Children with chronic kidney disease, even after transplantation, may be at risk for bone problems due to an imbalance of calcium and phosphorus in the blood, especially as their kidneys progressively fail to function. While some drug and diet treatments are available to prevent such bone disease, many children refuse to take them due to bad taste and tummy cramps. If calcium and phosphorus status remain abnormal for a long time, hard crystals can form in the blood vessels, eventually clogging them and resulting in heart problems. Investigators are studying possible new methods to help the kidneys maintain a normal balance of nutrients in the blood which is important for growing healthy bones and the prevention of side effects in blood vessels that can lead to heart disease. One method is to improve the team work of a hormone FGF-23 and a protein called Klotho that together stimulate the kidneys to increase phosphate removal. Investigators propose that this problem may be due to low blood zinc levels which often occur in children with kidney disease. Thus, in this study, investigators propose to first measure zinc in blood from children with chronic kidney disease (CKD) or who have had kidney transplants to assess zinc and phosphate status, the hormone FGF-23 and its assistant Klotho. If zinc status is low, the children will receive zinc supplementation for 3 months. After treatment with zinc, the same blood measurements will be repeated to determine if the zinc supplements have helped the hormones to remove phosphate from the body. If this pilot project is successful, investigators will then consider a larger scale project involving adult patients as well as pediatric patients from other pediatric centers. This project will also guide investigators as to whether they need to introduce zinc measurements as part of routine testing of CKD and transplant patients. In addition to measuring zinc levels in study participants, trace elements (TE) will also be measured. These include heavy metals such as cadmium, chromium, nickel, vanadium, copper, lead, manganese and selenium. Very little is known about levels and metabolism of TE in CKD especially before dialysis. In adults, cadmium, chromium, nickel, and vanadium probably accumulate in hemodialysis patients, while copper and lead may accumulate. Manganese, selenium are probably deficient. The study will allow investigators to obtain the information about TE in this group of pediatric patients.

Inclusion Criteria:

• Children between 4 and 18 years of age; diagnosis of CKD; renal transplant recipient with declining renal function (eGFR<90 ml/min/1.73 m2).

Exclusion Criteria:

• Children with CKD or kidney transplant younger than 4 years. Kidney transplant recipients with eGFR>90 ml/min/1.73 m2.