Association between predicted level of water turnover deficit and all-cause and cause-specific mortalities among Japanese adults: The Japan Collaborative Cohort Study.

Background: Although water is essential for maintaining health and life, the association between water turnover as an indicator of daily water requirement and water deficit and mortality is unclear.

Objective: We aimed to investigate this association in Japanese adults.

Methods: A total of 63,488 (36,739 females and 26,749 males) Japanese aged 40-79 years in the Japan Collaborative Cohort Study in 1988-1990 were followed up their mortality through 2009. Water turnover was calculated considering lifestyle and environmental factors using an equation previously developed by the International Doubly Labeled Water Database Group. Participants were classified into sex-specific quintiles based on their water turnover. Water deficit levels were calculated from water turnover and dietary water consumption, which were estimated using a validated questionnaire.

Results: The average daily water turnovers were 2,950 mL and 3,466 mL in females and males, respectively. During 19.4 years of median follow-up (1,039,914 person-years), 12,551 deaths were recorded. After adjusting for lifestyle and medical history, the bottom quintile was associated with higher hazard ratios (HRs) of mortality from all causes (females: HR, 1.26; 95% confidence interval, 1.12, 1.41; males: HR, 1.18; 95% CI, 1.07, 1.29) and cardiovascular disease (CVD). In spline analyses, the water turnover range with the lowest HR for all-cause mortality was 3,000-3,300 mL/day in females and the water turnover at which the HR for all-cause mortality plateaued was approximately 3,500-3,700 mL/day in males. Water deficiency levels were associated with all-cause and CVD mortality in both sexes.

Conclusion: This study showed an L-shaped association between water turnover and deficit and all-cause and CVD mortalities (especially coronary heart disease and ischemic stroke) in adults. These findings may be useful for setting the target values of water requirement, and differences between dietary requirements and actual intake may bridge the knowledge gap in water-mortality associations.