Effects of Dietary Phosphorus on Phosphorus and Calcium Whole-Body Balance and Kinetics in Moderate CKD

The aim of the study is to look at the effects of dietary phosphorus on phosphorous and calcium whole-body balance and kinetics in moderate chronic kidney disease (CKD). N = 14 enrolled subjects will be randomly assigned to a cross-over order of (A) Low P Diet, High P Diet. Each cross-over phase will be 19 days and consist of a 7-day outpatient, controlled diet period, followed by a 5- day inpatient, controlled diet, balance, and kinetic study period, followed by a second 7-day outpatient, controlled diet period.

Inclusion Criteria: * Men or women, ages 30-75 years old, any race or ethnicity * Moderate CKD, defined by KDIGO as eGFR category 3b (30-44 mL/min) or 4 (16-29 mL/min) with albuminuria categories A1-A3 * Serum intact parathyroid hormone above assay normal limit * Female subjects must be postmenopausal (\>12 months since last menstrual period), surgically sterile, or confirmed not pregnant by pregnancy test * Must be on stable doses of medications (except those noted in exclusion criteria) for at least 4 weeks prior to the study * Willing to discontinue supplements (e.g., vitamin D, calcium, multivitamin/minerals, or others) upon enrollment until completion of the study * Adequate vitamin D status defined as serum 25D \> 20 ng/mL based on National Academy of Medicine (then Institute of Medicine) criteria. Exclusion Criteria: * Plans to initiate dialysis within 6 months * Hypercalcemia defined as corrected serum calcium \>9.8 mg/dL within past 3 months * Hyperkalemia defined as serum potassium \>5.5 mg/dL within past 3 months * Hyperphosphatemia defined as serum phosphate \>5.5 mg/dL within past 3 months * Intestinal disease that alters absorption or normal intestinal function including celiac disease, small bowel resection, bariatric surgery, or chronic diarrhea/malabsorption * Serious, uncontrolled underlying systemic disease including diabetes, lupus, hypertension in the opinion of their physician * Pregnant or breastfeeding * Prescribed and taken a phosphate binder medication, calcitriol, vitamin D analogs, calcimimetics, PTH analogues, and other medications that may alter Ca and P metabolism within past 4 weeks * Non-English speaking