Chapter 11: Caffeine
More people use caffeine on a daily basis than any other psychoactive drug. Caffeine belongs to a class of compounds known as xanthines, which are the oldest known stimulants. Other closely related xanthines are theophylline and theobromine.
Three plants containing xanthines have been used by humans for thousands of years. These plants are coffee, which originated in the Middle East; tea, which was first grown in China; and cacao, which is native to Mesoamerica. All three played important cultural and economic roles, and all were at one time promoted for medicinal uses.
Coffee is made by infusing coffee beans that have been picked, sorted, roasted, and ground. After its introduction into Europe, coffee drinking spread widely, and coffeehouses became important places of relaxation and commerce. In the United States, use of coffee grew after the American Revolution, when tea drinking declined due to taxation and its association with the British Crown. Per capita coffee consumption among Americans is currently about 25 gallons per year, including all the varieties of instant, flavored, decaffeinated, and specialty coffees now available.
All types of tea come from hand-picked leaves of the same plant, Camellia sinensis. Variations occur based on the amount of oxidation the leaves undergo after they are picked: black tea comes from fully oxidized leaves, while green tea leaves are only minimally oxidized. Tea was imported into Europe and America beginning in the seventeenth century and economic policies played an important role in its popularity in different countries. Currently, per capita tea consumption among Americans is about 1 pound per year, and iced tea accounts for three-quarters of all tea consumed in the U.S. In addition to caffeine, tea contains very small amounts of the xanthine theophylline.
Chocolate was introduced into Europe from Mexico. The bitter liquid obtained from drying and roasting cacao kernels was an important part of Aztec and Mayan culture. Chocolate drinking spread slowly into Europe, and chocolate was often sold alongside coffee and tea in early coffeehouses. The sweet dessert we now call chocolate was developed in the nineteenth century by mixing cocoa butter, sugar and chocolate powder. Chocolate contains the xanthine theobromine as well as a small amount of caffeine.
Although coffee and tea remain popular drinks, Americans now consume more caffeine from soft drinks. Coca-Cola was developed as a nerve tonic in the late 1800ís and named for two of its flavoring ingredients, coca leaves and cola nuts. Although Coca-Cola originally contained a small amount of cocaine, today the coca leaves used in flavoring the beverage are free of cocaine. Other soft drinks, energy drinks, and over-the-counter drugs are all major sources of caffeine.
If taken orally, caffeine is rapidly absorbed into the body. Peak levels are reached in 30 minutes, and the half-life is about 3 hours. Caffeine exerts a stimulating action in several brain regions by blocking inhibitory receptors for adenosine. Tolerance and dependence occur in regular users, with headache and fatigue being the most common withdrawal symptoms.
The physiological effects of caffeine include stimulation of the central nervous system and skeletal muscles, sleep disturbances, elevated mood and constriction of blood vessels in the brain. This last effect may explain the ability of caffeine to reduce migraine headaches. Caffeine may reverse the effects of fatigue on both physical and mental tasks, but it may not improve performance in a well-rested individual, especially on a complex task.
In terms of health, there is no clear evidence that moderate caffeine consumption is dangerous. However, high consumption of caffeine may have negative reproductive effects, including reducing a woman's chance of becoming pregnant, and it may increase the risk of heart attack, especially in someone with other risk factors. Caffeine is not very toxic, but excessive use can have many unpleasant symptoms, including nervousness, tremors, heart palpitations, and gastrointestinal disturbances. Symptoms disappear when caffeine intake is reduced.