Verdict: The best-supported legal endurance aid, but it improves performance, not your underlying VO2max
Safety: Jitters, poor sleep, stomach upset and raised heart rate, more likely at high doses
Key takeaways
Caffeine at 3–6 mg per kg reliably improves endurance, with time trials about 2–3 percent faster on average
It does not build aerobic fitness, but taken before a lab test it can nudge peak values up, so keep caffeine the same before repeat tests
Very high doses add side effects without extra benefit, and late-day caffeine can hurt sleep
Caffeine is the most popular performance aid in the world, and unlike most supplements, it clearly works for endurance. What it does not do is raise your VO2max in the sense of building fitness. Here is what the research shows, including what to do before a VO2max test.
What it is
Caffeine is a stimulant found in coffee, tea, energy drinks, gels and pills. It works mainly by blocking adenosine receptors in the brain, which reduces the sense of effort, fatigue and pain. That is why the same pace feels easier and you can hold a hard effort for longer.
Does it raise VO2max?
Not in the way training does. VO2max is set mainly by how much oxygen your heart pumps and your muscles use, and caffeine does not change that capacity.
There is one nuance for test days. In a randomised, double-blind crossover study, Ruíz-Moreno and colleagues (2020) gave 13 active people 3 mg/kg of caffeine or a placebo 60 minutes before a ramp cycling test. With caffeine, maximal power rose from 258 to 271 W, and peak oxygen uptake was slightly higher (3.33 vs 3.18 L/min with placebo). The likely explanation is that people pushed a little further before stopping, not that their fitness changed overnight.
So caffeine can nudge the number on the day. If you compare tests over time, keep your caffeine routine identical. For test preparation, see VO2max tests explained and what to expect from a VO2max test.
Does it improve endurance performance?
Yes. The ISSN position stand (Guest et al., 2021) concludes that aerobic endurance shows the most consistent moderate-to-large benefits from caffeine, though the size of the effect varies between people. It improves performance in both trained and untrained people, and also in the heat and at altitude.
A meta-analysis by Southward and colleagues (2018, Sports Medicine) pooled placebo-controlled studies of endurance time trials. In the corrected analysis of 44 studies, moderate doses of 3–6 mg/kg improved mean power output by about 2.9% and time-trial completion time by about 2.3%. Not everyone responded: a few studies found slower times or lower power with caffeine. In racing, though, 2% is often the gap between places.
How it is used in studies
- Dose: 3–6 mg per kg of body weight. The minimal effective dose may be as low as 2 mg/kg. Very high doses around 9 mg/kg cause more side effects and are not needed.
- Timing: most often 60 minutes before exercise. Caffeine gum may work faster than capsules.
- Forms: coffee, capsules, gum, gels, chews and energy drinks have all been shown to work, mainly in aerobic exercise.
- Habitual use: regular coffee drinkers still tend to benefit, though individual responses vary with genetics and usual intake.
Side effects, safety and anti-doping
Common side effects include jitters, anxiety, a racing heart, stomach upset and poor sleep. Afternoon or evening doses can cut into the sleep you need to recover (see VO2max and sleep). Take care with concentrated caffeine powders, where dosing errors are easy and dangerous.
Caffeine is not on the World Anti-Doping Agency prohibited list. However, supplements and pre-workouts can be contaminated with banned stimulants, as the IOC consensus statement (Maughan et al., 2018) warns. If you compete, choose third-party tested products, such as Informed Sport or NSF Certified for Sport.
This is not medical advice. If you have a heart rhythm problem, high blood pressure, anxiety, are pregnant, or take medication, talk to your doctor before using caffeine for exercise.
Who might benefit
- Runners, cyclists, triathletes and rowers racing anything from a few minutes to several hours.
- People doing hard interval sessions like 4×4 intervals who want them to feel more manageable.
- Anyone already drinking coffee, who can simply time a cup before a key session.
It is less suitable for people sensitive to stimulants or who train late in the day. Caffeine pairs reasonably with beetroot nitrate, although extra benefits from combining them are not certain.
Bottom line
Caffeine is the most reliable legal performance aid for endurance, typically making time trials a few percent faster. It does not build aerobic fitness, but it may slightly raise a VO2max test result on the day, so stay consistent before repeat tests. For real VO2max gains, train consistently, track progress with the VO2max calculator, and compare other options on the supplements page.
Frequently asked questions
Does caffeine increase VO2max?
It does not raise your aerobic fitness. Caffeine mainly lowers perceived effort, so you can push harder. In one lab study it slightly raised peak oxygen uptake during a maximal test, likely because people went a bit further before stopping.
Should I take caffeine before a VO2max test?
Either is fine, but be consistent. Because caffeine may slightly raise peak values and time to exhaustion, use the same caffeine routine before every test so results are comparable.
Is caffeine good for cardio?
Yes, for performance. The ISSN says aerobic endurance shows the most consistent benefits from caffeine, typically at 3–6 mg per kg of body weight.
How much caffeine should I take before a run?
Studies mostly use 3–6 mg per kg about an hour before, and as little as 2 mg per kg may help. For a 70 kg person, 3 mg per kg is about 210 mg.
Sources
- Guest NS, et al. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr, 2021.
- Southward K, et al. The effect of acute caffeine ingestion on endurance performance: a systematic review and meta-analysis. Sports Med, 2018 (and correction, 2018).
- Ruíz-Moreno C, et al. Acute caffeine intake increases muscle oxygen saturation during a maximal incremental exercise test. Br J Clin Pharmacol, 2020.
- Maughan RJ, et al. IOC consensus statement: dietary supplements and the high-performance athlete. Br J Sports Med, 2018.