A new study published in Nephrology, Dialysis, Transplantation investigated the effects of nutritional vitamin D supplementation on markers of bone and mineral metabolism, that is, serum levels of fibroblast growth factor-23 (FGF23), Klotho, bone alkaline phosphatase (BAP) and sclerostin, in two cohorts with chronic kidney disease (CKD). This study enrolled 80 vitamin D-deficient children – 40 with mild to moderate CKD from the ERGO study (a randomized trial of ergocalciferol supplementation [estimated glomerular filtration rate (eGFR) 55 mL/min/1.73 m2]) and 40 with advanced CKD from the observational Cardiovascular Comorbidity in Children with Chronic Kidney Disease (4C) study (eGFR 24 mL/min/1.73 m2). In each study, vitamin D supplementation was started in 20 children, while 20 matched subjects not receiving vitamin D served as controls. The findings showed that before vitamin D supplementation, children in the ERGO study had normal FGF23 and BAP but decreased Klotho and sclerostin, whereas 4C patients had increased FGF23, BAP and sclerostin but normal Klotho levels. While, vitamin D supplementation further increased FGF23 in 4C but not in ERGO patients. Serum Klotho and sclerostin normalized with vitamin D supplementation in ERGO but remained unchanged in 4C patients. On the other hand, BAP levels remained unchanged in all patients. In the total cohort, significant effects of vitamin D supplementation were noted for Klotho at eGFR 40-70 mL/min/1.73 m2. Hence, it was inferred that Vitamin D supplementation normalized Klotho and sclerostin in children with mild to moderate CKD but further increased FGF23 in advanced CKD.