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Ask Stacy: Is Caffeine Good for Endurance Athletes?

Dr. Stacy Sims does a scientific deep-dive on caffeine and its effect on performance.


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Caffeine. The ubiquitous legal lift that so many athletes put into play on a regular basis. Got to hit the pool deck at 5:30 a.m.? Coffee! 3 p.m. lull? Coffee! Hard brick workout? Caffeinated gum or energy chews! Caffeine is so widely used on a personal level that WADA and the IOC dropped it from the banned substances list because setting a definitive threshold may have led to a positive result for some people just doing their normal thing: drinking coffee and tea. 

When we look at the question, “Is caffeine good for endurance athletes?” there is a plethora of research that demonstrates that yes, in fact, endurance performance does benefit from the strategic use of caffeine. However, the magnitude of performance enhancement following caffeine ingestion differs substantially between individuals, with the spectrum of responses ranging between highly ergogenic to ergolytic (helpful versus impairs exercise performance). 

We do know the main performance benefits of caffeine appear to come from its influence on the central nervous system and resulting in reduced perception of effort (exercise “feels” easier) and/or reduced perception of fatigue. Previous beliefs that caffeine increases fat use during exercise and spares glycogen are now considered a secondary pathway of performance enhancement. To understand how caffeine helps, it is important to understand how it is metabolized before we talk about dosage. Caffeine is quickly absorbed through the GI (gastrointestinal) tract and moves into the bloodstream, with elevated levels appearing 15-45 minutes post-consumption and peak concentrations usually found in the blood one-hour post-ingestion. Within three to six hours, due to tissue uptake and urinary clearance, 50-75% of circulating levels are eliminated. Typically, generalized guidelines recommend ingestion of 3–9 mg/kg body weight of caffeine approximately 60 minutes prior to exercise, and suggest there are no additional benefits associated with higher doses. More recent research indicates that lower doses (1-3mg caffeine per kg/body weight) can boost performance.    

Why the variance in timing and efficacy, though? Primarily, it has to do with your genetics and the form of caffeine consumed. There are two important genes that affect sensitivity and responsiveness to caffeine: ADORA2A and CYP1A2. A polymorphism in the gene that encodes ADORA2A allows individuals to be categorized as having a “high” (TT genotype) or “low” (CT or CC genotype) sensitivity to caffeine. The CYP1A2 is responsible for drug metabolism in the liver, and here, with caffeine, individuals with the AA genotype are considered “fast” metabolizers and those with AC or CC genotypes are considered “slow” metabolizers.   

To put it into performance terms, research examining the impact of ADORA2A genotype on the efficacy of caffeine in enhancing exercise performance found that caffeine improved 10-minute time trial cycling performance in all participants with “high” sensitivity, but in only one participant with “low” sensitivity to caffeine (Loy et al, 2015). A large randomized-controlled trial examined the CYP1A2 genotype and exercise performance found caffeine enhanced 10K time trial cycling performance among only “fast” caffeine metabolizers, and a performance decrease in the slow metabolizers with the AC genotype (Guest et al, 2018). Similarly, “fast” metabolizers demonstrated a greater improvement in 40K time trial performance following caffeine ingestion than “slow” metabolizers (Womack et al, 2012).

Once you’ve established your sensitivity and if you are a fast or slow metabolizer, you can start to think about what form of caffeine you want to use. A seminal study from 1998 looked at the effects of coffee versus other forms of caffeine on endurance performance. In their study, aerobically conditioned runners performed five treadmill runs to exhaustion at approximately 85% VO2max after receiving one of the following treatments 60 minutes prior: caffeine capsules plus water, regular coffee, decaffeinated coffee, decaffeinated coffee plus caffeine in capsule form, and placebo. Caffeine in capsule form significantly increased work capacity allowing them to run an additional 2-3K, as compared to the four other treatments. Bear in mind, though, this was all done in men, and the CYP1A2 is more active in men than in women (thus they can clear caffeine faster than women). 

In short, the simple answer to the question, “Is caffeine good for endurance athletes?” is: It depends on who you are, what your tolerance is, and how you metabolize caffeine.