Effect of Dietary Protein Source on Phosphaturia, PTH and FGF23 in Patients With CKD 3 and 4
Effect of Dietary Protein Source on Phosphaturia, PTH and FGF23 in Patients With CKD 3 and 4
Phosphorus is a substance in the blood that comes from food and is normally cleared from the
 body by the kidneys. In patients with kidney disease, excess phosphorus may build up in the
 body as you eat. This leads to problems with bones and blood vessels over time. In this
 study, we will compare the blood and urine before and after eating one week of a diet with a
 protein from plants (soy and grains) and before and after another one week of diet with
 protein from animals (meat and dairy products). The amount of phosphorus that the kidney puts
 out in the urine, and the changes in blood hormones in response to the diet will be measured
 at the beginning and end of each week on the two diets.
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).
Inclusion Criteria:

 -  age >18 years

 -  eGFR 20-45 by modified MDRD equation

 -  protein/creatinine ratio <5

 -  blood pressure <150/95

 -  not taking calcium binder or supplements, vitamin D, or phosphate binders

 -  normal serum phosphorus and calcium corrected for albumin and intact PTH <100pg/ml

 -  medically stable

 -  able to give informed consent and come for all visits

 Exclusion Criteria:

 -  history of significant liver disease or cirrhosis

 -  medically unstable

 -  unable to tolerate diets