Effect of repeated hot water immersion on cognitive performance, cerebrovascular function, sleep and biomarkers of neurodegeneration in older adults

Ageing is associated with cognitive decline and increased risk of developing neurodegenerative disease. Repeated passive heating, using hot water immersion (HWI), may improve cognitive performance via improved cerebral oxygenation, but this is yet to be examined in older adults. Twelve healthy older adults (aged: 69.2 ± 10.0 years; body mass index: 25.2 ± 4.1 kg m⁻²) completed a 6-week pre–post intervention study consisting of two to three weekly 1 h HWIs in 40°C water. Rectal temperature was maintained in a target range of 38.5–39.0°C during HWI. Cognitive performance (working memory via 1 and 2-back, inhibition via 2-choice reaction time, logical reasoning via logical relations) and cerebral oxygenation (Δoxyhaemoglobin, Δdeoxyhaemoglobin, Δtotal haemoglobin and Δtissue saturation index) were assessed during the first and final HWI sessions (pre-, immediately post- and 3 h post-HWI). Common carotid artery blood flow (CCA-BF), sleep quality (7-day baseline and final week), plasma [amyloid-β] 42 (Aβ42), and [phosphorylated tau] (p-tau), were measured pre- and post-intervention. Repeated HWI improved 1-back (P = 0.023) and logical reasoning (P = 0.002) performance, but not 2-back or 2-choice reaction time (P > 0.05). Cerebral oxygenation was acutely reduced immediately post-HWI (all parameters P < 0.05), but returned to baseline 3 h post-HWI, with no chronic adaptation. CCA-BF, sleep quality, [Aβ42] and [p-tau] all remained unchanged at 6 weeks (P > 0.05). Repeated HWI improves cognitive domains of logical reasoning and working memory without altering cerebral oxygenation, CCA-BF, sleep or neurodegenerative biomarkers. Further investigation into the underlying mechanisms for cognitive performance improvements via HWI is warranted.