Caffeine and its related purine alkaloids, such as theophylline and theobromine, are widely distributed in plants around the world. The most commonly consumed caffeine-containing beverage is tea, made from the leaves of Thea sinensis. This bush, native to southern China and now widely cultivated in other countries, contains caffeine and small amounts of theophylline and theobromine. Over half of the world’s population drinks tea, and the average citizen in China consumes 2 to 3 cups of tea every day. Other sources of caffeine include cocoa and chocolate from the seeds of Theobroma cacao, coffee from the fruit of Coffea arabica and related species, and cola-flavored drinks containing extracts of the nuts of Cola acuminata.
Statistics show that caffeine consumption is around 300 mg per day per inhabitant in the Nordic countries and in Britain, with tea being the most important source of caffeine in China. Caffeine consumption is of a similar magnitude in most countries, making it the most widely used psychoactive drug after ethanol.
Classical pharmacological studies of caffeine during the first half of the twentieth century confirmed that methylxanthines have stimulant and antisoporific effects that elevate mood, decrease fatigue, and increase work capacity. Further research revealed other important pharmacological properties of methylxanthines, which were used for therapeutic purposes for many years but have since been replaced by more effective agents. However, there has been a renewed interest in natural methylxanthines and their synthetic derivatives in recent years due to increased knowledge of their cellular mechanisms of action.