**1. Introduction**

Vitamin D is a fat-soluble steroid hormone, which regulates calcium and phosphate metabolism. The skin exposed to UVB radiation produces pre-vitamin D, which binds to the DBP protein (vitamin D binding protein), and is transported to the liver cells, where is hydroxylated to 25(OH)D (calcidiol). Subsequently, in the proximal tubules of the kidney, the 1α-hydroxylase 25(OH)D converts it to 1.25(OH)D (calcitriol). The production of 25(OH)D unlike 1.25(OH)D is not strictly regulated. Due to the relatively long half-life (about 3 weeks) and chemical stability, this metabolite is an indicator of the level of vitamin D resources in the organism [1,2]. Patients with idiopathic hypercalciuria and urolithiasis were until recently included in the group of people whose supplementation with vitamin D is considered unsafe due to the possibility of increased calciuria and the formation of new stones in the urinary tract [3].

Idiopathic hypercalciuria (IH) is one of the most common metabolic causes of urolithiasis, both in children and adults (30–60% adults and 40–80% children). Urinary calcium excretion in

children is considered increased when above 4 mg/kg body weight/24 h. [4]. The calcium-creatinine (Ca/Cr) ratio, calculated from the second morning urine sample, can also be used to estimate the level of hypercalciuria. The reference values of this indicator depend on age and range from 0.8 in infants to 0.2 in adults [4]. Symptoms of hypercalciuria are non-characteristic, including abdominal pain, haematuria, erythrocyturia [5–7]. The hypercalciuria may be the cause of formation of stones in urinary tract.

Urolithiasis (UL) is a condition in which in the urinary tract deposits are formed from chemicals that are normal or pathological constituents of urine [8–11]. In Europe, the incidence of this disease is estimated at about 4% in the adult population and 1–2% in children [8,12].

Both urolithiasis and hypercalciuria predispose to skeletal mineralization disorders, leading to a decreased bone density [8,9]. The human skeletal system is constantly changing. The most significant period is childhood and adolescence. Then, there is a rapid increase in the structure of the bone skeleton, which continues until the bone epiphysis is closed, i.e., until the age of 20. By this time, approximately 90% of bone mass is formed. The peak of bone mass is reached around 30 years of age. If insufficient, it is associated with an increased risk of osteoporosis in later life [9,13].

The gold standard in diagnosing bone density disorders is densitometry. In children, the bone density of the whole body is measured. The test result is expressed by means of indicators comparing the bone density of the tested person with the bone density of healthy people (Z-score). Z-score between (−2) and (−1) is considered as osteopenia in children, while osteoporosis is diagnosed at Z score > (−2), and accompanying clinical symptoms [1,9].

The identification of patients who are at risk of osteopenia and osteoporosis among patients with urolithiasis and idiopathic hypercalciuria allows the implementation of a preventive strategy that includes appropriate supplementation with vitamin D [1,14].

In this study, we evaluated the effect of vitamin D supplementation in children with idiopathic hypercalciuria on 25(OH)D blood level, caciuria, development of new stones in urinary tract and bone mineral density.

#### **2. Materials and Methods**

#### *2.1. Patients*

The research project was approved by Bioethics Committee at the Military Institute of Medicine (Resolution No. 26/WIM/2013 of 22 May 2013). The study included 36 children (18 boys, 18 girls) with urolithiasis in the course of idiopathic hypercalciuria and low levels of vitamin D, hospitalized in Pediatrics, Nephrology, and Paediatric Allergology Department of Military Institute of Medicine. Inclusion criteria were age 5–16, urolithiasis in the course of idiopathic hypercalciuria, good cooperation with medical staff. Exclusion criteria were chronic kidney disease, urinary tract infections, urinary tract defects, systemic diseases, bone diseases (except osteopenia), endocrine disorders, and patients treated with glucocorticosteroids.

The written consent of legal guardians was obtained and, in the case of children over 15 years of age, also the consent of the child.

#### *2.2. Experimental Study*

The medical interview included information about the onset of the problem with urolithiasis, accompanying diseases, and medications. All studied children were subjected to physical examination. The study was performed in four stages (Figure 1). Seven patients were followed up for 1 year, and 29 children for 2 years. The 1 year observation period was associated with reaching adulthood during the study period or resigning from the study after a year of observation.

Studied patients were supplemented with cholecalciferol tablets. We recommended to take the pill with a meal. During the study, vitamin D doses were adjusted to the season of the year and serum 25(OH)D concentrations (400 IU or 800 IU vitamin D dose) (Figure 1). Urolithiasis activity was assessed by ultrasound examination of the urinary system (no stones vs. new ones). In all cases in which calciuria would increase significantly during the treatment, intention was returning the patient to previous treatment and discontinuation of vitamin D.

#### **Figure 1.** Study stages.
