This randomized, double-blinded, placebo-controlled clinical trial was conducted in Bandar Abbas, Iran, between the years 2016 and 2017. According to the suggested formula for calculation of sample size for randomized clinical trials, and considering type I error of 5% (α = 0.05) and type II error of 10% (β= 0.10, power=90%), and using data from a previous study (20), the sample size was calculated as 25 subjects in each group. Subjects were selected from those who were referred to the Diabetes Clinic of Hormozgan University of Medical science, Bandar Abbas, Iran based on the inclusion and exclusion criteria. Inclusion criteria were as follows: pregnant women aged 18–40 years, diagnosis of GDM after the first trimester by the one-step standard diagnostic test (a single 75-g oral glucose tolerance test) according to the International Association of the Diabetes and Pregnancy Study Groups (IADPSG) criteria (21), and written informed consent to participate in the study. Whereas, the exclusion criteria were: preeclampsia, insulin therapy, hypothyroidism, urinary tract infection, chronic kidney disease, liver diseases, and smoking. Among 61 referred pregnant women with GDM, 52 eligible patients were invited to take part in the study. Participants were assigned to the intervention (n=25) and control groups (n=25) by computer randomization (fig. 1). The research has retrospectively been registered at the Iranian Registry of Clinical Trials (IRCT) with the registration number: IRCT20150607022585N3 and is accessible at the following website: www.irct.ir.
First, goals, methods, and the stages of the project were fully explained to the participants and written informed consent was obtained from every patient. The duration of the study was 6 weeks for each participant. The intervention group received two vitamin D supplements (D-VIGEL 50000IU CAP, Daana Pharma Co, Iran) at the beginning of the study and 21 days later, along with 1000 mg/d calcium supplement (Jalinous Pharmaceutical Co, Iran). The control group received two placebo capsules (Daana Pharma Co, Iran) at the same time and 1000 mg/d calcium supplement (Jalinous Pharmaceutical Co, Iran). In order to modify the effect of diet on study outcomes, all women were consulted by a clinical dietician individually and received personalized diet plans according to their condition. Furthermore, they were asked not to change their daily physical activity.
Data on the clinical and gestational status of participants were recorded by an endocrinologist in a checklist. A general information questionnaire was used to collect data on the general characteristics of the attendees. Anthropometric indices including height (without shoes with the precision of 0.5 cm), weight (with light clothes and with the precision of 100 g using Seca scale), BMI (by dividing weight (kg) by height squared (m2) was calculated. Fasting blood samples were collected twice (at the study baseline and at the end of the intervention) to measure lipid profile, fasting plasma glucose (FPG), insulin, 25-hydroxy vitamin D, calcium, and phosphorus. Commercial kits and chemiluminescent immunoassay technology were applied for the measurement of serum 25-hydroxy vitamin D concentration (DiaSorin Inc, USA) and serum insulin level (DiaSorin Inc, Italy). Fasting plasma glucose (FPG), serum cholesterol, triglyceride, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), calcium, and phosphorus were measured using commercial kits (Pars Azmun Inc, Iran). Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) was calculated using the suggested formula (22).
Statistical Package for the Social Sciences (SPSS) software (version 25.0, Armonk, NY: IBM Corp.) was used for data analysis. The normality of data distribution was assessed using the Shapiro-Wilks test. To compare quantitative variables which were measured at baseline and the end of the intervention, paired t-test (for data with normal distribution) and Wilcoxon test (for non-normally distributed data) were applied. Independent t-test (for data with normal distribution) and Manne-Whitney test (for non-normally distributed data) were used to compare quantitative variables between two groups. To remove the potential effect of baseline values on our results, the analysis of covariance test was used. P-value≤0.05 was considered statistically significant.