Serum levels of vitD, Mg and Ca were not related to FEV1. Most participants in this study were vitD-, Mg- and Ca sufficient. Women had higher se vitD than men. Se Mg, but not se vitD and se Ca, was associated with QoL in COPD. Prospective randomized studies are needed to substantiate these finding. Clinical trials ID at www.clinicaltrials.gov: NCT01564953.

Thank you to Dr. Rutten for the comment on our manuscript "Serum magnesium and not vitamin D is associated with better QoL in COPD: A cross-sectional study". Dr. Rutten mentions that effects of a nutrient will only be visual in case of deficiency, and once sufficient, there is no additional benefits of the nutrient. In our study, most participants were serum vitamin D, magnesium and calcium sufficient, and few were nutrient deficient based on serum values. In the manuscript we have mentioned, that based on the small group with subnormal serum magnesium and the lack of participants with very low serum vitamin D, our results cannot rule out an association between lung function and serum vitamin D or serum magnesium in patients with deficiency of the given nutrient. Concerning the use of univariate correlations to show the relationships between nutrients and lung function or QoL without correction for other factors, we have explained that serum vitamin D was measured as 25-(OH)-D3 as it reflects the concentration of both sunlight-induced synthesis and intake of vitamin D in the previous 3e4 weeks. We also measured ionised calcium and serum magnesium. We did not correct for other factors as the measurements of these nutrients were defined separately. Lung function was measured as FEV1 in % predicted, and participants were recruited during summer time to minimize possible seasonal variation of both vitamin D and risk of exacerbations. As this study is a cross-sectional and not a longitudinal intervention study, we cannot evaluate a relationship between the parameters and health outcome over time.