Latent association between low urine pH and low body weight in an apparently healthy population

Low urine pH, a plausible predictor for chronic kidney disease and metabolic disorders, is often observed in obese individuals. However, the association between low urine pH and low body weight is equivocal. We examined clinical parameters including urine pH and body mass index (BMI) in a cross-sectional study of 3629 apparently healthy Japanese adults aged 25-80 years who underwent a health-screening check-up. Urine pH was lower and the prevalence of proteinuria was significantly higher in subjects with BMI of ≥ 27.0 kg/m(2) compared with those with BMI of 21.0-22.9 kg/m(2). By contrast, hematuria was more prevalent in subjects with BMI of ≤ 20.9 kg/m(2). Logistic regression analysis showed that BMI of ≥ 27.0 kg/m(2) was significantly associated with low urine pH (≤ 5.5), which remained significant after adjustment for relevant confounders including age, sex, proteinuria, estimated glomerular filtration rate, urine density, hematuria, smoking status, and daily alcohol drinking. However, the association disappeared after further adjustment for serum uric acid. In contrast, the association between low urine pH and BMI of ≤ 19.0 kg/m(2) was significant after adjustment for age and sex and rather strengthened by the further adjustment for serum uric acid. In conclusion, low urine pH may be independently associated with low BMI. However, the underlying mechanisms of low urine pH in low body weight may differ from those in high body weight.