## 4 Discussion  
Vitamin D deficiency in SSc has been widely reported [17; 18; 19; 20; 21]. In the population studied, its prevalence reached 50%, and the association between vitamin D deficiency and a higher prevalence of arterial hypertension was one of the strongest findings in this study. It is surprising given the lack of similar data in the present literature, to the best of the authors' knowledge. There is some evidence that vitamin D demonstrates a blood pressure lowering effect; nevertheless, it is inconsistent and requires more research [22]. Furthermore, we found a strong link between vitamin D deficiency and proteinuria. A similar effect was demonstrated in the past in other rheumatic diseases, such as systemic lupus erythematosusus, but not in SSc. In fact, vitamin D supplementation was shown to reduce proteinuria in lupus [23]. There are very little data with regard to kidney abnormalities in SSc beyond the scleroderma renal crisis. [PERSON] et al. found albuminuria in 25% of SSc subjects, as well as an increased total protein excretion in 17% of the same population. Thus, they suggested that it should be more thoroughly investigated, as it was related to kidney involvement and systemic vasculopathy [24]. Skin involvement, measured using a widely employed mRSS score, was not correlated with vitamin D deficiency in all the investigated patients, similarly to other findings [25; 21; 26; 27]. However, in the dcSSc subgroup alone, mRSS negatively correlated with vitamin D levels. [PERSON] et al. and [PERSON] also reported an inverse relationship between mRSS and serum vitamin D levels, even though they investigated smaller groups of patients (\(n\) = 38 and \(n\) = 39, respectively) [19; 28]. SSC-related ILD (lung fibrosis) was significantly correlated with vitamin D deficiency, confirming the findings of [PERSON], [PERSON], and [PERSON] et al. [28; 29; 30]. Vitamin D has been shown to have antifibrotic effects through different molecular mechanisms including decreased serum levels of IL-13 and IL-17, directly through T regulatory lymphocytes, TGF-\(\beta\)/SMAD signaling, and the renin-angiotensin system [31]. In our study, the link between pulmonary fibrosis and vitamin D deficiency seemed particularly strong in patients with dcSSc. The increased levels of CRP among vitamin D-deficient patients in our study were in line with the observations reported by [PERSON] et al. [20]. Yet, [PERSON] et al. and [PERSON] et al. found no association with CRP [26; 29]. However, this could be explained by smaller sample sizes in these two studies. It lies in the nature of CRP that a plethora of transient factors might confound its level (at the low values). Consequently, we cannot draw definite conclusions with such small sample sizes and a relatively weak link, which was not confirmed with the [PERSON]'s correlation. Moreover, [PERSON] et al. demonstrated increased levels of interleukin 6 that positively (albeit weakly) correlated with the modified Rodnan skin score (r = 0.291, \(p\) = 0.041) in their SSc group [32]. Interestingly, [PERSON] et al., as well as [PERSON] et al. [30], reported a link between increased ESR and vitamin D deficiency, which we could not corroborate. The limitations of the study include the lack of information on the enrollment season. In fact, a seasonal influence variation in vitamin D levels has been reported [33; 34], and there is a study that demonstrated the influence of seasons on the disease activity in Thai SSc patients [35]. However, there was no specific seasonal trend in the course of the recruitment in our research, and the recruitment process spread over six years, thus there is little risk of seasonal bias. This study was monocentric, but it had a moderate number of patients that were evaluated prospectively as compared with other published studies. Nevertheless, further work in larger groups is required to confirm the obtained results, link the serial 25(OH)D3 measurements with the disease evolution in cohort studies, and investigate its usefulness in interventional studies.