The Effects of Acute and Repeated Hot Water Immersion on Cardiovascular, Cognitive and Muscle Function and Heat Shock Proteins in Healthy Older Adults

Abstract

Ageing per se reduces cardiovascular, muscle and cognitive function, and impairs the cytoprotective heat shock protein (HSP) response to heat stress, increasing risk of morbidity and mortality. Whilst frequent exercise improves and lowers both disease risk and all-cause mortality, participation and adherence decline with age, often due to physical limitations or medical contraindications. Passive heat therapy using hot water immersion (HWI) has emerged as a potential method to improve health outcomes, but research exploring the time course response of its chronic effect on health outcomes is limited and has primarily focused on younger and clinical populations. It was hypothesised that acute HWI would transiently increase HSPs and biomarkers of inflammation in the blood, improve cardiovascular function and reduce cognitive performance, and that repeated HWI would improve cardiovascular and muscle function; improve cognitive performance in response to acute heat stress; and increase basal levels of HSPs while reducing inflammation in the blood in healthy older adults.
Participants were immersed to the clavicle in 40°C water for 60 minutes each HWI to achieve and maintain a target rectal temperature of 38.5-39.0°C. For acute HWI, HSPs, inflammation, cardiovascular function and cognitive performance were assessed across 48 hours. For repeated HWI, participants were randomised to a six-week HWI intervention (12-18 60-minute sessions over six weeks) or control condition with a six-week washout prior to crossover.
Acute HWI in healthy older adults had no effect on plasma [HSPs] or [inflammatory biomarkers] but transiently impaired vascular endothelial function 3 hours post-HWI. Working memory performance following HWI was transiently increased. All acute measures returned to baseline 48 hours post-HWI. Repeated HWI in healthy older adults had no effect on vascular endothelial function or blood pressure, but reduced resting HR. Repeated HWI had no effect
on muscle function but improved working memory and logical reasoning, while basal levels of HSPs and inflammatory biomarkers were not altered.
In conclusion, this thesis has provided evidence that acute and repeated HWI in 40°C water may have the potential to improve cognitive performance in healthy older adults, but further work is warranted to elucidate the mechanisms for these improvements. Furthermore, acute and repeated HWI did not improve cardiovascular or muscle function in healthy older adults. Future studies are needed to investigate its impact in older clinical groups such as those who have chronic inflammation or mobility issues.

Date of Award	16 Jun 2026
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Ant Shepherd (Supervisor), Jo Corbett (Supervisor) & Zoe Saynor (Supervisor)