Does the mineral content of tap water correlate with urinary calculus composition?

The association between the mineral content of drinking water and urolithiasis remains elusive. The aim of this study is to investigate whether the mineral composition of tap water correlates with urinary calculus composition. Patients with calculi that underwent biochemical analysis at two urological centres in the North-West of England between November 2015 and December 2020 were included. Calculus composition was reviewed with respect to patient demographics, serum biochemical variables, and water mineral composition data obtained from the local water supply company using patient postcodes. 1711 urinary tract calculi from 1518 patients, living in 87 water supply zones were included. Water sodium concentration was an independent predictor of mixed calcium oxalate/uric acid calculi (OR 1.157, p < 0.001) and a negative independent predictor of calcium oxalate monohydrate (OR 0.896, p = 0.001) and dihydrate (OR 0.742, p = 0.034) calculi. Moreover, the magnesium-to-calcium ratio of tap water was a negative independent predictor of calcium oxalate monohydrate calculi (OR < 0.001, p =  < 0.001), while tap water magnesium concentration inversely correlated with the percentage of calcium oxalate within calculi (rs = - 0.054, p = 0.026). Total water hardness did not independently predict calculus type. Many factors are implicated in the formation of urinary calculi. This study is the first to assess calculus composition in relation to tap water mineral content using postcode data on a case-by-case basis. Though total water hardness did not independently predict calculus composition, the interesting findings relating to water sodium and magnesium concentrations are in need of closer scrutiny in larger scale studies.