High-Intensity Training in the Heat: Does Thermal Stress Further Enhance Performance Adaptations? A Systematic Review and Meta-Analysis

High-intensity training (HIT) and heat exposure both increase physiological strain, but whether their combination provides an additional performance benefit beyond comparable training in cooler conditions remains unclear. This systematic review with random-effects meta-analysis examined whether HIT in the heat enhances performance and synthesised evidence on physiological adaptation, recovery, retention and testing context. The meta-analysis was restricted to direct, matched heat-versus-cool or thermoneutral training comparisons; a structured narrative synthesis considered the broader eligible literature. PubMed/MEDLINE, Scopus, Web of Science and SPORTDiscus were searched for repeated high-intensity training under heat stress. Forty reports representing 34 study families were identified, and 13 direct heat-versus-cool contrasts entered the meta-analysis. The pooled estimate favoured HIT in the heat (Hedges’ g = 0.40, 95% CI 0.02 to 0.78; I² = 43.3%; p = 0.039), but heterogeneity was moderate and the estimate was sensitive to influential contrasts. Thermoregulatory, perceptual and cardiovascular adaptations were more consistently observed than a clear incremental performance benefit. Overall, direct evidence suggests a potentially positive additional performance effect, but this inference is constrained by the small heterogeneous evidence base and variation in training and testing protocols. HIT in the heat may therefore be useful for athletes and tactical personnel who need to develop fitness and heat tolerance concurrently, even though the magnitude of any additional conditioning effect relative to cooler training remains uncertain. This preprint has not been peer reviewed.