Caffeine stimulates the central nervous system and is claimed to improve performance.
Consuming the amount of caffeine commonly found in 2½ cups of coffee (approximately 330 mg) 60 minutes before exercising was found to extend endurance in moderately strenuous activity. Also, the work felt easier to study participants. The effects of caffeine vary from individual to individual and also according to the amount consumed. For example, people who drink little or no coffee (which contains caffeine) are more sensitive to its effect compared to heavy coffee drinkers.
Caffeine causes an increased heart rate and promotes the release of stomach acid. In some people, caffeine causes nervousness. Caffeine is a diuretic, causing water loss. This can be a problem in hot weather because the risk for dehydration increases if you do not increase your water intake to offset the losses. Stopping caffeine intake suddenly can result in a nasty headache and lethargy. These caffeine withdrawal symptoms have even been seen in individuals consuming low amounts of caffeine—less than 226 mg per day, which is found in about 1½ cups of coffee.