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Kidney Week

Abstract: FR-OR034

Fractional Phosphorus Absorption Is Inappropriately Normal and Does Not Correlate with 24-Hour Urine Phosphorus in Patients with Moderate CKD Compared with Healthy Adults

Session Information

Category: Bone and Mineral Metabolism

  • 402 Bone and Mineral Metabolism: Clinical

Authors

  • Stremke, Elizabeth, Purdue University, West Lafayette, Indiana, United States
  • Wiese, Gretchen, Purdue University, West Lafayette, Indiana, United States
  • Wastney, Meryl E., Metabolic Modeling Services, Blenheim, New Zealand
  • Moe, Sharon M., Indiana University School of Medicine, Indianapolis, Indiana, United States
  • Moorthi, Ranjani N., Indiana University School of Medicine, Indianapolis, Indiana, United States
  • Hill Gallant, Kathleen M., Purdue University, West Lafayette, Indiana, United States
Background

Intestinal phosphorus (P) absorption in patients with CKD is understudied, yet current therapies focus on reducing intestinal P absorption to lower the risk of cardiovascular dysfunction and mortality. Rodent studies suggest that intestinal P absorption remains inappropriately normal in early CKD, despite elevations in FGF23 and declining calcitriol. We have previously shown that 24h urine P is not related to net P absorption from metabolic balance studies in CKD patients (Stremke et al. CJASN 2018), and thus more direct measures of absorption are needed.

Methods

In this controlled feeding study, we aimed to determine P absorption in patients with moderate CKD vs healthy subjects using a 33P radioisotope tracer method. CKD and controls were matched for age (+/-10y), sex, and race. Subjects ate a controlled study diet of ~1500 mg/d P, ~1400 mg/d Ca, ~3200 mg/d K, ~2400 mg/d Na, and 0.8 g/kg/d protein for 1 week. The final two study days consisted of a clinical research center (CRC) inpatient P absorption test utilizing oral and IV doses of 33P. Fractional P absorption was determined by multi-compartment kinetic modeling. 24h urine P (uP) was determined by ICP-OES as a 2-day average. Paired analyses were performed to determine differences between CKD and controls. The relationship between fractional P absorption and 24h uP was determined by Pearson’s correlation.

Results

N=6 CKD patients (eGFR=29-55 mL/min/1.73m2) and N=6 controls completed the study. Fractional P absorption was similar between CKD patients and controls (0.68 vs 0.66, p=0.91). 24h uP also did not differ (856 vs 878 mg/d, p=0.91). 24h uP did not relate to fractional P absorption overall (r=0.07, p=0.37), nor within either group (CKD, r=0.09, controls, r=0.11, p>0.50).

Conclusion

Fractional P absorption is similar between moderate CKD patients and healthy control subjects consuming a controlled diet. This supports the animal data that P absorption is maintained at inappropriately normal levels in early/moderate CKD despite hormonal changes that should suppress P absorption. We also show no association between CRC collected 24h uP and fractional P absorption supporting that 24h uP is not reflective of P intake or net P absorption in CKD.

Funding

  • NIDDK Support