Caffeine or 1, 3, 7-trimethylxanthine is the most widely consumed

pharmacologically active substance in the world. Its major action

is to stimulate the central nervous system. Chemically speaking

caffeine belongs to the alkaloid family and the methylxanthine

group. Caffeine is found in over 60 plant species of which the most

well-known are cocoa-beans, tea and coffee. The closely related

substances theophylline or 1, 3-dimethylxanthine and theobromine

or 3, 7-dimethylxanthine are also found in a variety of plants.

Caffeine occurs naturally in tea, coffee, cocoa and chocolate

products and is added to soft drinks and a variety of both

prescription and over-the-counter drugs. Standard values for the caffeine content of these foods and beverages have been set. Thus an average sized cup (150 ml) of ground roasted coffee contains around 85 mg, instant coffee 60 mg, decaffeinated coffee 3 mg, leaf or bag tea 30 mg, instant tea 20 mg and cocoa or hot chocolate 4 mg caffeine.

Caffeine absorption from the gastrointestinal tract is rapid and virtually complete about 45 minutes after ingestion. The peak plasma caffeine concentration is reached 15-120 minutes after ingestion. Intakes of 5-8 mg caffeine/kg body weight give plasma caffeine concentrations of 8-10 mg/l. The half-life of caffeine in the plasma is 2.5-4.5 hours in young and elderly men increasing to 80 hours in newborn infants and over 100 hours in premature infants. The caffeine half-life is reduced by 30-50% in smokers, doubles in women taking oral contraceptives and rises to 15 hours in the last trimester of pregnancy. The metabolism of caffeine is species specific. In humans about 80% of caffeine is demethylated to paraxanthine and about 16% converted to theobromine and theophylline in the liver. Further demethylation and oxidation forms urates and uracil derivatives. About a dozen caffeine metabolites can be recovered in the urine but less than 3% of ingested caffeine.

A number of mechanisms for the effects of caffeine have been suggested. Caffeine stimulates the release of intracellular calcium and inhibits the activity of cyclic nucleotide phosphodiesterases at concentrations between 0.5 and 1 mM. These concentrations are well above those seen in the blood in response to normal intakes of caffeine suggesting that the effects of caffeine are mediated by other mechanisms. It is now generally accepted that physiological concentrations of caffeine (about 100 ?M) act by antagonising the effects of adenosine. Caffeine acts at A1 adenosine receptors which are negatively linked to adenyl cyclase and A2a adenosine receptors which are positively linked to adenyl cyclase. Hence, competition between caffeine and adenosine at cell surface A1 and A2a adenosine receptors leading to changes in the intracellular concentration of cyclic AMP is the likely mechanism underlying the physiological effects of caffeine.

Caffeine is a mild stimulant of the central nervous system and as a result can influence human behaviour. This includes effects on aspects of mood such as alertness and anxiety, mental performance and sleep. Studies of effects of caffeine on human behaviour often use very high intakes and the relevance of the results of these studies to normal levels of caffeine consumption are questionable. In addition, such studies often look at the effects of a single dose of caffeine and the relevance of this to long-term caffeine consumption can also be questioned as the body may well adapt over the long-term. It is also clear that there is considerable individual variation in the response of human behaviour to caffeine ingestion and extrapolation of the results from a small study population to the general population is likely to produce spurious results.