Background/Aim: There are several factors, such as race, age, sex, and geographical variations, associated with renal stone formation. Although it is known that the prevalence of urolithiasis in Japanese children is low, the reason remains obscure. We hypothesize that the low prevalence of urolithiasis is associated with the urinary calcium excretion. The aim of our study was to investigate the prevalence of hypercalciuria in Japanese children. Methods: This investigation is a population-based school survey. A group of 529 healthy Japanese children was screened for hypercalciuria by measurement of the urinary Ca/Cr ratio using the morning fasting urine. In addition, the urinary Na/Cr ratio was also calculated for each subject. Results: Hypercalciuria regarded as an urinary Ca/Cr value of more than 0.17 was noted only in 3 out of 529 children (0.6 %), while most cases (494/529, 93.4%) demonstrated hypocalciuria (urinary Ca/Cr <0.05). The mean urinary Ca/Cr value was 0.024 in all subjects combined. Linear regression analysis revealed a positive direct correlation between urinary Ca/Cr and Na/Cr values (rs = 0.14, p < 0.01). The urinary Ca/Cr ratio was not related to age in either sex. Conclusions: The present study demonstrates that the prevalence of hypercalciuria in Japanese children is low as compared with other countries, even though absorptive hypercalciuria and dietary hypercalciuria might be missed in this setting. This low prevalence of hypercalciuria may be associated with the lower prevalence of urolithiasis in Japanese children. As it is suggested that a low dietary intake of calcium and sodium may play some role in the low urinary calcium excretion, a randomized, controlled study comparing the efficacy of different modes of therapy, such as a low-calcium diet and/or a low-salt diet, might provide valuable information for the prevention of urolithiasis.

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