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The body's water balance is changed by food and beverage intake, metabolism, and excretion. In this study, we performed a cross-sectional study that investigated the changes of water intake and water output in healthy Japanese young and elderly people and handicapped adults. Water balance was assessed by water intake from foods and beverages, metabolic water production, non-renal water losses (NRWL), and urine volume. Most of the parameters did not change with aging in healthy adults. Estimated total water intake (ml/kg/day) increased with aging. In the healthy men, healthy women, and handicapped adults, daily water intake (median [interquartile range]) accounted for 49.4 (41.4-59.9) ml/kg, 42.9 (38.7-51.8) ml/kg, and 50.9 (43.8-74.0) ml/kg, respectively. Water loss from the kidney accounted for 19.2 (16.2-29.2) ml/kg, 22.0 (16.2-26.6) ml/kg, and 27.5 (22.7-47.2) ml/kg, respectively. NRWL accounted for 26.6 (18.5-35.2) ml/kg, 22.4 (16.2-28.8) ml/kg, and 23.5 (19.8-28.5) ml/kg, respectively. Our findings suggest that a daily total water intake of more than 50-55 ml/kg is required to prevent dehydration in healthy and handicapped adults. J. Med. Invest. 70 : 195-199, February, 2023.