Comment on “Association between drinking water type and kidney stone risk in U.S. adults a cross-sectional analysis of NHANES 2009–2016 data”

Dear Editor, Qin et al examine a practical and under-studied question: whether drinking water type (tap vs bottled) is associated with kidney stone prevalence in U.S. adults. Using four NHANES cycles (2009–2016; n = 10 246), the authors report a consistent inverse association between tap water intake and self-reported nephrolithiasis, while bottled water intake shows no statistically significant relationship[1]. This commentary was prepared in accordance with the TITAN 2025 guideline to support transparent and responsible reporting[2]. The central observation is clinically intuitive. In fully adjusted models, higher tap water intake was associated with lower odds of kidney stones (Q5 vs Q1 adjusted OR: 0.678; P-value for trend = 0.0177), and restricted cubic splines suggested a largely linear decrease in risk with increasing tap water consumption[1]. The authors strengthen confidence by triangulating methods: k-means clustering identified a “high tap/low bottled” pattern with the lowest stone prevalence, and propensity score–matched analyses reproduced the inverse association (Q5 vs Q1 adjusted OR: 0.563)[1]. These findings sit well within established prevention principles: increasing fluid intake to achieve higher urine volume is a cornerstone of stone prevention, supported by randomized evidence showing reduced recurrence with higher water intake[3]. Guideline recommendations similarly emphasize targeting a urine volume of at least ~2.5 l/day for most stone formers[4]. Against this background, Qin et al raise an important hypothesis that water source (not just volume) may correlate with stone risk at a population level. However, several limitations temper causal inference and messaging. The outcome is self-reported “ever kidney stones,” which cannot distinguish incident from recurrent disease or stone composition, and the cross-sectional design makes reverse causation plausible (stone formers may alter hydration and preferentially choose bottled water)[1]. Moreover, “tap” and “bottled” are heterogeneous exposures: mineral content (calcium, magnesium, bicarbonate, and sodium) varies widely by municipality, filtration practice, and brand, yet direct water chemistry was unavailable – so mechanistic attribution should remain cautious. Overall, Qin et al provide a useful population-level signal that higher tap water intake is associated with lower stone prevalence, whereas bottled water appears neutral on average[1]. Prospective studies integrating water chemistry, urinary risk profiles, and stone phenotype are now needed to determine whether the association reflects hydration volume alone, mineral composition, or residual behavioral confounding – and whether prevention advice should be tailored to local water composition rather than “tap vs bottled” per se.