Brazil study: Does your morning coffee boost performance or not

Many people who drink an extra cup of coffee before a run assume that more caffeine leads to better performance. This idea is common in fitness culture. However, a new systematic review and meta-analysis suggests the link between caffeine and endurance is more complex. The study indicates that most of the benefit may come from smaller doses, not the large amounts many athletes use.

About the study

Caffeine has been studied as a performance aid for decades. The evidence that it helps is well known. What is less clear is whether more caffeine gives more benefit or simply more side effects. Researchers analyzed 48 randomized, placebo-controlled trials with 689 participants. They compared how low, moderate, and high caffeine doses affected aerobic time-trial performance.

Low doses were up to 3 mg per kilogram of body weight. Moderate doses were 4 to 6 mg per kilogram. High doses were anything above 6 mg per kilogram. The team only included studies using pure caffeine in capsule or liquid form. They did not use coffee, energy drinks, or gum because the caffeine content in those can vary by more than 50 percent.

Low doses nearly matched moderate doses

Low caffeine doses, roughly 1.3 to 3 mg per kilogram of body weight, produced a statistically significant improvement in time-trial completion time. Moderate doses of 4 to 6 mg per kilogram also improved performance. But when researchers looked at actual race-time improvements, the real-world difference between the two dose ranges was small. Low doses corresponded to a mean improvement of about 2.14 percent. Moderate doses came in at about 2.18 percent.

Doubling or tripling caffeine intake may not get a person meaningfully further across the finish line. The moderate-dose group showed more variability across studies, meaning the results were less consistent. The low-dose group showed a more uniform effect, suggesting a more reliable performance boost. Researchers noted that two studies had a large influence on the moderate-dose results. No studies using high caffeine doses above 6 mg per kilogram met the inclusion criteria for time-trial performance, so that area remains unclear.

Why caffeine affects people differently

Genetics plays a role in how the body responds to caffeine. A gene called CYP1A2 controls an enzyme that breaks down about 95 percent of the caffeine a person consumes. Depending on the version of this gene, caffeine may work very differently. People with the CC variant of CYP1A2, commonly classified as slower caffeine metabolizers, may see a weaker or less consistent performance response. Those with AA or AC variants tend to show energetic benefits from caffeine more often.

Early evidence also points to other genetic factors, including variations in adenosine receptors. These are the receptors in the brain that caffeine blocks to reduce the sense of effort. This research is still developing, but it helps explain why two people can take the same dose and have completely different experiences.

Finding the right pre-workout caffeine amount

If a person uses caffeine before workouts or races, a straightforward starting point is that less may be enough. A low dose of roughly 1.3 to 3 mg per kilogram of body weight, taken about 60 minutes before exercise, produced significant and consistent performance improvements across the studies. For a 70 kg person, this is about 90 to 210 mg.

A standard cup of coffee contains anywhere from 80 to 200 mg of caffeine depending on how it is brewed. This means a morning cup may already put a person in that effective range. Most studies used caffeine capsules, which deliver a precise dose, but the numbers are a useful reference.

The case for starting low is also supported by side effects that come with higher doses. High caffeine intake is commonly linked to anxiety, heart palpitations, headaches, poor sleep, and gastrointestinal issues. If moderate doses offer only a small real-world advantage over low doses, the balance of risk and benefit favors the smaller amount. This is especially true for people sensitive to caffeine or those training in the evening.

Low caffeine doses of 1.3 to 3 mg per kilogram improved endurance time-trial performance by roughly 2 percent. This nearly matched the benefit of moderate doses of 4 to 6 mg per kilogram. Because higher doses come with greater side-effect risk and no proportional performance gain, starting low is a sensible strategy for most people.