Caffeine belongs to a group of lipid-soluble compounds called methylxanthines. Together, caffeine and related substances such as theophylline and theobromine are naturally found in coffee beans, tea leaves, chocolate, cocoa beans, and cola nuts. Depending on the preparation technique, a serving of brewed coffee usually contains between 60 to 150 mg of caffeine, while instant coffee typically has about 60 to 100 mg of caffeine. Teas usually provide around 20 to 70 mg, and soft drinks typically have approximately 35 to 55 mg of caffeine.87,88
Caffeine stimulates the nervous system, which results in the release of epinephrine from the adrenal medulla as well as heart rate and contractility elevations and peripheral vasodilation. Effects of caffeine administration also include increased release of calcium from the sarcoplasmic reticulum and elevations of cellular cyclic adenosine monophosphate (AMP), which is responsible for the activation of the hormone-sensitive lipase that results in the mobilization of fatty acids from fat cells. Caffeine can also block adenosine receptors; adenosine usually has a calming effect and blocking adenosine might partially explain the stimulating effects of caffeine. It also acts as a mild diuretic.