Vitamin D and COPD: seasonal variation is important.

Janssens et al 1 have demonstrated the relationship between vitamin D status and lung function in patients with chronic obstructive pulmonary disease (COPD). However, in their study there was only one assessment of vitamin D status per patient. Given the minimal component of diet to vitamin D status (serum 25-hydroxyvitamin D (25(OH)D) concentration), 2 it is mostly determined by sunlight exposure and has a seasonal variation in healthy individuals. 3 In a study of 24 patients with COPD (mean (SD) age 69 (5.8) years and smoking history 43 (15.8) pack-years) with measurements undertaken in the same individuals at the end of summer (August/September) and winter (March/April), we have also shown that vitamin D status correlated with forced expiratory volume in 1 s (FEV 1) (r¼0.486, p¼0.016), which remained significant (p¼0.048) when corrected for age, gender, body mass index and activity level. However, we also showed that the relationship between spirometry (FEV 1 and forced vital capacity (FVC)) and vitamin D status For spirometry there were non-significant seasonal trends with a mean difference of 0.05 l (95% CI À0.15 to 0.12) and 0.07 l (95% CI À1.1 to 0.25) for FEV 1 and FVC, respectively (table 1). Janssens et al 1 showed that patients with COPD have poorer vitamin D status than healthy smokers. Our results confirmed this finding and, additionally, showed differences between the vitamin D status both in summer and winter compared with age-, gender-, month-and geographical region-matched controls from the UK National Diet and Nutrition Survey (NDNS). Furthermore, in the winter there were three subjects who had 25(OH)D concentrations >50 nmol/l but none with a concentration >75 nmol/l, a concentration regarded by many as being appropriate to define vitamin D sufficiency. 2 In the summer, eight patients had a 25(OH)D concentration >50 nmol/l and only one patient had a concentration >75 nmol/l. We did not find any association between oily fish intake and vitamin D status. Sunlight exposure is also determined by the distance from the equator and therefore the findings from one region may not represent those of another. In a study which evaluated the vitamin D status in patients with asthma compared with healthy controls (which showed no significant difference in status between these groups), 5 the serum 25(OH)D concentration was twice as high in patients from Aberdeen (latitude 578N) than in those from Norwich (latitude 528N). We therefore believe that assessing vitamin D status in …