The purpose of this study is to learn more about how the kidneys control the blood levels of phosphorus in patients with early chronic kidney disease. The ultimate goal is to use this information to design improved treatment strategies for phosphorus-related problems for the millions of patients with chronic kidney disease.

Phosphorus is a mineral found in dairy products, nuts, and meat that is essential for bone health and many other important functions inside the body's cells. The kidneys are responsible for keeping the blood levels of phosphorus normal. Healthy kidneys do this by spilling excess phosphorus into the urine. In patients with chronic kidney disease, the kidneys are unable to spill an adequate amount of phosphorus so that excess phosphorus can accumulate in the walls of blood vessels leading to heart disease, their leading cause of death. A recently discovered hormone called FGF-23 helps control the blood levels of phosphorus by "telling" the kidney how much phosphorus to spill in the urine. The purpose of this study is to learn more about how FGF-23 helps the kidneys control the blood levels of phosphorus. The ultimate goal is to use this information to design improved treatment strategies for phosphorus-related problems for the millions of patients with chronic kidney disease.

Inclusion Criteria:

• subjects with stage 3a, 3b and 4 CKD
• subjects have to be aged 18 years or older
• subjects have to have normal serum phosphate levels (< 4.6 mg/dl)
• subjects have to have sufficient 25-OH Vit0amin D levels (≥ 20 ng/ml)

Exclusion Criteria:

• subjects with rapidly advancing renal failure who thus might develop hyperphosphatemia or end stage renal disease requiring initiation of dialysis during the study period
• subjects expected to require dialysis initiation within the follow up period
• subjects with hyperphosphatemia > 4.6 mg/dL
• subjects with any previous or current treatment with phosphate binders or active vitamin D (doxercalciferol or calcitriol)
• subjects with malnutrition, defined as a serum albumin < 3.0 mg/dl, in order to avoid worsening protein-malnutrition status during the restricted study period since phosphate restricted diets are also low in protein
• subjects with chronic gastrointestinal diseases that could impede their absorptive ability such as Crohn's disease, ulcerative colitis and sprue
• subjects with liver disease (ALT or AST > 100 U/L) or cholestasis (direct bilirubin > 1.0 mg/dl) because this can limit their ability to absorb fat soluble vitamins such as vitamin D
• subjects with anemia, defined as a hematocrit <27% at the screening visit
• subjects wht have been hospitalization within the previous 4 weeks
• subjects who are pregnant
• subjects who are breastfeeding mothers
• subjects with primary hypoparathyroidism
• subjects with primary hyperparathyroidism
• subjects with previous subtotal parathyroidectomy
• subjects with previous outpatient counseling by a renal nutritionist
• subjects who are unable to independently provide written informed consent - prisoners, mentally incompetent, minors

2 weeks

In addition to limited power, the small sample size led to imbalances in baseline laboratory tests, which added further variability to the analyses. .