This may help you.
Many runners make a cup of coffee a part of their prerun/prerace routine. But what effect does caffeine really have on performance?
There are several ways caffeine may improve performance in endurance events. Caffeine stimulates the central nervous system which facilitates neuromuscular function by improving muscle contraction/reaction time. This effect on the CNS may also lead to a decreased perception of fatigue.
Another potentially beneficial effect of caffeine for runners is an increased concentration of free fatty acids in the blood and increased uptake and utilization of these fatty acids by muscle tissue. This in turn, spares the muscles limited supply of glycogen. The importance of this effect is illustrated by the term "hitting the wall" which is the point at which a person's glycogen stores have been completely depleted.
All of this makes caffeine sound like it's the ultimate aid for the long distance runner. However, there is a downside to the use of caffeine.
The first and best known negative effect is that caffeine is a potent diuretic. Staying well hydrated is one of the keys to quality runs in training and competition. Therefore, ensure that you increase your fluid intake if you are going to take caffeine prior to working out. Caffeine also increases the secretion of acid into the stomach which can lead to cramps and stomach pain - definitely two things you don't need in the middle of a long run.
Before you decide whether or not caffeine is for you, there are a few other factors to consider. One is that tolerance is easily built up. Therefore, the effect that caffeine may have on your performance will be limited if you are a regular coffee, cola or tea drinker. So give yourself two or three caffeine free days before trying it out in your program. As well, like all other changes to your running program, try it out first in training and not in competition.
If you do decide to use caffeine, the optimum dose for positive effects is 2-3mg/kg of body weight. (There are about 80 mg in a cup of brewed coffee and 40mg in a can of Coke.) The effects of caffeine peak around 30 minutes after ingestion and continue for 2-3 hours.
(c) Susan Glen, M.Sc. nutrition