Background Vitamin D deficiency may be a risk factor for exacerbations of chronic obstructive pulmonary disease (COPD). Studies investigating the prevalence and determinants of vitamin D deficiency among COPD patients in the UK are lacking.

Methods We conducted a cross-sectional study in 278 COPD patients aged 40–85 years screened for eligibility to participate in a clinical trial of vitamin D supplementation. Lifestyle and demographic data were collected by questionnaire and a blood sample was collected for analysis of serum 25-hydroxyvitamin D (25[OH]D) concentration and DNA extraction. Serum 25(OH)D concentration was determined by liquid chromatography – tandem mass spectrometry. Thirty-seven single nucleotide polymorphisms (SNP) in 13 vitamin D-related genes (DBP, DHCR7, CUBN, LRP2, CRTAM, LTA4 H, CYP2R1, CYP3A4, CYP27A1, CYP27B1, CYP24A1, VDR, RXRA) were typed using Taqman allelic discrimination assays. Logistic regression was used to identify environmental and genetic factors associated with risk of vitamin D deficiency (25[OH]D concentration < 50 nmol/L).

Results Mean serum 25(OH)D concentration was 45.4 nmol/L (SD 25.3); 171/278 (61.5%) participants were deficient. The following factors independently associated with increased risk of vitamin D deficiency: BMI >30 kg/m² (OR 1.87, p = 0.04) and blood draw during winter and spring seasons (OR 3.00, p < 0.01; OR 2.50, p < 0.01, respectively). The following factors independently associated with reduced risk of deficiency: consumption of a vitamin D supplement, 100–400 IU/day (OR 0.42, p < 0.01); and a sunny holiday abroad no more than 2 months prior to blood draw (OR 0.27, p = 0.02). None of the 37 SNP investigated independently associated with vitamin D deficiency.

Conclusions Vitamin D deficiency was highly prevalent among COPD patients in this study. Obesity and winter and spring sampling were risk factors for deficiency. Recent travel to a sunny country and consumption of vitamin D supplements were protective. Genetic variants in the vitamin D pathway that have previously been shown to associate with risk of vitamin D deficiency in healthy adult populations were not associated with deficiency in this patient group.