Cross-adaptation from heat stress to hypoxia: A systematic review and exploratory meta-analysis.
Level 1 - systematic review of randomized trials
Systematic review and exploratory meta-analysis of human intervention studies
PubMed 38471285 · doi:10.1016/j.jtherbio.2024.103793
What was done
A systematic review and exploratory meta-analysis evaluated the effect of heat acclimation on physiological, perceptual, and physical performance outcomes during rest, submaximal, and maximal exercise under hypoxic conditions. Scopus and MEDLINE were searched for peer-reviewed English-language human studies meeting PICO criteria. Risk of bias and study quality were assessed using the COSMIN checklist. Nine studies comprising 79 recreationally trained male participants were included. The primary heat acclimation protocol was fixed-intensity exercise averaging 9 ± 3 sessions of 89 ± 24 minutes at 39 ± 2 °C and 32 ± 13% relative humidity.
What was found
Heat acclimation produced moderate beneficial effects on resting oxygen saturation (g = 0.60) and on submaximal exercise outcomes in hypoxia: heart rate (g = -0.65), core temperature (g = -0.68), and skin temperature (g = -0.72). Small effects were observed during maximal exercise for ventilation (g = 0.24), peak power (g = 0.32), and time trial completion time (g = -0.43). No effect was found on perceptual measures. Confidence intervals and exact p-values were not reported in the abstract.
Why it matters
Heat acclimation may serve as an accessible cross-adaptation tool to reduce submaximal physiological strain at altitude when direct hypoxic pre-exposure is logistically unfeasible.
Limits
The total evidence base is small (9 studies, 79 participants) and restricted entirely to recreationally trained males, limiting applicability to females and highly trained athletes. Substantial methodological variation exists across studies, and underlying mechanisms were not determined.
Cited by
- supports Hormetic stressors induce cross-adaptation, meaning that exposure to one physical stressor confers biological resilience against other distinct stressors through shared and overlapping cellular pathways.