It is not known whether vitamin D levels make a significant contribution to muscle dysfunction in COPD.

In 104 COPD patients (mean(SD) FEV1 44(22)%predicted) and 100 age and sex-matched controls serum 25(OH)D, 1,25(OH)2D and parathyroid hormone levels were measured and related to quadriceps strength and endurance. In a subset (26 patients, 13 controls) quadriceps biopsy was performed and mRNA expression of myogenic regulatory factors (mrf) and fibre specific myosin heavy chains (MHC) determined.COPD patients were weaker and less physically active than controls. 25(OH)D levels were similar in both groups (48.5(25.5)nmol·l(-1) COPD vs. 55.4(28.3)nmol·l(-1) control, p=0.07) but PTH levels were significantly higher in patients (5.2(2.3)pmol·mL(-1) vs. 4.4(2.0)pmol·l(-1), p=0.01). 1,25(OH)D was significantly correlated with strength in controls but not COPD patients and not with quadriceps endurance assessed using repetitive magnetic stimulation in COPD (n=35) or control (n=35) subjects. In controls but not COPD, muscle biopsy analysis showed a negative relationship between 25(OH)D and MyHCIIa expression (r(2)=0.5, p=0.01) and a positive relationship between mrf4 and MyHCIIa expression (r(2)=0.5, p=0.009), and myf5 and MHCI expression (r(2)=0.72, p=0.004).

In contrast to healthy controls, muscle strength is not associated with vitamin D levels in COPD which may represent vitamin D resistance.