## 2 Materials and Methods  
The study was cross-sectional and prospective in design. It was performed between June 2012 and October 2018 within the population of outpatients and inpatients of the Department of Rheumatology and Internal Medicine in Poznan, Poland. The population studied consisted of 68 patients with SSc, classified using the criteria of the American College of Rheumatology and the European League Against Rheumatism (ACR-EULAR) 2013 criteria [14]. There were no exclusion criteria.  
History taking and physical examination of all patients were carried out by the same physician (P.Z.), who has experience in the care of SSc patients. Clinical data were collected using MEDS (Minimal Essential Data Set) in line with the EUSTAR (EULAR Scleroderma Trials and Research group) methodology [15], also by the same practitioner. The following features were assessed: gender, age, time of disease since the onset of [PERSON]'s phenomenon, time of disease since the onset of other symptoms characteristic of SSc, duration of the disease since the diagnosis, skin involvement using the modified Rodnan skin score (mRSS), [PERSON]'s phenomenon, digital ulcerations, synovitis, joint contractures, muscle weakness, esophageal symptoms (heartburn and dysphagia), intestinal manifestations (diarrhea, constipation, and bloating), arterial hypertension, scleroderma renal crisis (ever), dyspnea, palpitations, and teleangiectasias.  
A specialist cardiac (echocardiography) and pulmonary examination (pulmonary function tests and high-resolution computed tomography) was carried out. The results were interpreted by certified specialist physicians: a cardiologist (T.M.K.) and a pulmonologist (T.P.), both with considerable experience in SSc diagnostics. A number of characteristics were sought: heart conduction blocks; palpitations, systolic, and diastolic dysfunction; ejection fraction; pulmonary arterial hypertension using Doppler ultrasonography; total lung capacity; pulmonary restriction using pulmonary function tests; and the presence of SSc-related interstitial lung disease (ILD) using HRCT.  
Sera of patients were secured and assessed for the levels of 25(OH)D3. A radio immuno assay method (RIA) was used to measure serum concentration of 25(OH)D3 in patients with SSc using 25-OH Vitamin D3 RIA CT kit by DiaSource (Nivelles, Belgium). In line with recommendations from [PERSON] and the assay manufacturer, vitamin D deficiency was defined as 25-hydroxyvitamin D3 levels <20 ng/mL [16]. In addition, we determined the erythrocyte sedimentation rate (ESR), C-reactive protein concentration, serum creatinine concentration, the estimated glomerular filtration rate (chronic kidney disease epidemiology collaboration equation; eGFR CKD), as well as the presence or absence of proteinuria.  
This study obtained the consent of the Bioethics Committee at the Poznan University of Medical Sciences (No. 624/12). All patients participating in the study gave their informed consent in writing according to the local law regulations.  
Statistical analyses were conducted in Statistica 13.3 (TIBCO, Palo Alto, CA, USA). Data are presented as means \(\pm\) standard deviation (SD) or percentage, unless stated otherwise. The Mann-Whitney U test was used to compare continuous or ordinal variables, whereas logical (Boolean) variables were compared using the [PERSON]'s exact test. The Spearman's rank-sum coefficient was calculated to investigate possible correlations. The significance threshold was set at 0.05. Two-sided \(p\)-values are presented throughout.