By Anthony McDaniel, M.D.
Caffeine (also spelled caffein) is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant. Caffeine was isolated from coffee in 1820 by a German chemist, Friedlieb Ferdinand Runge, and in 1821 by French chemists working independently; viz., by Robiquet and by Pelletier and Caventou. It was Pelletier, noting that the drug had been isolated from coffee (French: café), who coined the word "cafeine", which became the English word "caffeine". Caffeine is found in varying quantities in the beans, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants. It is most commonly consumed by humans in infusions extracted from the bean of the coffee plant and the leaves of the tea bush, as well as from various foods and drinks containing products derived from the kola nut. Other sources include yerba mate, guarana berries, and the Yaupon Holly.
In humans, caffeine acts as a central nervous system (CNS) stimulant, temporarily warding off drowsiness and restoring alertness. Caffeine is the world's most widely consumed psychoactive substance, but, unlike many other psychoactive substances, is legal and unregulated in nearly all jurisdictions. Beverages containing caffeine, such as coffee, tea, soft drinks, and energy drinks, enjoy great popularity; in North America, 90% of adults consume caffeine daily. The U.S. Food and Drug Administration (FDA) lists caffeine as a "multiple purpose generally recognized as safe food substance". Caffeine has diuretic properties when administered in sufficient doses to subjects that do not have a tolerance for it. Regular users, however, develop a strong tolerance to this effect, and studies have generally failed to support the common notion that ordinary consumption of caffeinated beverages contributes significantly to dehydration.
Because adenosine, in part, serves to regulate blood pressure by causing vasodilation, the increased effects of adenosine due to caffeine withdrawal cause the blood vessels of the head to dilate, leading to an excess of blood in the head and causing a headache and nausea. This means caffeine has vasoconstriction properties. Reduced catecholamine activity may cause feelings of fatigue and drowsiness. A reduction in serotonin levels when caffeine use is stopped can cause anxiety, irritability, inability to concentrate, and diminished motivation to initiate or to complete daily tasks; in extreme cases it may cause mild depression. Together, these effects have come to be known as a "crash". Withdrawal symptoms — possibly including headache, irritability, an inability to concentrate, drowsiness, insomnia and pain in the stomach, upper body, and joints — may appear within 12 to 24 hours after discontinuation of caffeine intake, peak at roughly 48 hours, and usually last from one to five days, representing the time required for the number of adenosine receptors in the brain to revert to "normal" levels, uninfluenced by caffeine consumption. Analgesics, such as aspirin, may relieve the pain symptoms, as may a small dose of caffeine.
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