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It’s commonly acceptable to dope with this performance-enhancing substance every morning: Caffeine, found in naturally occurring and manufactured products, is a naturally occurring central nervous system stimulant, the most widely consumed psychoactive substance in the world. For sports performance, the dosing conversation is pretty simple: milligrams per kilogram of body weight. While that guideline works reasonably well for events lasting a few hours, it starts to get more complicated once a race lasts the entire day. Is thinking in milligrams per kilogram still the right idea for endurance athletes, or should milligrams per hour be part of the conversation as races get longer?
How caffeine works
Most people think caffeine gives them energy. In reality, it just prevents them from feeling tired. Adenosine is a neurotransmitter. Over the course of a day, it accumulates and acts as an inhibitory signal tied to fatigue and drowsiness. Caffeine binds to adenosine receptors and occupies them without activating them; this blunts feelings of fatigue, supports cognitive function, and reduces the perception of effort during exercise. With caffeine, pace or power output feels more manageable than it otherwise would, which is the primary driver of caffeine's ergogenic benefit in endurance sport.
Genetic Differences in Metabolism
Due to genetic factors, people can experience the same dose of caffeine differently. Caffeine is processed almost entirely by the liver enzyme CYP1A2. Roughly 99 percent of an oral dose is absorbed within about 45 minutes, and peak plasma concentration can occur anywhere from 15 minutes to two hours after ingestion. The average half-life is around five hours, but for individuals, the half-life can range from about 1.5 to 9.5 hours.
Genetics explain a large share of that variability. That is the biological reason two athletes taking an identical mg/kg dose can walk away with completely different experiences, from a mild lift in one person to anxiety and tachycardia in another. It also explains why advice has shifted away from trusting a population-average number and toward testing an individual dose in training before relying on it in competition.
Effect of Delivery Method
The caffeine delivery method affects how quickly levels peak in the blood. Capsules generally peak around 60 minutes after ingestion. Caffeinated gum peaks much faster, often within 15 to 20 minutes, because it is absorbed through the buccal mucosa rather than the gut. Gels and coffee tend to sit somewhere in between and vary more from person to person. This is an important variable to consider when planning for an event. For something like a marathon, a capsule or gel taken about an hour beforehand lines up well with the capsule-style timing curve. For a shorter-duration, more intense event like a Hyrox, gum's faster onset may work better than a slower-absorbing caffeine form.
Sex differences?
While caffeine is beneficial for both males and females, female sex hormones may alter caffeine metabolism. Some evidence points to slower caffeine clearance during the late luteal phase and with hormonal contraceptive use, tied to estrogen's effect on metabolism. This suggests that it’s possible women can experience caffeine differently through different cycle and life phases. However, the studies behind this are small. The honest conclusion is that current evidence does not support building a phase-specific caffeine strategy around the menstrual cycle. Individual testing in training remains the best approach for determining a proper dosing strategy. What the research does support is the need for more well-controlled, hormonally verified research that has not been done yet.
Caffeine in Ultra Distance Racing
A recent meta-analysis found that moderate doses of caffeine, in the 4 to 6 mg/kg range, produced a consistent improvement in time-trial performance. Lower doses of 1 to 3 mg/kg also produced a significant benefit, just to a lesser extent than doses in the 4-6mg/kg range. The studies covered a wide range of effort durations, from time trials as short as three minutes up to about two hours. This is important because it means almost none of the dose-response evidence comes from events resembling a half-iron, full-iron, or ultra-distance effort.
In fact, most caffeine dosing research comes from studies lasting one to three hours, so caffeine guidance applied to ultra-distance events is often more extrapolated than it is directly tested in a lab. The most relevant studies come from military research, where the effect of caffeine was studied on sustained wakefulness while combating multi-day fatigue.
Case studies, however, can give us some insight, and they’ve shown athletes using far more caffeine than the standard 3 to 6 mg/kg. Recently, Josh Amberger raced the UNBOUND XL, a gravel race that took roughly 24 hours. He consumed around 1610mg caffeine, which works out to about 23 mg/kg or an average of 67 mg/hr. Yes, this is a lot. But in the context of ultra-racing, where extreme sleep deprivation and mental fatigue compound over many hours, that kind of intake can look more reasonable rather than considered reckless.
Another case study looked at elite ultra-marathon runners in a 100-mile single-stage race. These athletes reported average total intakes of around 912 milligrams spread across 15 to 19 hours of running. Again, these athletes consumed significantly more than current guidelines suggest. Cases like this suggest different caffeine advice may be necessary. For ultra races, lower, sustained doses may work better, especially later in the event, rather than repeating full 4 to 6 mg/kg boluses. Repeat low-dosing around 50 mg/hr appears to be well tolerated in this context, and is best reserved for overnight segments when circadian rhythms are disrupted. Caffeine intake is a well-supported strategy for maintaining performance during a period of overnight sleep loss. The benefit shows up even when caffeine is introduced relatively late into a long session. But, the timing matters. Caffeine may benefit the late stages of a race, but a caffeine gel taken 15 minutes prior to the end of the event is unlikely to take effect before the race is over.
Why mg/hr or mg/kg/hr start to matter
This is where the opening question comes back. For an event lasting roughly one caffeine half-life, about five hours for an average metabolizer, a single well-timed pre-race dose can be enough to get to the finish line, without any maintenance dosing at all. Once a race extends beyond that duration, a single mg/kg number doesn’t capture the impact of caffeine metabolism.
It may be important to consider caffeine concentration: an initial amount distributed across body water, then declining on a predictable half-life curve, with each subsequent dose stacking on top of whatever is left from the last one. Athletes taking repeated doses across a very long race are not experiencing the extreme peak plasma concentrations they would with the same dose at a single time point; they are managing a rising and falling total load in the body. That may be why it’s important to consider mg/kg/hr or even simply mg/hr for pacing repeat doses across an ultra-distance event. The variable that matters in long-distance racing is cumulative concentration, not just the size of the last dose taken.
Putting it into practice
For most performance contexts, the guidelines remain at 3 to 6 mg/kg, but the event duration should shape caffeine dosing decisions. For ultra-distance events, lower, spaced doses rather than one large bolus may be a better strategy, especially when timed around the onset of sleep deprivation and dark hours. Individual response varies substantially based on genetics, habitual caffeine intake, and potentially hormone status in female athletes. The dose that works well for a training partner is not a guarantee for anyone else. The only reliable way to find an individual effective dose is to test it repeatedly in training long before race day. Lastly, caffeine is a performance tool, not fuel. It doesn't substitute for the carbohydrates and calories that actually supply energy.