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Study finds 24-hour urine phosphorus is unrelated to dietary phosphorus intake in patients with CKD
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Among patients with moderate CKD on a tightly controlled dietary intake, 24-hour urine phosphorus was highly variable and unrelated to dietary phosphorus intake or absorption and was inversely related to phosphorus retention, according to research that appears in the Clinical Journal of the American Society of Nephrology.
“In our study, we were able to show that a single 24-hour urine phosphorus was highly variable within and among patients with moderate-stage chronic kidney disease, even while consuming a consistent study diet with the same amount of phosphorus in a tightly controlled inpatient research setting. At least two measures are required to void the day-to-day variability,” Kathleen M. Hill Gallant, PhD, RD, co-author of the study from the department of nephrology at Indiana University School of Medicine, told Healio Nephrology. “Importantly, we also demonstrated that 24-hour urine phosphorus did not relate to net intestinal phosphorus absorption determined from full balance studies in these patients under steady-state conditions with complete and meticulously timed and pooled urine collections. Our study was small with only eight patients, but these net phosphorus absorption data in CKD patients are wholly unique and call into question the validity of 24-hour urine phosphorus as a biomarker of absorption in patients with CKD.” Hill Gallant added that results of this study should be confirmed with a larger population.
According to the study, the day-to-day variation in 24-hour urine phosphorus within and among subjects was wide. To achieve reliability of at least 75%, two 24-hour urine measures were needed. Estimating dietary phosphors intake from one 24-hour urine measure resulted in underestimation up to 98% in some patients, while others showed overestimation up to 79%.
“It makes a lot of sense physiologically that urine phosphorus excretion wouldn’t equal net intestinal absorption in patients with CKD. It has been shown that the kidney is the major regulator of phosphate, and in healthy people is able to completely excrete absorbed phosphate loads,” Hill Gallant said. “Thus, as kidney function declines, either net absorption becomes disconnected with renal excretion or the intestine is able to respond and completely compensate for the reduced kidney function, which seems unlikely.” – by Jake Scott
Disclosures: The authors report no relevant financial disclosures.
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