Lower Vitamin D Levels During Second Trimester Are Associated With Developing Gestational Diabetes Mellitus: an Observational Cross-sectional Study

In this observational cross-sectional study, 40 women with GDM and 40 healthy pregnant between 20-40 years in age, in their 24-28th gestational week applied to Gulhane Education and Research Hospital (Ankara, Turkey) were recruited between January 2014 to March 2014. Inclusion criteria were singleton pregnancy, aged 20-40 years, gestational age 24-28 weeks, non-history of acute or chronic diseases. The exclusion criteria included alcohol using, smoking, history of GDM, diagnosed with type 1 and 2 diabetes mellitus, preeclampsia, polycystic ovary syndrome, thyroid and parathyroid disease, metabolic bone disease, kidney disease, abnormal liver function, multiple pregnancy, or women receiving drugs related to calcium and vitamin D metabolism (with the exception of routine vitamin supplements prescribed during pregnancy). We randomly selected the 40 pregnant women diagnosed GDM by two-step procedure based on Carpenter-Coustan criteria. Women with GDM did not receive any type of the medications such as insulin or medical nutrition therapy. We also randomly selected 40 healthy pregnant women with normal serum glucose levels in the same period. This study must recruit 40 women for each group to have 80% study power with 5% type I error level to detect a clinically significant difference.

The study was approved by the ethic committee of Gulhane Education and Research Hospital, Ankara, Turkey with the number 50687469-1491-2721-13/1648.4-2882. Before commencing the research, not only women got clued in information about the study procedure and objective but also a written informed consent was obtained from all participants.

GDM screening at this hospital involves a two-step procedure. At the outset, 50 g 1 hour glucose challenge test (GCT) was used to screen for GDM at 24th and 28th weeks of gestation. Women with abnormal glucose levels ≥130 mg/dL (7.2 mmol/L) after 1 hour GCT underwent a standard 100 g 3 hour oral glucose tolerance test (OGTT) performed in the morning after eight hours overnight fasting. The diagnosis of GDM was confirmed if at least 2 of 4 glucose levels exceed Carpenter-Coustan criteria: fasting ≥ 95 mg/dL (5.3 mmol/L), 1 hour ≥ 180 mg/dL (10.0 mmol/L), 2 hours ≥ 155 mg/dL (8.6 mmol/L), and 3 hours ≥ 140 mg/dL (7.8 mmol/L).

Demographic and individual information such as maternal age, gestational week, parity were recorded. Weight and height of women with light clothes and without shoes were measured via a digital Seca Scale. BMI (Body Mass Index) was calculated by using the following formula: BMI=Weight (kg)/[Height (m) × Height (m)] and evaluated based on classification of (World Health Organisation) WHO as follows: >18.5 kg/m2 defined as underweight, 18.5-24.9 kg/m2 defined as normal weight range, 25.0-29.9 kg/m2 defined as overweight and ≥30.0 kg/m2 defined as obese. Investigators collected blood samples from each women between 24th and 28th weeks of pregnancy during their scheduled antenatal visit to determine the serum 25-hydroxyvitamin D [(25OHD)] concentration.

Levels of 25(OH)D on serum was measured by high performance liquid chromatography (HPLC) in biochemistry laboratory at Gulhane Education and Research Hospital. Investigators used clinical cutoffs based Institute of Medicine (IOM) criteria to categorize 25(OH)D levels: severe deficiency (<10 ng/mL), deficiency (10-20 ng/mL) and insufficiency (20-30 ng/mL).