Low 1,25-dihydroxyvitamin D levels are associated with severe COVID-19: an observational study in hospitalized patients. - Canbolat Unlu E; Karabela SN; Yesilbag Z; Korkusuz R; Borcak D; Salim Dogdas E; Senoglu S; Isiksacan N; Kart Yasar K (Frontiers in nutrition, 2026)