Caffeine: What It Actually Does to Your Brain and Endurance

A look at how caffeine affects exercise capacity and mental sharpness, based on the strongest evidence available.

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By the Ration Editorial Team · Reviewed against primary research · Updated July 13, 2026

Caffeine is a natural stimulant found in coffee beans, tea leaves, cacao, and kola nuts.

It belongs to a class of compounds called methylxanthines, which also includes theobromine, found in chocolate.

People consume it mainly through coffee, tea, energy drinks, and supplements, often to feel more alert or to push harder during exercise.

Caffeine's main trick is blocking adenosine receptors in the brain.

Adenosine is a molecule that builds up throughout the day and makes you feel drowsy. By occupying its receptors, caffeine delays that sleepy signal, which is why you feel more awake and focused after a cup of coffee^[6][7].

In muscle and fat tissue, caffeine also appears to increase the availability of fatty acids for fuel and may reduce the perceived effort of exercise, both of which help explain its endurance effects^[4].

This is where caffeine has some of its most consistent evidence.

Multiple meta-analyses have looked at caffeine's effect on VO2max, the measure of how much oxygen your body can use during intense exercise, along with related endurance markers.

• ↑VO2maxAHuman trials

  Research may raise VO2max · strong agreement · 5 studies

  View 5 studies on PubMed

  • Effects of Caffeine Intake on Endurance Running Performance and Time to Exhaustion: A Systematic Review and Meta-Analysis (2022)
  • Effects of caffeine chewing gum supplementation on exercise performance: A systematic review and meta-analysis (2023)
  • The Effect of Acute Caffeine Ingestion on Endurance Performance: A Systematic Review and Meta-Analysis (2018)
  • Wake up and smell the coffee: caffeine supplementation and exercise performance-an umbrella review of 21 published meta-analyses (2020)
  • Effect of Pre-Exercise Caffeine Intake on Endurance Performance and Core Temperature Regulation During Exercise in the Heat: A Systematic Review with Meta-Analysis (2022)
• ↑Cognitive PerformanceAHuman trials

  Research may raise Cognitive Performance · strong agreement · 4 studies

  View 4 studies on PubMed

  • The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness (2010)
  • The combined effects of L-theanine and caffeine on cognitive performance and mood (2008)
  • A review of caffeine's effects on cognitive, physical and occupational performance (2016)
  • Is caffeine a cognitive enhancer? (2010)

A meta-analysis of caffeine chewing gum found measurable improvements in exercise performance, suggesting the effect holds even with fast-absorbing forms^[1].

Caffeine also seems to help endurance performance in the heat, with research pointing to benefits for time to exhaustion even when core temperature regulation is a challenge^[2].

For runners specifically, a systematic review found caffeine improved both endurance running performance and time to exhaustion^[3].

An umbrella review pooling 21 separate meta-analyses concluded that caffeine's ergogenic effects are among the most reproducible in sports nutrition^[4].

An earlier systematic review of acute caffeine ingestion reached a similar conclusion for endurance tasks^[5].

Together, these studies represent one of the more consistent bodies of evidence in performance nutrition, which is why the effect on VO2max earns a high evidence grade in our rankings.

Caffeine's reputation as a mental sharpener holds up reasonably well in the research.

A broad review of caffeine's effects on cognitive, physical, and occupational performance found benefits for vigilance, reaction time, and mood, particularly during sleep deprivation or long shifts^[6].

Another review asked directly whether caffeine counts as a cognitive enhancer, concluding that it reliably improves attention and alertness, though effects on memory and higher reasoning are less clear^[7].

Some of the more interesting findings come from combining caffeine with L-theanine, an amino acid found in tea.

• One randomized controlled trial found the combination improved cognitive performance and increased subjective alertness compared to either substance alone^[8]
• A separate RCT found the pairing improved both performance on demanding cognitive tasks and self-reported mood^[9]
• The theory is that L-theanine may offset some of caffeine's jittery, anxious edge while preserving its focus benefits

Doses used in exercise research typically range from about 3 to 6 mg per kg of body weight, taken roughly 30 to 60 minutes before activity^[3][4][5].

For a 70 kg (154 lb) person, that works out to roughly 210 to 420 mg, which is about 2 to 4 cups of brewed coffee.

Cognitive studies have generally used smaller doses, often in the 40 to 200 mg range, sometimes paired with 100 to 200 mg of L-theanine^[8][9].

Common sources and forms include:

• Brewed coffee, roughly 95 mg per 240 mL cup
• Tea, roughly 30 to 50 mg per cup, often alongside naturally occurring L-theanine
• Energy drinks and pre-workout powders, which vary widely in dose
• Caffeine chewing gum, shown to be absorbed quickly and effective for performance^[1]
• Caffeine anhydrous pills or capsules, commonly used in research settings

For most healthy adults, moderate caffeine intake is considered safe.

Common side effects at higher doses include jitteriness, increased heart rate, anxiety, and trouble sleeping if taken later in the day.

People who are pregnant, have anxiety disorders, heart arrhythmias, or are sensitive to stimulants may want to be more cautious with dose and timing.

Tolerance also builds with regular use, so habitual coffee drinkers may need higher doses to feel the same alertness or performance boost seen in research on less frequent users^[4].

• Endurance athletes looking for a modest, well-supported boost to performance and time to exhaustion^[2][3][4][5]
• People doing shift work or sleep-deprived tasks who need short-term alertness^[6]
• Anyone doing sustained, demanding cognitive work, especially when paired thoughtfully with L-theanine^[8][9]
• People who tolerate caffeine well and are not sensitive to its cardiovascular or anxiety-related effects

Caffeine has some of the strongest, most reproducible evidence of any performance compound, both for endurance exercise and for alertness and focus.

The effect sizes are real but modest, not miraculous, and they depend heavily on dose, timing, and individual tolerance.

For most people, a cup or two of coffee before a workout or a demanding mental task reflects roughly what the research supports.