Elucidation of the etiology and characteristics of pink urine in young healthy subjects

Background: Pink urine syndrome (PUS) is attributed to the precipitation of uric acid caused by low urinary pH (U-pH). However, the reasons for the lower U-pH are unclear. Objectives: To investigate the occurrence of PUS and verified the cause of U-pH reduction. Methods: Participants comprised 4,940 students who had undergone a physical examination. Data on the presence [PUS (+)] or absence [PUS (−)] of PUS, as well as age, gender, body mass index (BMI), blood pressure (BP), heart rate (HR), and U-pH were collected. Of these participants, 300 randomly selected individuals were evaluated for their waist circumference, as well as their levels of urinary C-peptide, angiotensinogen, methylglyoxal, thiobarbituric acid-reactive substances (TBARS), and Na⁺ excretion. Independent risk factors of lower U-pH were decided by a multiple-regression analysis. Results: PUS was observed in 216 students (4.4 %). A greater number of men comprised the PUS (+) group compared with the PUS (−) group, and subjects in this group had high BMI, BP, and HR values, as well as low U-pH. A logistic regression analysis revealed that the BMI and U-pH were independent risk factors for PUS (+). The decrease of U-pH was closely related to the progress of chronic kidney disease (CKD). BMI value was related to PUS (+) in the CKD (−) subjects. On the other hand, low U-pH was related to PUS (+) in the CKD (+) subjects. All factors other than HR showed a significant negative correlation with U-pH. However, multiple-regression analysis revealed that TBARS and angiotensinogen were independent risk factors. Conclusion: Obesity and lower U-pH were each independently related to PUS, whereas increased intrarenal oxidative stress and exacerbation of the renin–angiotensin system activation were associated with the lowering of U-pH. U-pH low value is related to potential CKD.