Vitamin D status is associated with muscle strength and quality of life in patients with chronic obstructive pulmonary disease (COPD): a seasonal prospective observation study

Background: Owing to hospitalization, reduced functional capacity and consequently, less sunlight exposure, sub-optimal vitamin D status (25-hydroxyvitamin D [25(OH)D] <50 nmol/L) is prevalent among chronic obstructive pulmonary disease (COPD) patients.
Objective: This study aimed to investigate seasonal changes in vitamin D status and any associated changes in fat free mass (FFM), muscle strength and quality of life (QoL) in COPD patients.
Methods: COPD patients living in Northern Ireland (n 51) completed study visits at the end of winter (March/April) and at the end of summer (September/October), corresponding to the nadir and peak of vitamin D status, respectively. At both time-points, serum concentration of 25(OH)D was quantified by LC-MS/MS, FFM (kg) was measured using bioelectrical impedance and muscle strength (kg) was measured using handgrip dynamometry. QoL was assessed using the validated St George’s Respiratory Questionnaire.
Results: Mean±SD 25(OH)D concentration was significantly higher at the end of summer compared to the end of winter [52.5±30.5 nmol/L vs 33.7±28.4 nmol/L, P<0.001]; and housebound patients had significantly lower 25(OH)D concentration compared to non-housebound patients at the end of summer [42.9±4.2 vs 57.2±9.9 nmol/L; P<0.001]. Muscle strength (at both time-points) and QoL (end of summer only) were positively predicted by 25(OH)D concentration, independent of age, sex and smoking status.
Conclusion: This study highlights the need for health policies to include a recommendation for year-round vitamin D supplementation in housebound COPD patients, and wintertime supplementation in non-housebound patients, to maintain optimal 25(OH)D concentrations to protect musculoskeletal health. Furthermore, an optimal vitamin D status may have potential benefits for QoL in these patients.