Plasma homocysteine responses to repeated bouts of exercise-heat stress during a standard 10–14-day heat acclimation protocol

Introduction: Homocysteine (Hcy) is a clinically (e.g., cardiovascular, neurodegenerative diseases) relevant biomarker in plant and animal models and an intriguing target to consider in thermopriming and heat tolerance. Methylene tetrahydrofolate reductase (MTHFR) is an Hcy regulatory enzyme for which function-altering alleles are prevalent among global populations and may affect physiological function during exercise/heat stress. Purpose: We pursued a pilot study to establish normal Hcy range in thermally naïve and acclimated individuals to evaluate the effect of physiological acclimation (HA) on plasma Hcy among fit men and women. We hypothesized that heat acclimation would reduce plasma homocysteine levels in healthy individuals. Methods: 18 participants ((mean ± standard deviation (SD)): males (n=9): 21.9±2.9 years, VO2max 48.2±7.2 ml•kg -1 •min -1 ; females (9), 21.9±3.0 years, 47.6±7.0 ml•kg -1 •min -1 ) completed a 5-day HA protocol and two heat tolerance tests (HTT1, HTT2) pre- and post-HA. Baseline was defined as naïve state (pre-HTT1-pre-HA, resting). Whole blood was collected before (pre), during (mid), and after (post) HTT1s, HTT2s, and HA days 1 and 5 (HA1, HA5) pre- and post-physical activity for a total of 14 time points. Plasma Hcy concentration ([Hcy]) was assayed by commercially-available fluorometric Hcy kits (catalogue ab228559, Abcam). Results: Rectal temperature (mean difference post-pre HA, -0.187°C) and heart rate (-18 BPM) decreases demonstrated physiological acclimation. [Hcy] significantly decreased post-HA (vs. baseline p<0.05). [Hcy] was lower (vs. baseline) at mid-HTT2-pre-HA(-5.58µM), pre-HA1(-5.58µM), post-HA1(-5.57µM), pre-HA5(-5.61µM), post-HA5(-5.58µM), and mid-HTT2-post-HA(-5.50µM). Conclusion: We defined a pilot data set of [Hcy] responses across multiple days of exercise-heat stress among healthy individuals and observed that repeated exposures reduce [Hcy]. We observed variability among our healthy participants which should be considered in future experimental designs. Future research will coordinate genetic testing to assess incidence of MTHFR allelic variants among the study populations and associations with exercise-heat stress and acclimation effects on [Hcy]. Funding Source: DoD20029 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.