The relationship between low maternal serum vitamin D levels and glycemic control in gestational diabetes assessed by HbA1c levels: an observational cross-sectional study

Vitamin D acting on its own receptors can produce a number of desired biological effects via different mechanisms and, therefore, contributes to the improvement of human health [1]. After absorption, vitamin D is bound to the binding protein and carried to the liver via the bloodstream. From there it begins two hydroxylation processes; beginning in the liver it is transformed into 25(OH) vitamin D (calcidiol), and finally, in the kidneys, where it is transformed into 1,25 dihydroxy-vitamin D (calcitriol), which is the biologically active form of vitamin D. The levels of serum active vitamin D; 1,25dihydroxy-cholecalciferol (1,25(OH)₂ 2D) increase during pregnancy as depicted by several studies, the general conclusion linked this serum rise to increased renal hydroxylation of 25-hydroxy-cholecalciferol (25(OH) D) to 1,25(OH)₂ 2D under stimulation of pregnancy [2].

There’s a great association between vitamin D deficiency and type 2 diabetes mellitus [3]. In addition, some papers reported a prevalence of inadequate 25(OH)D levels in 41% of the women with GDM, and they consequently proposed routine 25(OH) D testing of all pregnant women when screening for GDM or earlier, and treatment of women who are found to be deficient [4].

This proposition was the rationale behind this study; we found a need to study the status of vitamin D deficiency amongst Egyptian females and its relation to the glycemic control in pregnant women with GDM, and we aimed at producing recommendations concerning this issue to be implemented in the current Egyptian antenatal care program.

We included in this study the assessment of the glycated haemoglobin (HbA1c). The glycation of haemoglobin was first used 25 years ago to assess the level of glycaemia in subjects with diabetes [5]. Since then, assay of HbA1c has become the ‘gold standard’ by which the effectiveness of glycemic control is judged in clinical practice [6].

This cross-sectional observational study was conducted at Cairo University hospital; Kasr al-Aini, the obstetrics and gynecology department, during the period between June 2012 and June 2013. It was approved by the Scientific & Ethical Committee of the Obstetrics and Gynecology Department of Cairo University. We approached the pregnant women, in the third trimester, attending the antenatal care clinic of Cairo university hospital during this period, the target age group was between 20-40 years. We first enrolled 80 pregnant women with established diagnosis of gestational diabetes. Those women were diagnosed as having GDM as depicted by estimating the fasting and 2-hours post-prandial blood glucose levels in previous antenatal care visit; they were already receiving treatment for their condition, through a collaborative approach, involving the Diabetes Mellitus outpatient clinic and the internal medicine department of Cairo University. We then included a second group; consisting of randomly selected 80 pregnant women, those women had normal fasting and 2-hours post-prandial blood glucose levels blood glucose levels, during their routine antenatal care visits. The need to include a control group emerged when we were faced by the high incidence of vitamin D deficiency among our included cases of the first group. A question was raised; is this deficiency caused by impaired glucose tolerance, or it is a common finding in the population study. We included an equal number to the previously enrolled pregnant diabetic women, in order to obtain statistical equality. Written Informed consents were obtained from each participant prior to enrollment; the consent form was written in Arabic language and followed the world health organization guidelines (WHO). All consent forms were signed by one of the investigators, and were collected and filed. Patients giving history of type 1 or 2 diabetes mellitus antedating pregnancy were excluded, as well as any medical disorders (such as hypertension or cardiac disease) or medical complications related to pregnancy. All the patients included were subjected to detailed history taking with special focus on maternal age, parity, gestational age at diagnosis of gestational diabetes, previous history or family history of diabetes, history of gestational diabetes in previous pregnancies. Data concerning insulin regimens, insulin types and doses (if diagnosed of having gestational diabetes) were also noted. Body Mass Index (BMI) was calculated for all subjects as follows; weight in kilograms divided by their height in meters squared (18.5 or less: underweight, 18.5-24.9 is normal, 25-29.9: overweight, 30-39.9: Obese and 40 or greater is extremely obese). The BMI in the control group was matched to that in the GDM group for sakes of statistical adjustment. Laboratory investigation included Vitamin D3 level measured by radio immunoassay. Vitamin D deficiency was defined conservatively as <25 ɳmol/L, insufficiency as 25–50 ɳmol/L and sufficiency as >50 ɳmol/L. HbA1c was measured by radioimmunoassay to assess the glycemic control. The normal level for glycosylated hemoglobin is less than 7%. Diabetics rarely achieve such levels, but tight control aims to come close to it. Levels above 9% show poor control, and levels above 12% show very poor control. Fasting serum insulin (in mIU/L), fasting blood glucose and 2 hours postprandial, interpreted in mg/dL were included within the laboratory investigations. The insulin sensitivity as a reflection of insulin resistance was measured using the Quantitative insulin sensitivity check index (Quicki). This index correlates well with glucose clamp studies (r =0.78), and it has the advantage of that it can be obtained from a fasting blood sample, hence it is the preferred method for several types of clinical research [7]. It is calculated as {1/( log fasting insulin + log fasting blood sugar). Blood samples were collected from pregnant women with gestational diabetes during pregnancy during routine antenatal care visits and were analyzed in Cairo University Clinical Pathology Department’s laboratories. All results were statistically analyzed and tabulated.

