Reduced Serum Vitamin D Levels Are Associated with Insulin Resistance in Patients with Obstructive Sleep Apnea Syndrome

Background and objectives: Obstructive sleep apnea syndrome (OSAS) is associated with cardiovascular and metabolic risk factors, such as insulin resistance. Furthermore, OSAS has been associated with decreased levels of vitamin D (Vit D). The aim of the study was to assess the association between Vit D levels (expressed as 25(OH)D serum levels) and insulin resistance in patients with OSAS. Materials and Methods: Serum 25(OH)D levels were measured in consecutive subjects who had undergone polysomnography and pulmonary function testing. OSAS patients were divided into those with (homeostatic model assessment [HOMA-IR] 2) and without insulin resistance (HOMA-IR < 2). Results: Overall, 92 patients (81 males) were included in the study. OSAS patients with insulin resistance significantly differed from those without insulin resistance in terms of the body-mass index (BMI) (36.3 +/- 5.8 compared to 32 +/- 5.6 kg/m(2), respectively, p = 0.001), apnoea-hypopnoea index (AHI) (57.4 +/- 28.9 compared to 40.9 +/- 27.9 events/h, respectively, p = 0.009) and indices of hypoxia during sleep. Patients with OSAS and insulin resistance had lower levels of serum 25 (OH) D compared with OSAS but without insulin resistance (19.3 +/- 11.5 vs 26.7 +/- 12.2 ng/mL, respectively, p = 0.005). Regression analysis demonstrated a negative association of 25(OH)D levels ( = -0.048, odds ratio [OR]: 0.953, 95% confidence interval [CI]: 0.913-0.995, p = 0.030) and a positive association of BMI ( = 0.110, OR: 1.116, 95% CI: 1.007-1.237, p = 0.036) with insulin resistance. Conclusions: Vit D insufficiency was significantly more frequent among OSAS patients with insulin resistance. Both low 25(OH)D levels and high BMI were associated with the risk of insulin resistance in this population.