Heat acclimation-induced intracellular HSP70 in humans: a meta-analysis.
Level 1 - systematic review of randomized trials
Systematic review and meta-analysis of human intervention studies
PubMed 31823288 · doi:10.1007/s12192-019-01059-y
What was done
Authors searched several databases for human studies measuring changes in intracellular HSP70 (protein and mRNA) in response to heat acclimation. They analyzed the overall effect of heat acclimation on HSP70 expression, tested differences between protein and mRNA subgroups, and evaluated potential moderators including frequency (days), duration, core temperature, and exercise intensity across 12 included studies (118 participants, mean age 24 years, 98% male).
What was found
Heat acclimation produced a significant overall increase in HSP70 expression (g = 0.97, 95% CI: 0.08 to 1.89). Subgroup analysis showed an increase in HSP70 protein expression (g = 1.51, 95% CI: 0.71 to 2.31), whereas mRNA expression was not increased (g = -0.39, 95% CI: -1.36 [upper limit not reported in abstract]). Frequency of heat acclimation (number of days) was the only identified variable that significantly moderated HSP70 protein expression.
Why it matters
This review demonstrates that heat acclimation induces intracellular HSP70 protein upregulation in humans, providing quantitative evidence for cellular thermotolerance that depends principally on the number of heat exposure days.
Limits
The total sample size was small (118 participants across 12 studies) and almost entirely young men (mean age 24, 98% male), limiting generalizability to females and older populations. The 95% confidence interval for the primary outcome was broad (0.08 to 1.89), the abstract omits the upper bound for the mRNA confidence interval, and tissue type or sample origin is not specified in the abstract.
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