Research about the effects of caffeine on sport performance is extensive. But it is also conflicting. What is caffeine's affect on endurance, strength, recovery, hydration? How does caffeine affect cognitive performance? How does it work in your body? Does it make a difference if you combine it with carbohydrates? How much caffeine should you consume? In what form? Does caffeine affect men and women differently?
To answer questions like these, KHF staff often turn to peer-reviewed, scientific journals—such as the Journal of the International Society of Sports Nutrition (JISSN). From time to time, the ISSN publishes "position papers." These are not statements of policy, but rather the highlights, and summaries, of the scientific literature.
Following is a summary of the ISSN's findings on caffeine and performance. [For the complete paper, go here.]
Actually, we don't know. Or better, there's no simple answer.
Caffeine is absorbed by your gut, metabolized by your liver, and enters your bloodstream within 15-45 minutes of consumption. It peaks in about an hour, can hang around for 2-10 hours, and then is either absorbed by your body (tissues) or gets flushed out through your kidneys.
This is where things get fuzzy. Its not clear whether caffeine affects your muscles, nerves, or both during exercise. If its neural, it may work by suppressing adenosine, which promotes sleep and suppresses arousal. If muscular, it may affect muscle contraction. But because it crosses the blood brain barrier, its difficult to determine in exactly which system (nervous, skeletal, or muscular) caffeine has the greatest effect.
At least during endurance activities, caffeine may decrease your reliance on glycogen (glucose) and increase your ability to use fat for muscle energy. Caffeine may also increase endorphins in your body—decreasing your perception of pain and leading to the feeling known as "runner's high." One other effect of caffeine may be to increase how long it takes before you feel muscle fatigue—particularly in your legs.
During endurance (90+ minutes) activities, most of us consume some form of glucose—whether in the form of gels, sports bars, or carbohydrate-containing drinks. But combine that with a moderate amount of caffeine (5mg/kg, or 225-320mg), and you may find your performance improving 5-10%, especially if you're doing a time trial. However, in this case, less is worse: lower doses of caffeine (3mg/kg) don't seem to have the same effect.
When it comes to rebuilding your glycogen stores after an intense workout, caffeine may also be helpful. In fact, it may improve your bodies ability to replenish glycogen by as much as 66%. However, as we mentioned above, it takes between 3 to 6 hours for caffeine to clear your bloodstream. So be careful that post-activity caffeine consumption doesn't interrupt your sleep regimen.
This really depends upon your level of training, regular caffeine consumption, and type of exercise. But if we had to pick a magic number, we'd guess around 200 - 400mg of caffeine. As we mentioned above, the positive performance effects of caffeine kick in around 3mg/kg (of body weight). But many studies peg the greatest benefits for endurance athletes and enthusiasts around 6mg/kg.
Use the Caffeine Calculator (sidebar) to approximate the optimal range for you.