Water turnover increased by 640 mL/day during summer compared to spring in free-living older adults, while total energy expenditure decreased significantly, indicating seasonal adaptation to hot weather.
Key Findings
Results
Water turnover increased significantly from spring to summer in free-living older adults.
Water turnover increased from 2.939 (0.625) L/day in May to 3.579 (0.943) L/day in August.
The increase was +0.640 L/day (P < 0.001).
Mean temperatures during measurement were 19°C in May (spring) and 29°C in August (summer).
Study used doubly labeled water (DLW) method in 26 older Japanese adults aged ≥65 years.
Results
Total energy expenditure decreased significantly from spring to summer in older adults.
TEE decreased from 2271 (280) kcal/day in May to 2123 (470) kcal/day in August.
The decrease was -149 kcal/day (P = 0.036).
TEE was measured using doubly labeled water in both seasons.
Mean (SD) age of participants was 73.7 (5.4) years.
Results
Total body water increased significantly from spring to summer in older adults.
Total body water increased from 31.1 (4.6) kg in May to 31.9 (5.2) kg in August.
The increase was +0.8 kg (P = 0.009).
This was measured alongside doubly labeled water assessments of water turnover and TEE.
The increase in total body water may reflect seasonal physiological adaptation.
Background
Older adults are particularly susceptible to the impact of extreme heat, and adequate water intake is essential to prevent dehydration.
The study was motivated by escalating effects of climate change and global warming posing increasing burdens on human health.
Water turnover of older adults in hot environments was previously unknown prior to this study.
The study measured both water turnover and physical activity using DLW and a triaxial accelerometer.
Methods
Environmental conditions differed substantially between the two measurement periods.
In May (spring), mean values of maximum, mean, and minimum temperatures were 24°C, 19°C, and 14°C, with mean humidity of 57%.
In August (summer), mean values of maximum, mean, and minimum temperatures were 35°C, 29°C, and 25°C, with mean humidity of 66%.
A total of 26 older Japanese adult males and females aged ≥65 years participated.
Kim H, Nakayama Y, Yoshida T, Yokoyama K, Watanabe Y, Itoi A, et al.. (2026). Hydration, water requirements, and energy balance from spring to summer in free-living older adults: a doubly labelled water study.. Scientific reports. https://doi.org/10.1038/s41598-026-38832-w