Caffeine, the most widely used ergogenic aid in sports, primarily stimulates the release of epinephrine, also known as adrenaline. In one study, caffeine ingestion increased plasma epinephrine levels by 207% and norepinephrine by 75% compared to placebo. This surge in stress hormones is the main driver behind caffeine’s performance-boosting effects, increasing heart rate, blood flow to muscles, and the availability of fuel for exercise.
How Caffeine Triggers Epinephrine Release
Caffeine doesn’t directly force your adrenal glands to pump out adrenaline. Instead, it works by blocking a brain chemical called adenosine. Under normal conditions, adenosine acts like a brake pedal on your nervous system, slowing down neural activity and promoting relaxation. Caffeine fits into the same receptors that adenosine uses but doesn’t activate them, effectively removing that brake.
With adenosine’s calming influence blocked, your sympathetic nervous system (the “fight or flight” system) ramps up. This triggers the adrenal glands to release epinephrine and norepinephrine into the bloodstream. Through this same mechanism, caffeine also indirectly boosts the release of dopamine, serotonin, and acetylcholine, all of which contribute to feelings of alertness and focus.
What Epinephrine Does for Performance
The epinephrine spike from caffeine sets off a chain of physiological changes that benefit athletic performance. Your heart rate increases, airways dilate, and blood is redirected toward working muscles. One of the most significant effects is increased fat mobilization. Research shows that caffeine roughly doubles the rate of lipid turnover in the body, meaning your muscles gain access to more fatty acids as fuel. When researchers blocked the adrenaline pathway using a beta-blocker, this fat-burning effect was significantly reduced, confirming that the sympathetic nervous system is the primary route through which caffeine boosts metabolism.
For endurance athletes, this shift toward burning fat can spare glycogen (stored carbohydrate in muscles), potentially delaying the point of exhaustion. For strength and power athletes, the heightened nervous system activation translates to faster reaction times, greater force output, and reduced perception of effort.
Caffeine’s Effect on Cortisol
Cortisol, the body’s main long-term stress hormone, responds to caffeine in a more nuanced way than epinephrine. On its own, caffeine does not raise cortisol levels at rest. In one controlled study, cortisol measurements taken 60 minutes after caffeine ingestion were identical to placebo values in both men and women, as long as no additional stressor was present.
The story changes during exercise. While moderate exercise alone didn’t raise cortisol, combining caffeine with physical exertion produced a delayed cortisol increase, with levels climbing significantly about 40 minutes after the workout ended. This effect appeared equally in men and women. For most athletes, this temporary cortisol bump isn’t a concern, but it’s worth noting that caffeine amplifies the hormonal stress response when paired with intense physical demands.
Testosterone and Growth Hormone
If you’re hoping caffeine will boost anabolic hormones like testosterone, the evidence is mixed at best. During resistance exercise, testosterone rises regardless of whether you’ve consumed caffeine. Studies comparing caffeine and placebo conditions found no significant difference in testosterone levels after a strength training session. One small study using caffeinated chewing gum (roughly 3 mg per kilogram of body weight) reported increased testosterone alongside improved sprint performance in cyclists, but this finding hasn’t been widely replicated.
Growth hormone tells a less favorable story. Caffeine ingestion before resistance exercise actually blunted the growth hormone response at multiple time points after training compared to a placebo condition. The post-exercise growth hormone spike was significantly lower with caffeine. The practical significance of this single-session hormonal dip is unclear, but it’s a reminder that caffeine’s hormonal effects aren’t uniformly positive.
Dosing and Timing
The hormonal and performance benefits of caffeine consistently show up at doses of 3 to 6 mg per kilogram of body weight. For a 70 kg (154 lb) person, that’s roughly 210 to 420 mg, or about two to four cups of brewed coffee. Some research suggests doses as low as 2 mg per kilogram may produce ergogenic effects, though the minimum threshold for a meaningful hormonal response isn’t firmly established.
Timing matters. Caffeine reaches its peak concentration in the blood within 30 to 60 minutes of ingestion, with a range as wide as 15 to 150 minutes depending on individual metabolism, stomach contents, and the form consumed. For most people, taking caffeine about 60 minutes before the moment you want peak performance provides the best window for elevated epinephrine levels to align with your activity.
Tolerance Changes the Hormonal Response
If you drink coffee every day, the hormonal surge from caffeine becomes less dramatic over time. Research on habitual coffee drinkers has found that the cortisol response to caffeine is blunted or absent entirely compared to people who rarely consume it. One randomized crossover trial in regular coffee drinkers found no effect of caffeine on cortisol, androstenedione, or DHEA levels at all.
The epinephrine response follows a similar, though not identical, pattern. Habitual users still experience some sympathetic nervous system activation, but the magnitude is smaller. This is why athletes who want the full hormonal kick from caffeine sometimes cycle off it for several days before competition, allowing sensitivity to return. Even a brief withdrawal period of two to four days can partially restore the response, though the exact timeline varies between individuals.