiffness is a powerful independent marker of cardiovascular disease in diabetic patients [39]. IR might also link arterial stiffness and vitamin D deficiency in type 2 diabetes. The role of vitamin D in glucose metabolism is to increase insulin secretion from pancreatic β-cells and increase insulin sensitivity in peripheral target tissues [40]. Several interventional trials were performed to evaluate the effect of vitamin D supplementation in nondiabetic insulin-resistant subjects [17,18,19]. Insulin sensitivity and IR were improved significantly and fasting insulin was decreased after daily supplementation with 4,000 IU of daily vitamin D in insulin-resistant Asian females compared with a placebo control. IR was improved when the endpoint serum 25(OH)D reached ≥ 32 ng/mL [17]. However, in other trials there was no improvement in glucose metabolism, insulin secretion, or inflammatory markers despite 25(OH)D levels approaching 70 ng/mL [18,19]. There was no amelioration in glycemic control, lipid profiles, or HOMA-IR in the current study.

Vitamin D supplementation improved flow-mediated vasodilation (FMD) in several randomized trials [4,20,21,22]. However, other studies reported no beneficial effects on endothelial function [41,42,43] and arterial stiffness [23] in nondiabetic subjects. In the current study, vitamin D supplementation had no beneficial effects on arterial stiffness (PWV and AIx) in type 2 diabetic patients.

The elevated PTH levels in the vitamin D-deficient state might aggravate endothelial dysfunction. The inverse association between vitamin D and PWV might be dependent on serum PTH levels. Pirro et al. [44] showed that elevated PTH levels in vitamin D-insufficient, postmenopausal females were a significant predictor of aortic stiffness, irrespective of cardiovascular risk factors. Yiu et al. [24] recently reported the results of very high-dose vitamin D (5,000 IU/day) supplementation for 12 weeks in patients with type 2 diabetes with suboptimal vitamin D (< 30 ng/mL) levels. Although the PTH levels were decreased in the intervention group compared with the placebo group, vitamin D supplementation did not improve vascular function, as determined by FMD or baPWV [24]. All of the participants in the current study were vitamin D-deficient (< 20 ng/mL). However, most of the participants had PTH levels within the normal range. After high-dose vitamin D supplementation for 24 weeks, the vitamin D state was improved significantly in the vitamin D-treated group. However, unlike the results reported by Yiu et al. [24], there were no differences of PTH levels. Aloia et al. [45] studied the threshold of vitamin D levels that increased PTH levels. The threshold was 16.8 ng/mL (42 nmol/L), and 25(OH)D levels above this point did not show clinical or statistically significant PTH changes [45]. At the end of the current study, the mean vitamin D level was 18.4 ng/mL in the placebo group. The nonsignificant changes in PTH between groups might be due to higher levels of vitamin D during the summer in the placebo group than the threshold. In the current study, vitamin D supplementation had no beneficial effect on PTH levels, which might explain the lack of improvement in arterial stiffness (baPWV and Aix).

Increased arterial stiffness is affected primarily by blood pressure, which causes repetitive pulsatile stress on the arterial wall and leads to both structural and functional changes in the artery. Arterial stiffness is also increased in metabolic syndrome and insulin-resistant states [46]. In the current study, there was no improvement in any of the components of metabolic syndrome, including glycemic control (HbA1c), lipid profiles, or blood pressure after vitamin D supplementation. There were also no effects on IR (HOMA-IR), inflammatory marker (hsCRP), or PTH in the vitamin D group. The lack of reduction in arterial stiffness might have arisen from the negative effects of vitamin D supplementation on these arterial stiffness-related cardiovascular risk factors. The intervention period might also have been of insufficient duration to make any structural or functional improvement in the arteries.

This study had several strengths over other similar works. We did not change the patients' medications, including antidiabetic, antihypertensive, antilipid, or antiplatelet medications, as doing so could have affected the results. In addition, all of the participants were vitamin D-deficient. However, this study had several potential limitations. First, the results might have been confounded by the coadministration of calcium because the KFDA has approved only compounds that combine oral vitamin D3 and calcium. Excess calcium in the serum might have caused pathological changes by influencing calcification modulators such as pyrophosphate and by binding to calcium-sensing receptors on vascular smooth muscle cells [47]. Second, the current study included relatively few participants. As such, our study did not have sufficient power to detect small differences in metabolic parameters, HOMA-IR, or arterial stiffness between groups. Third, the baseline PTH levels in the majority of participants were normal, which suggests that the vitamin D deficiency in the study participants was not severe. It is also possible that the dose of vitamin D used was insufficient to change PTH levels significantly, which might have affected arterial stiffness. Fourth, this study started during winter and ended in summer, 6 months later. During this period, the gradual increase in serum 25(OH)D levels due to the increased sun exposure might have reduced the effect of vitamin D supplementation, reducing the differences between the groups.

High-dose vitamin D supplementation in vitamin D-deficient patients with type 2 diabetes might be effective and safe at elevating 25(OH)D levels. However, we found no beneficial effects on IR or arterial stiffness. Additional long-term, high-dose, interventional trials in type 2 diabetic patients with severe vitamin D-deficiency should be performed to evaluate the effects of vitamin D supplementation on cardiovascular risk factors, such as IR and arterial stiffness.