Caffeine is a socially acceptable, legal drug consumed by all groups in society. Caffeine is often referred to as nutritional ergogenic aid, but it has no nutritional value. Ingested caffeine is quickly absorbed from the stomach and peaks in the blood in 1-2 hours.
Caffeine has the potential to affect all systems of the body, as it is absorbed by most tissue.
The remaining caffeine is broken down in the liver and by-products are excreted in urine. In the 1980's, many studies found that caffeine did not alter exercise metabolism, and implied that it had no ergogenic effect, without actually measuring performance. A few reports did examine caffeine and performance during endurance exercise and generally found no ergogenic effect. By the end of the decade, it was suggested that caffeine did not alter metabolism during endurance exercise and may not be ergogenic. These studies employed well-trained, elite or serious, recreational athletes.