Effect of caffeine ingestion on fluid balance during exercise in the heat and during recovery

Background. The effect of ingestion of a common stimulant, caffeine, on fluid balance during exercise and recovery is not fully known.  Objectives. To determine the effect of caffeine on fluid balance during exercise in the heat and during a 3-hour recovery period thereafter.  Methods. In a randomised, controlled design, caffeine-naive participants ( N =8) pedalled on a bike to achieve 2.5% baseline body mass loss in a hot environment in four separate conditions: with (C+) or without (C–) caffeine ingestion (6 mg/kg of body mass) prior to exercise, followed by (W+) or without (W–) 100% fluid replenishment (water) of the body mass loss during a 3-hour recovery period (yielding C+W+, C+W–, C–W+ and C–W–, respectively).  Results. Mean (standard deviation) urine production was not different ( p >0.05) regardless of rehydration status: 230 (162) mL (C+W–) v. 168 (77) mL (C–W–); and 713 (201) mL (C+W+) v. 634 (185) mL (C–W+). For the 3-hour recovery, caffeine ingestion caused higher hypohydration during rehydration conditions ( p =0.02), but practically the mean difference in the loss of body mass was only 0.2 kg.  Conclusion. In practical terms, there was no evidence that caffeine ingestion in moderation would impair fluid balance during prolonged exercise in the heat or during 3 hours of recovery.