# Hydration, water requirements, and energy balance from spring to summer in free-living older adults: a doubly labelled water study

## TL;DR

This study found that older adults increase their water turnover during hot summer months, indicating a need for higher hydration.

## Contribution

The study provides new empirical data on seasonal changes in water turnover and energy expenditure in older adults using doubly labeled water.

## Key findings

- Water turnover increased by 640 mL/day in summer compared to spring.
- Total energy expenditure decreased during summer despite higher temperatures.
- Older adults showed seasonal adaptation through increased water turnover.

## Abstract

The escalating effects of climate change, particularly global warming, are posing an increasing burden on human health. Older adults are particularly susceptible to the impact of extreme heat. Adequate water intake is essential to prevent dehydration in hot environments. Therefore, it is important to understand water turnover (WT) and intake. WT of older adults in hot environments remains unknown. This study aimed at investigating the seasonal effects on WT, total energy expenditure (TEE), and physical activity using doubly labeled water (DLW) and a triaxial accelerometer. A total of 26 older Japanese adult males and females aged ≥ 65 years participated in the study. WT and TEE were measured using DLW in May and August 2012. The mean values of maximum, mean, and minimum temperatures and mean humidity of the measurement days were 24 °C, 19 °C, 14 °C, and 57% in May (spring) and 35 °C, 29 °C, 25 °C, and 66% in August (summer) 2012, respectively. The mean (standard deviation, SD) age of the participants was 73.7 (5.4) years. Total body water increased significantly from 31.1 (4.6) to 31.9 (5.2) kg (+ 0.8 kg, P = 0.009) from May to August. TEE decreased significantly from 2271 (280) to 2123 (470) kcal/day (- 149 kcal/d, P = 0.036), while WT increased significantly from 2.939 (0.625) to 3.579 (0.943) L/day (+ 0.640 L/d, P < 0.001). WT increased by 640 mL/day during summer compared to that during spring, when the average temperature was 19 °C. Our findings indicate that WT increases during hot weather in older adults, reflecting seasonal adaptation.

The online version contains supplementary material available at 10.1038/s41598-026-38832-w.