| Summary: | Abstract Introduction Consuming intracellular osmolytes, like betaine (BET), may attenuate symptoms of heat stress. The purpose of this study was to examine the effects of BET supplementation on fluid balance and heat tolerance after a 7‐day loading period and during passive heat exposure. Methods A double‐blind, placebo controlled, crossover study compared BET or placebo consumption (50 mg·kg−1, twice daily) for 7 days in young, recreationally active men (N = 11). Results During the loading period, no significant interactions were found for any marker of fluid balance between or within conditions. During heat exposure, significant time effects but no condition x time interactions, were found for plasma characteristics (i.e., volume, osmolality, sodium, albumin, and total protein). Plasma volume was significantly increased by min 30 in both conditions (PLA: +6.9. ± 5.0%, BET: +10.2 ± 7.4%) and remained elevated for the remainder of the experimental trial, but was not significantly different between conditions. After 60 min of passive heat exposure, both conditions experienced a similar increase in core temperature (PLA: +0.32 ± 0.22°C, BET: +0.31 ± 0.21°C; p = 0.912). Conclusions Supplemental BET did not improve markers of fluid balance or heat tolerance during 7 days of loading or during passive heat exposure.
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