Association of whole body water mass and its change with mortality: evidence from the UK Biobank cohort study

Whole body water mass (WBWM) may reflect both body composition and fluid status. However, the association between WBWM and the risk of mortality remains unclear. This study aimed to assess the association of baseline WBWM and annual change in WBWM (ΔWBWM) on mortality using data from the UK Biobank cohort. Of the 502,356 participants recruited from 2006 to 2010, a total of 489,793 participants who had completed data on baseline WBWM were eligible for the study and followed up till 2022. Cox proportional hazards regression model was used to assess the association of baseline WBWM with all-cause and cause-specific mortality as well as the joint association of baseline WBWM and ΔWBWM on mortality. Baseline WBWM was classified into three categories (low, medium, and high) using sex-specific tertiles. We further categorized ΔWBWM into three groups (decrease, stable, and increase) using zero and the sex-specific median values of negative ΔWBWM. The median age of the participants was 58 years. Over a median follow-up of 13.7 years, 40,139 (8.2%) deaths were documented. A V-shaped association was found between baseline WBWM and mortality from all causes, cardiovascular diseases (CVD), and other causes, while a less pronounced but positive association was observed between WBWM and cancer mortality when WBWM exceeded the sex-specific threshold. Compared to medium baseline WBWM, hazard ratios (HR) for low baseline WBWM in relation to all-cause, CVD, and other causes mortality were 1.069 (95% confidence interval [CI] 1.042–1.096), 1.084 (95% CI: 1.022 − 1.149), and 1.221 (95% CI: 1.167–1.279), respectively. Individuals with high baseline WBWM had 8.7% (95% CI: 5.8%–11.7%), 25.4% (95% CI: 18.3%–32.9%), and 7.5% (95% CI: 3.5%–11.6%) higher hazards of all-cause, CVD, and cancer mortality, respectively, compared to those with medium baseline WBWM. Joint associations of baseline WBWM and annual average change in WBWM were observed, with the lowest mortality risk observed in the medium and stable WBWM group. The lowest mortality risk was observed among participants with medium and stable WBWM. WBWM may serve as a potential marker for health assessment.