FGF-23 is a newly described protein that is an important regulator of phosphorus in the body. This protein increases in people with kidney disease and people who need dialysis have very high levels of FGF-23 in the blood. However, although some studies have indicated that FGF-23 levels go up with increased intake of phosphorus, no one knows if FGF-23 levels can be lowered in patients with kidney disease by preventing them from absorbing phosphorus from food. This study is designed to see what happens to levels of FGF-23 in the blood when patients with chronic kidney disease take medications to prevent phosphorus absorption. Since high levels of FGF-23 have been linked with increased rates of death in patients with advanced kidney disease, controlling the levels may, in the future, be a way to decrease heart disease in patients with kidney disease.

Inclusion Criteria:

• Inclusion criteria include pediatric patients, between the ages of 2 and 21 years, with CKD stages 2-4 (GFR 15-90 ml/min/1.73m2).

Exclusion Criteria:

• Exclusion criteria include: the use of phosphate binder therapy within the past 3 months, treatment with 25(OH)vitamin D or 1,25dihydroxyvitamin D, underlying metabolic bone disease, or underlying renal phosphate wasting disorder.

3 months