Vitamin D status assessed by serum 25-hydroxyvitamin D levels (25(OH)D) has been shown to be inversely associated with insulin resistance. The underlying basis for such association is less clear. In the present study, we assessed the prevalence of inadequate vitamin D levels and its relationship to glucose tolerance status and circulating adiponectin in healthy Thai population. The cohort of 246 subjects was classified into two groups according to 75 g oral glucose tolerance test (OGTT) results. There were 86 subjects and 160 subjects in normal glucose tolerance group (NGT) and abnormal glucose tolerance group (AGT), respectively. Anthropometric variables were recorded for each individual. Fasting blood samples were assayed for 25(OH)D, adiponectin, glucose, and insulin levels. Insulin resistance (HOMA-IR) and insulin secretion index (HOMA-B) were calculated by the homeostasis model assessment. Pearson and partial correlation analyses were performed. There were 35 males and 211 females with a mean age of 62.4 +/- 7.2 years in this study. The mean levels of 25(OH)D were 21.4 +/- 6.6 ng/ml. The prevalence of vitamin D deficiency defined by 25(OH)D levels less than 20 ng/ml and vitamin D inadequacy defined by 25(OH)D levels less than 30 ng/ml were 44.3 and 91.9%, respectively. The mean levels of 25(OH)D obtained from samples collected in the rainy season (19.4 +/- 4.6 ng/ml) were significantly lower than those collected in the winter (22.6 +/- 8.3 ng/ml) and summer period (23.1 +/- 4.6 ng/ml). AGT subjects had slightly lower average 25(OH)D levels than the NGT group (21.0 +/- 6.8 vs. 22.1 +/- 6.2 ng/ml, P = 0.09). 25(OH)D levels were positively associated with adiponectin levels (r = 0.20, P< 0.05) and negatively associated with HOMA-IR and BMI (r = -0.22, P < 0.01 and r = -0.22, P < 0.01, respectively) only in AGT subjects. An independent association between 25(OH)D and adiponectin levels was demonstrated after controlling for BMI (r = 0.17, P < 0.05). High prevalence of vitamin D inadequacy and seasonal variation of vitamin D status are found in Thai population. We demonstrated an association between insufficient vitamin D status and lower circulating adiponectin in subjects with abnormal glucose tolerance independently of adiposity which may indicate the role of adiponectin as a link between vitamin D status and insulin resistance.

Registry: ClinicalTrials.gov; Title: Obstructive Sleep Apnea and Gestational Diabetes: Incidence and Effects of Continuous Positive Airway Pressure Treatment on Glucose Metabolism; Identifier: NCT02108197; URL: https://clinicaltrials.gov/ct2/show/NCT02108197.

Serum oestradiol is more related to bone mass than free testosterone in normal men. Oestrogen-receptor gene polymorphism is also associated with bone mass in men independently of oestradiol levels. Serum oestradiol together with oestrogen-receptor genotype may partly determine bone mass in males.