The American Journal of Clinical Nutrition, 116, 491-499 The American journal of clinical nutrition, 116(2), 491-499. American Society for Nutrition The American Journal of Clinical Nutrition, 116(2), 491-499. OXFORD UNIV PRESS PRECOVID-study group 2022, ' Vitamin D supplementation in chronic obstructive pulmonary disease patients with low serum vitamin D : a randomized controlled trial ', The American Journal of Clinical Nutrition, vol. 116, no. 2, pp. 491-499 . https://doi.org/10.1093/ajcn/nqac083 PRECOVID-study group 2022, ' Vitamin D supplementation in chronic obstructive pulmonary disease patients with low serum vitamin D : a randomized controlled trial ', The American journal of clinical nutrition, vol. 116, no. 2, pp. 491-499 . https://doi.org/10.1093/ajcn/nqac083 The American Journal of Clinical Nutrition, 116, 2, pp. 491-499 The American Journal of Clinical Nutrition, 116(2), 491-499. Oxford University press