Introduction
Vitamin D deficiency is a major public health problem worldwide. The Indian subcontinent, situated between 8.40 -N and 37.60 N latitude, is exposed to adequate sunshine and ultraviolet B rays throughout the year. Because of such abundant exposure to sunlight, it has been presumed that Indians have adequate levels of vitamin D. Several studies in India show high prevalence of vitamin D deficiency in different populations1 and related to abdominal obesity.2 Previously, we had shown that patients with type 2 diabetes mellitus (T2DM) have lower levels of vitamin D as compared with non-diabetic individuals.3
Specifically, women in India are more likely to have vitamin D deficiency because many of them are confined to households and have high coverage of body part with clothes, thus they may not be exposed to sufficient sunlight. In a study in North India (Delhi), the prevalence of vitamin D deficiency was high in women, particularly those belonging to low socioeconomic status.4 Similar results have been reported for postmenopausal women in South India.5 It is important to note that low vitamin D levels and inadequate intake of calcium may predispose postmenopausal Indian women to bone fractures.6
Pre-diabetes is associated with abdominal obesity, insulin resistance, non-alcoholic fatty liver disease and metabolic syndrome, and such individuals are at an increased risk for developing T2DM and cardiovascular disease.7 A recent study in 15 states of India showed that the prevalence of pre-diabetes and T2DM was 10.3% (95% CI 10.0 to 10.6) and 7.3% (95% CI 7.0 to 7.5), respectively.8 Specifically, pre-diabetes was more prevalent than T2DM in all states in India excluding north (Punjab) and in the northeast. Importantly, Asian Indians with pre-diabetes are at a heightened risk for conversion to T2DM as compared with British Caucasians.9 In a previous study in North India (Delhi), we showed that the women and men had similar prevalence of diabetes (13.8% and 13.3%, respectively) but average blood glucose levels (mg/dL) were higher in women as compared with men (147.3±46.3 vs 141+46.8, respectively).10
The relationship between vitamin D and development of pre-diabetes has not been well understood. Cross-sectional and longitudinal studies have shown inverse association between vitamin D levels and incident diabetes.11 In Nurses’ Health Study (age, 30–55 years), living in USA, after multivariate adjustments, women who consumed more than 800 IU/day of vitamin D had a 23% lower risk for developing incident T2DM compared with women who consumed less than 200 IU/day (RR 0.77, 95% CI 0.63 to 0.94; p<0.01).12
Only a few studies have been done in India regarding insulin resistance, hyperglycemia and vitamin D deficiency. No association has been reported between vitamin D levels, body mass index (BMI), T2DM and hemoglobin A1C across the spectrum of glucose intolerance in Asian Indians residing in Western India.13 In a cross-sectional study in East India, individuals with pre-diabetes (n, 157) were investigated for vitamin D deficiency and insulin resistance. In this study, individuals with vitamin D deficiency showed significant inverse correlation to insulin resistance and positive correlation with insulin sensitivity.14 Further, same authors showed significantly lower levels of serum adiponectin and serum 25-hydroxy vitamin D (25(OH)D) and higher serum insulin levels in persons with pre-diabetes or T2DM as compared with controls.15 Of note, all above studies in India involved limited number of subjects and have included individuals from both genders. In view of propensity of Indian women to have obesity, metabolic syndrome and consequent hyperglycemia and low serum 25(OH)D levels, an investigation on larger number of women at high risk for development of diabetes is required.