Vitamin D plays a great role in bone metabolism and mineral homeostasis. Relevant data identify roles for the active form of vitamin D (1,25(OH)₂D3) in many biological processes including regulation of cellular growth, differentiation and metabolic modulations [8]. Vitamin D insufficiency has a known effect on bone density, neonatal vitamin D and calcium status, and childhood rickets [9]. In several studies, the relation between low vitamin D levels, insulin resistance and impaired insulin secretion was clearly demonstrated [10]. Moreover, specific receptors for 1,25(OH)₂D3 were detected in pancreatic ß cells, denoting a probable effect of vitamin D on the insulin secretion process [11]. Obesity is another significant risk factor for 25OHD deficiency [12, 13]. An analysis of the National Health and Nutrition Examination Survey (NHANES) for the years 2003/2004 data also demonstrated that 25 (OH) D deficiencies were highly prevalent in overweight and obese American subjects [14].

In our study we tried to find a correlation between glycemic control and vitamin D deficiency in pregnant women known to have gestational diabetes, diagnosed at previous visits, and we included normal pregnant population as a control. Vitamin D deficiency was the salient feature in both groups, with the mean serum vitamin D levels close in both groups. This could be explained by the fact that the great majority of the pregnant women, included in our study, belonged to poor economic standards which may affect a good supplementation of vitamin D; they have poor housing conditions, poor exposure to sun, insufficient food intake which might be referred to as vegetarian in other papers, repeated non spaced pregnancies and lack of medical service to provide good vitamin supplementation. Another important factor is that, in spite of the sunny climate in Egypt, all the included women were veiled, which hinder the passage of sunrays to the skin, and subsequently diminishing trans-cutaneous vitamin D absorption. We had a negative linear correlation between fasting blood glucose levels and vitamin D3. Our findings also support an independent inverse association between 25(OH)D and HbA1c in women with GDM, showing a potential interaction between 25(OH)D and blood glucose control in pregnancy. The higher levels of HbA1c, indicating poor glycemic control, were found in women with lower levels of 25(OH)D and this inverse relation was found to be statistically significant (r = - 0.492).

The possible explanation for such relationship, between Vitamin D deficiency and the impaired glycemic control, found in our study could be made attributed to the defect in the important role that Vitamin D plays in glucose homeostasis, and via different mechanisms. The mechanism of action of vitamin D in type 2 diabetes is thought to be mediated not only through regulation of plasma calcium levels, which regulate insulin synthesis and secretion, but it also improves insulin sensitivity of the target cells (liver, skeletal muscle, and adipose tissue). Additionally, Vitamin D enhances and improves β-cell function and protects them from detrimental immune attacks, directly by its action on β-cells, but also indirectly by acting on different immune cells, including inflammatory macrophages, dendritic cells, and a variety of T cells. Macrophages, dendritic cells, T lymphocytes, and B lymphocytes can synthesize Vitamin D, all contributing to the regulation of local immune responses [15].

This correlation between vitamin D and GDM were evaluated by various studies and the results were controversial [16–19]. Maghbooli et al demonstrated that maternal serum levels of 25(OH)D during 24-28 weeks of pregnancy were significantly lower in women with GDM compared with controls [16]. More recently, Makgoba and his colleagues did not find an association between first trimester maternal serum 25(OH)D levels and subsequent GDM development [17]. On the other way round, Clifton-Bligh demonstrated an inverse association between maternal serum 25(OH)D levels and fasting blood glucose, although the association between 25(OH)D and GDM was not statistically significant [18]. Additionally, a recent study of Indian women failed to demonstrate a relationship between maternal vitamin D status and risk of developing GDM [19]. More recently meta-analysis, published in 2013, linked gestational diabetes with insufficient 25(OH)D levels, it correlated in addition vitamin D deficiency to preeclampsia, preterm birth and small for gestational age (SGA) babies [20]. In 2013, a randomized controlled study from a Turkish group, on 234 women with GDM and 168 controls, came to a more specific conclusion; they found a statistical significance, between glycemic control and vitamin D levels, only in women with severe deficiency of 25(OH)D levels. They also concluded that only this group (with severe vitamin D deficiency) is at a higher risk of GDM [21]. We mostly attribute this disparity in the results and correlations to the population differences, the most influential factor being, from our point of view, the absence or very few numbers of pregnant women with severely deficient 25OHD.

An important limitation of the present study is the small number of patients with severe vitamin D deficiency and the single measurement of 25(OH)D levels in the third trimester of pregnancy. Other confounding factors included the seasonal variation, as the study extended over a whole year, the heterogeneity of dietary habits and the differences in socioeconomic standards between the included pregnant women. We believe however that these factors did not have a major impact on the statistical analysis of this study, considering that most, if not all, of our patients live in the poor socioeconomic areas.

In our study, there was high prevalence of vitamin D insufficiency and deficiency. Considering this we suggest routine testing of all pregnant women, and treatment of women who are found to be vitamin D deficient. We do not know whether adequate vitamin D replacement may contribute to prevention of GDM later in pregnancy, as our study suggests a significant inverse correlation between vitamin D status and glycemic control in all the pregnant women included. We recommend larger scale studies addressing this issue, with a special concern to the effectiveness of adequate replacement of vitamin D in preventing GDM.