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 measurement
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