Phosphorus Absorption in Healthy Adults and in Patients With Moderate Chronic Kidney Disease

Inclusion: Men and women ages 30-75 years old, any race Moderate CKD, based on the last set of labs done in the past year, defined as: Glomerular Filtration Rate (GFR) category G3a (eGFR 45-59 mL/min) with A2 or A3 albuminuria or proteinuria by positive dipstick or urine protein/creatinine ratio (UPCR); -or- G3b (eGFR 30-44 mL/min), with or without evidence of albuminuria (any A1-A3) or proteinuria. (Figure 1) -or- Healthy control with no diagnosis or evidence of CKD Female subjects must be post-menopausal, surgically sterile, or confirmed not pregnant by pregnancy test and not breastfeeding. Subjects must be on stable doses of medications (except those noted in exclusion criteria) for at least 6 weeks prior to the study Exclusion: Plans to initiate dialysis within 6 months Labs based on the last set done in the past year: Hypercalcemia defined as serum calcium \> 10.5 mg/dL Hyperkalemic \> 5.5 mg/dL Prescribed a phosphate binder medication Small bowel resection, bariatric surgery. Medically unstable or poorly controlled hypertension, diabetes, or gastrointestinal disorders in the opinion of the physicians on the study team Calcitriol, ergocalciferol, cholecalciferol, vitamin D analogs, calcimimetics, PTH analogues, calcium supplements, multivitamins/mineral and other medications that may alter phosphorus metabolism - must be off for at least 6 weeks prior to study