Background Vitamin D deficiency has been reported to associate with susceptibility to acute respiratory infections and exacerbations in asthma patients. Studies investigating the prevalence and determinants of vitamin D deficiency among asthma patients in the UK are lacking.
Methods We conducted a cross-sectional study in297 asthma patients aged 16–78 years who participated 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. Thirty-seven single nucleotide polymorphisms (SNP) in 13 vitamin D-related genes (DBP, DHCR7, CUBN, LRP2, CRTAM, LTA4H, 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 50.6 nmol/L (SD 24.9); 162/297 (54.5%) participants were deficient. The following factors independently associated with increased risk of deficiency: BMI of 25.1–30 kg/m2 (OR 2.20, p = 0.004) and >30 kg/m2 (OR 1.96, p = 0.03); blood draw during winter season (OR 3.26, p = 0.001); Fitzpatrick skin-type score of 1 (“extremely fair skin; always burn, never tan” – OR 5.15, p = 0.02). The following factors independently associated with decreased risk of deficiency: Caucasian ethnicity (OR 0.23, p = 0.02); >2 h of sun exposure/day (OR 0.48, p = 0.003); 100–400 IU vitamin D supplement, daily (OR 0.22, p < 0.001); and use of a tanning bed in previous year (OR 0.20, p = 0.006). The following genetic factors independently associated with increased risk of deficiency: carriage of the G allele for DBP SNP rs4588 (GT genotype: OR 1.82, p = 0.02; GG genotype: OR 17.4, p = 0.006); GT genotype for CYP2R1 SNP rs10500804 (OR 1.85, p = 0.02); AG genotype for CYP2R1 SNP rs10766197 (OR 1.82, p = 0.03); AA genotype for VDR SNP rs7975232 (OR 2.15, p = 0.02).
Conclusions Over half of participating asthma patients were vitamin D deficient. Obesity, winter sampling, fair skin, and SNP in DBP, CYP2R1 and VDR genes were risk factors for deficiency. Caucasian ethnicity, sun seeking behaviour, modest daily supplement doses and recent tanning bed use were protective.