Assessment of Inorganic Phosphate Intake by the Measurement of the Phosphate/Urea Nitrogen Ratio in Urine

Abstract

In chronic kidney disease (CKD) patients, it would be desirable to reduce the intake of inorganic phosphate (P) rather than limit the intake of P contained in proteins. Urinary excretion of P should reflect intestinal absorption of P(inorganic plus protein-derived). The aim of the present study is to determine whether the ratio of urinary P to urinary urea nitrogen (P/UUN ratio) helps identify patients with a high intake of inorganic P.A cross-sectional study was performed in 71 patients affected by metabolic syndrome with CKD (stages 2–3) with normal serum P concentration. A 3-day dietary survey was performed to estimate the average daily amount and the source of P ingested. The daily intake of P was 1086.5 ± 361.3 mg/day; 64% contained in animal proteins, 22% in vegetable proteins, and 14% as inorganic P. The total amount of P ingested did not correlate with daily phosphaturia, but it did correlate with the P/UUN ratio (p < 0.018). Patients with the highest tertile of the P/UUN ratio >71.1 mg/g presented more abundant inorganic P intake (p < 0.038).The P/UUN ratio is suggested to be a marker of inorganic P intake. This finding might be useful in clinical practices to identify the source of dietary P and to make personalized dietary recommendations directed to reduce inorganic P intake.

The basics of phosphate metabolism

“…In balance studies, at least two consecutive 24-hour urine collections were needed to predict Pi absorption with ≥75% reliability [ 38 ]. However, it has been suggested to normalize urinary Pi to urea nitrogen in urine, yielding a better association with dietary intake [ 39 ]. Also, random spot urine samples have been evaluated to assess the correlation with 24-hour urine Pi excretion.…”

The Role of Diet in Bone and Mineral Metabolism and Secondary Hyperparathyroidism

“…This inaccuracy, together with hidden dietary phosphate sources, raises further concern in predicting the average intake of phosphate in CKD patients by dietary recalls and it may at least partially explain the heterogeneous results in the literature on the effects of phosphate restriction. The utility of the urinary phosphate/urea nitrogen ratio in the identification of patients with higher inorganic phosphate intake was recently demonstrated [ 176 ]. This may be used as a useful tool for monitoring hidden dietary phosphate sources in CKD patients.…”

Effects of a single phosphate-enriched test meal on inflammasome activity and postprandial inflammatory markers in healthy subjects

“…Trautvetter et al [ 23 ], who analyzed data from 149 subjects in Germany, reported a habitual phosphate intake of more than 1300 mg per day. In contrast to phosphate in natural foods, phosphate from inorganic phosphate additives is more effectively absorbed in the intestine [ 24 ]. Although circulating Pi is tightly regulated in healthy subjects [ 25 ], our previous data show that compared to placebo, a single dose of 700 mg phosphorus administered orally as sodium dihydrogen phosphate (NaH 2 PO 4 , 3.53 g) to healthy subjects results in a significant increase in postprandial plasma Pi levels, which remain elevated over a period of 8 h post-meal (at 480 min: + 13%) [ 26 ].…”