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.

Background/Objectives: It would be desirable to reduce the intake of inorganic phosphate (P), which is easily absorbed and is associated with cardiovascular disease. The phosphate-to-urea nitrogen ratio (P/UUN) in urine should reflect the proportion of inorganic P ingested relative to protein intake. In this manuscript, we will refer to this parameter as P/UUN, which is conceptually equivalent to the phosphate-to-urea ratio (P/U). These studies aim to evaluate whether an increased intake of inorganic P translates into an increase in the P/UUN. Methods: A total of 18 healthy volunteers (Study-1) and 18 chronic kidney disease patients (Study-2) were included. At baseline, all participants completed a 3-day dietary survey, and on the third day collected a 24 h urine sample. In Study-2, blood samples were also obtained. Participants were then stratified into three groups (6 per group) for a 3-day dietary intervention: control group: maintained their usual diet; soda group: consumed soda zero, a source of added inorganic P; and processed cheese group: consumed fresh processed cheese, which includes both organic and inorganic P additives. At last visit, all participants again completed a 3-day dietary survey and collected a 24 h urine sample (and blood samples in Study-2). Dietary P intake was estimated using two tools: the diet calibrator and the Spanish Food Composition Database (BEDCA). Results: After the intervention, neither BEDCA nor the diet calibrator was able to provide an accurate measurement of inorganic P ingested. However, only in the soda group, P/UUN increased in both studies (p = 0.046 and 0.047). In Study-2, the levels of FGF23 and klotho remained unchanged (p = 0.9 and p = 0.7, respectively). Conclusions: These findings suggest that urinary P/UUN ratio may be a useful biomarker to monitor changes in inorganic P intake and could help to individualize dietary recommendations to reduce inorganic P exposure without restricting protein intake.