Effect of Dietary Protein Source on Phosphaturia, PTH and FGF23 in Patients With CKD 3 and 4

Chronic Kidney Disease-Mineral Bone disorder (CKD-MBD) is a constellation of problems related to alterations in mineral and bone homeostasis that occur in CKD stage 3-5D (estimated GFR 60-15 ml/min). The damaged kidney is unable to fully excrete a phosphorus load, leading to a compensatory secondary hyperparathyroidism to attempt to increase urinary phosphorus excretion in order to maintain serum phosphorus in the normal range. Eventually this compensation of elevated PTH becomes pathologic and leads to abnormalities in biochemistries, bone and vascular disease, all of which are associated with morbidity and mortality in patients with CKD. Prevention of these complications is key to improved patient outcomes. Unfortunately, this normal or high normal phosphorus does not reflect the "behind the scenes" appropriate and inappropriate compensation. The use of medication to bind phosphorus from food (phosphate binders) may prevent absorption of phosphorus across the intestine and prevent or change the elevations in PTH and other hormones like FGF23. Thus, either urinary excretion of phosphorus, or changes in hormone may be more appropriate end points to evaluate efficacy of phosphate binders than is serum phosphorus.

In healthy individuals, there is variation throughout the day (diurnal) in serum phosphorus and urine phosphorus excretion, but in dialysis patients, this variability appears to be lost. No data exists for patients with stage 3 and 4 (pre-dialysis) CKD. Intestinal phosphorus absorption is also dependent on bioavailability (amount of free phosphorus available to be absorbed), which differs depending on the protein source, as the phosphorus in grain/soy diets is less bioavailable than that from protein from animal/casein protein source. In our animal model of CKD, these differences in bioavailability impact urinary phosphorus excretion and serum levels of FGF-23, but not PTH. As phosphaturia, PTH, and FGF23 may become important end points for future clinical trials, understanding diurnal variability and the relationship to diet in patients with CKD 3 and 4 with normal serum phosphorus levels is critical. We hypothesize that dietary protein source will affect the hormonal response and diurnal phosphorus homeostasis in advanced CKD. To test this hypothesis, we will examine the following specific aims in a population of CKD stage 3 and 4 subjects from the Indiana University Affiliated Nephrology Clinics and determine

1. if the dietary protein source affects fasting serum and urinary phosphorus excretion
2. if the protein source affects post prandial changes in serum and urinary phosphorus in patients
3. if changes in plasma FGF23 and PTH correlate with urinary phosphorus excretion in response to different protein sources.

We will conduct a cross over study to assess blood and urine after one week of a diet that differs only in the source of the protein (and thus the bioavailability of phosphorus).