*2.4. Role of Vitamin D*

During the last decades, the role of Vit D on CV events has triggered a lot of studies where observational studies (OS) have reported an association of vitamin D deficiency with cardiovascular disease, including carotid intima-media thickness, peripheral vascular disease, and cardiovascular death. Vitamin D supplementation diminishes levels of inflammatory markers and lipids (particularly triglycerides), improves endothelial function (as measured by brachial artery flow-mediated dilatation) and blood pressure (BP) control in the general population with or without vitamin D deficiency [71]. Besides, nephrologists have supported supplementation with 1,25-dihydroxy vitamin D in patients with ESRD since the inactivation of Vit D with the progression of CKD was known. If not managed on time, 1,25(OH)2D deficiency might promote the classic view of mineral and bone disorders (MBDs) such as secondary hyperparathyroidism and osteitis fibrosa cystica. These abnormalities together with endothelial dysfunction and vascular changes from the early stages of CKD [72], results in further vascular calcification and arterial sti ffness [73]. Vitamin D has been shown to have anti-inflammatory and anti-oxidative properties and additionally downregulates the expression of renin, correlating with an increased prevalence of hypertension, heart failure, CV events, and a higher CV mortality rate in CKD [74–76].

In vitro data have shown a direct e ffect of vitamin D on endothelial function, related to decreased oxidative stress and increased levels of endothelial nitric oxide synthase (eNOS). These findings are supported by the promising results of a few randomized clinical trials which represented beneficial effects of nutritional vitamin D supplementation or paracalcitriol on endothelial function (brachial artery flow-mediated dilatation) in CKD stage 3–4 [77,78]. Other positive e ffects on Vit D supplementation were noticed on inflammation markers, intracellular cell adhesion molecule, vascular cell adhesion molecule, E-selectin parathyroid hormone, and arterial sti ffness [79].

A recent meta-analysis supports the positive e ffect of vitamin D intervention on endothelial function mainly in younger patients, apparently due to an earlier diagnosis, where vascular remodeling has not ye<sup>t</sup> been established. Limitations of this meta-analysis were the small number of studies included, and the short duration of intervention suggesting a need for larger and longer studies on this topic, with su fficient power to assess hard endpoints [80]. The controversies remain also on the impact of Vitamin D on cardiac structure and function.

Experimental studies through a specially engineered mouse model have shown that targeted deletion of the vitamin D receptor gene increased cardiomyocyte size and LV weight without fibrosis [81]. Similarly, an association between vitamin D deficiency and increased myocardial collagen content, impairment of cardiac contractile function, and increased cardiac mass was reported previously [82,83]. On the other hand, beneficial e ffect of treatment with activated vitamin D on attenuation of myocardial hypertrophy [84] and prevention of heart failure [85] in experimental models were not supported neither by Primo and Opera trials, which showed that 48 or 52 weeks of treatment with paricalcitol, respectively, at a dose that adequately controls secondary hyperparathyroidism did not regress LV hypertrophy or improve LV systolic and diastolic dysfunction in CKD stage 3–5. Moreover, the promising e ffect of lowering CV-related hospitalizations needs further confirmation [86,87].

In addition, based on the data of the recent meta-analysis including 38 studies involving 223, 429 patients (17 RCTs, *n* = 1819 and 21 OSs, *n* = 221,610) it could be concluded that that the existing RCTs that used the intention-to-treat principle do not provide an adequate or conclusive evidence that Vit D supplementation a ffects the mortality of patients with CKD while in observational studies Vit D

treatment was significantly correlated with a 38% reduction in all-cause mortality and 45% reduction in CV mortality. The di fferent findings between the RCTs and OSs demonstrate that confidence on neither should be absolute and the conclusion was that large-size RCTs with a proper dose and su fficient treatment time, in the true vitamin D-deficient patients with CKD are needed in the future to assess, prospectively, any potential di fferences in survival [88].

### **3. Importance of New CKD-MBD Biomarkers in Early Cardiovascular Risk Assessment**

Considering significant CV risk and mortality in patients with CKD, there is a growing attempt to find a reliable biomarker that would timely detect not only kidney disease but also define patients under higher risk to reduce CV mortality.

Compared to the "older" CKD-MBD biomarkers and already established in clinical routine, phosphate and PTH, which however display increased levels when CKD is already advanced, newer biomarkers, FGF23, Klotho, and sclerostin, give a bit more hope as there is growing evidence suggesting that their disturbed serum levels can detect initial CKD (Table 1).
