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In-Exercise Caffeine Improves Exercise Performance in the Heat without Exacerbating Hyperventilation and Brain

Akira Katagiri1, Rikuto Otsuka1, Takeshi Nishiyasu

  • 1Institute of Health and Sport Sciences, University of Tsukuba, Tsukuba City, Ibaraki, JAPAN.

Medicine and Science in Sports and Exercise
|June 23, 2025
PubMed
Summary

Consuming caffeine during prolonged exercise in the heat enhances endurance performance. This strategy improves late-stage cycling duration without negatively impacting body temperature or cerebral blood flow.

Keywords:
BRAIN BLOOD FLOWCENTRAL FATIGUECONTROL OF BREATHINGERGOGENIC AIDTIME TO EXHAUSTION

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Area of Science:

  • Exercise Physiology
  • Nutritional Science
  • Environmental Physiology

Background:

  • Pre-exercise caffeine enhances endurance, but heat may negate this effect due to increased physiological strain.
  • High caffeine levels in heat can cause hyperthermia, hyperventilation, and cerebral hypoperfusion.
  • In-exercise caffeine intake delays blood caffeine elevation, potentially mitigating negative effects in heat.

Purpose of the Study:

  • To test if in-exercise caffeine improves performance in prolonged exercise in the heat.
  • To assess caffeine's impact on respiratory and cerebral blood flow responses during exercise in heat.
  • To evaluate caffeine's effect on body temperature during heat-stress exercise.

Main Methods:

  • 12 healthy adults completed cycling tests in 35°C heat.
  • Participants consumed caffeine (5 mg/kg) or placebo during moderate-intensity exercise in a crossover design.
  • Performance was measured by time to exhaustion during high-intensity cycling.

Main Results:

  • In-exercise caffeine intake led to longer high-intensity cycling duration (218 vs. 165 sec).
  • Perceived exertion was lower with caffeine during moderate exercise.
  • Minute ventilation, cerebral blood flow, and esophageal temperature remained similar between caffeine and placebo trials.

Conclusions:

  • In-exercise caffeine intake can improve late-stage exercise performance in the heat.
  • This ergogenic effect occurs without worsening hyperthermia, hyperventilation, or cerebral hypoperfusion.
  • Timing caffeine intake during exercise is a viable strategy for heat performance.