Effect of two vitamin D repletion protocols on 24-h urine calcium in patients with recurrent calcium kidney stones and vitamin D deficiency: a randomized clinical trial

This study was a parallel-group randomized controlled clinical trial undertaken in the kidney stone prevention clinic of Shahid Labbafinejad medical center, from April 2018 to May 2020. The patients were recurrent calcium stone formers who had serum 25-hydroxyvitamin D (25(OH)D) level of 10–20 ng/mL, aged 18–70 years, had body mass index (BMI) of less than 30 kg/m² [12], 24-U Ca below 300 mg [6], normal serum calcium levels who agreed to participate in the study. The recurrent calcium kidney stone was defined as having at least two episodes of radiopaque stones in the past medical history of the patient [13, 14]. The following cases were not included in our study: patients who were pregnant or lactating, using calcium supplements or other forms of vitamin D supplements or any medical drug which may affect serum or urinary calcium, had any history of kidney stone passage or gross hematuria within two months before the study, had known history of primary hyperparathyroidism or diseases that affect vitamin D and calcium metabolism (such as sarcoidosis or some other chronic granulomatous disorders), diabetes mellitus, and any malignancy or malabsorption. Patients who had 24-h urine volume under or over-collection [15] were also excluded from the study.

The study was approved by the ethics committee of the Urology and Nephrology Research Center, Shahid Beheshti University of Medical Sciences (IR. SBMU.UNRC.1395.26), and Tehran University of Medical Sciences (IR.TUMS.VCR.REC.1397.193). The trial was registered in the Iranian Registry of Clinical Trials (IRCT) (IRCT20160206026406N4). All the study protocols were performed in accordance with the 1964 Declaration of Helsinki, and all participants gave written informed consent before the study.

Study participants received either a daily dose of 2000 IU oral cholecalciferol for 12 weeks (maintenance dose) or a weekly dose of 50,000 IU oral cholecalciferol for eight weeks (loading dose). The final assessment was performed 12 weeks after the beginning of the study. These doses were selected according to loading treatment and maintenance therapy recommendations for VDD management, published by the endocrine society clinical practice guidelines [16]. In addition, all patients received general nutritional consultation for preventing kidney stone recurrence according to the European Association of Urology guidelines [17] with emphasis on taking adequate calcium from dairy products (800–1200 mg/day), salt intake restriction to less than 5 g/day, and consumption of moderate amounts of animal proteins (0.8–1 g/kg/day) [18‐20].

Because of the differences in the study protocol, participants and researchers were not blind to the study protocol, and only the statistician was blinded for the statistical analyses.

Study variables included demographic, anthropometric, the duration and episodes of kidney stone disease, serum 25(OH)D, parathyroid hormone (PTH), calcium, and phosphate concentrations, 24-h urine analysis for volume and concentration of urea, creatinine, calcium, sodium, potassium, phosphate, magnesium, citrate, oxalate and uric acid, and also participant`s dietary intake data. Demographic, anthropometric, and kidney stone history data were collected at the beginning of the study. Serum and 24-h urine samples were taken at the beginning and the end of study. All the laboratory measurements were performed as we reported in our previous study [9]. The LithoRisk® software (Biohealth, Italy) was used to calculate relative supersaturations of calcium salts, i.e., calcium oxalate (CaOxSS) and calcium phosphate (CaPSS). Dietary intakes were assessed by two 24-h dietary recalls on non-consecutive days at the beginning, the sixth week of intervention, and the end of study. The recalls were analyzed using Nutritionist 4 software (N-Squared Computing), modified according to Iranian food composition.

The sample size was calculated using the following equation:

Considering the type one error of 0.05, type 2 error of 0.20, and \(\begin{array}{*{20}c} {effect\begin{array}{*{20}c} {\,} \\ \end{array} \begin{array}{*{20}c} {size = \frac{\mu_1 - \mu_2 }{\sigma }} \\ \end{array} } \\ \end{array}\) of 0.75, the minimum required sample size was 28 in each group. Patients were randomized into study groups with an allocation ratio of 1:1 with a simple randomization method. A random sequence was generated using a computer program to produce the comparable groups and eliminate the source of bias in treatment assignments.

All the statistical analyses were performed using SPSS software version 24.0 (IBM, Chicago, Illinois, USA). Data are shown as mean ± standard deviation or median (IQR) for quantitative variables and frequency (percentage) for qualitative ones. The Kolmogorov–Smirnov test was used to assess the normality of data distribution. According to normality test results, the independent sample T-test or Mann–Whitney test was used to compare the mean outcome quantities between the two study groups. The Chi-Square test or Fisher exact test were also used to compare qualitative factors between the two groups. Serum variables, 24-h urine analysis, and supersaturation changes were compared between groups using a univariate general linear model (GLM) with generalized estimating equations (GEE) approach. Finally, multivariable GEE analysis was performed to assess the 24-h urinary calcium changes by adjusting confounding variables. Regression coefficient (β) were reported along with 95% confidence interval. The β (regression coefficient) signifies how much the mean of a dependent variable (for example the level of serum parathyroid hormone) changes when the participants received loading vitamin D repletion therapy (50,000 IU cholecalciferol/oral/weekly) compared with the maintenance vitamin D repletion protocol (2000 IU cholecalciferol/oral/daily). A p- value < 0.05 was considered statistically significant.