BACKGROUND: Heat-related illness compromises health and performance in endurance athletes during training and competition. Betaine (BET) is a nutrient that has been previously identified in animal models to act as an osmolyte and attenuates the effects of thermal stress. However, much of the prior research has only assessed the efficacy of preloading BET in passive heat models. Therefore, the purpose of this study is to examine the effects of preloaded BET in an active heat model. METHODS: Eight endurance-trained males (age 26.4 ± 6.8 years; VO2 Peak 55.5 ± 4.8 mL/kg/min) completed 60 min of cycling at 70% VO2 peak in a hot environment (33° C, 35% RH) after a 7-day supplement loading protocol (50 mg/kg, 2x daily) of placebo (PLA) or BET in a double blind, randomized, counterbalanced, crossover study. Core temperature and thermal sensation were measured at rest and every 10 minutes throughout the active heat protocol. Nude body weight was measured prior to- and immediately post-exercise to calculate sweat rate. No fluid ingestion was allowed during this time. Blood samples were collected at rest, 30 minutes, and immediately after exercise. Visual analog scales were administered before and immediately after exercise to quantify sensations of thirst. Bioelectrical impedance assessed fluid compartments before and after the respective supplementation weeks. RESULTS: Area under curve analysis identified BET as having a smaller overall increase in core body temperature compared to PLA (p = 0.012). Further analysis showed ending core temperature was significantly lower in BET (-0.023 ° C; p = 0.029) than PLA. BET also resulted in a significant increase in sweat rate (mean difference = 0.19 ± 0.20 L/hr; p = 0.02). Blood assessments revealed BET had lower hematocrit at the mid-exercise timepoint compared to PLA (BET: 48.3%; PLA: 50.8%; p = 0.02). Increases in total body water (TBW) and intracellular fluid (ICF) in the BET condition approached significance compared to PLA (TBW: +1.69 L, p = 0.055; ICF: +1.39L, p = 0.066). No significant differences were found between conditions in subjective measures of thermal sensation or thirst (p = 0.318; p = 0.862). CONCLUSION: BET supplementation may have the capacity to mitigate the rise in core body temperature and maintain plasma volume during exercise in an uncompensable heat stress environment, despite having no significant effect on subjective sensations of heat stress.

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