Dynamic responses in the human methylome to exertional heat exhaustion, heat injury, and heat stroke.

Exertional heat illness encompasses a continuum from heat exhaustion (EHE) to heat stroke (EHS), yet the molecular mechanisms remain poorly understood. DNA methylation offers a stable epigenetic signature linking environmental stress to gene regulation and long-term physiological outcomes. We profiled genome-wide DNA methylation in blood from active-duty service members hospitalized for EHE (n = 36), heat injury (EHI; n = 18), or EHS (n = 50). Blood was collected longitudinally and analyzed using the Illumina 850 K array, with normalization and time-course clustering (TCseq) to identify co-regulated CpG networks. Ingenuity Pathway Analysis was applied differentially methylated probes (p < 0.01, Δβ > 0.04). At diagnosis, both EHI and EHS shared hypomethylation in pathways regulating heat sensing, oxidative defense, and vascular tone. Over time, EHI exhibited adaptive methylation changes supporting neuronal repair, glucocorticoid regulation, and cytoskeletal stability. In contrast, EHS demonstrated sustained downregulation of metabolic and cardiovascular regulators, persistent inflammasome activation, and oxidative imbalance. Early patterns were shared between EHI and EHS, reflecting a common acute stress response, but later responses diverged: EHI showed partial epigenetic recovery, while EHS exhibited sustained metabolic suppression and inflammasome activation. These time-dependent methylation signatures identify potential molecular targets for promoting recovery and preventing long-term complications of heat illnesses.