caffeine

caffeine, naturally occurring nitrogenous organic compound of the alkaloid group, widely known for its effects on the central nervous system, particularly its ability to increase alertness. Caffeine is found in tea, coffee, guarana, maté, kola nuts, and cacao.

Chemical characteristics

Pure caffeine (trimethylxanthine) occurs as a white powder or as silky needles, which melt at 238 °C (460 °F); it sublimes at 178 °C (352 °F) at atmospheric pressure. It is very soluble in hot water; upon cooling, the solution deposits crystals of caffeine monohydrate. Caffeine is generally less soluble in organic solvents than in hot water. It is odorless and has a bitter taste.

Amount in common beverages

Caffeine is present in ground coffee in amounts ranging between 0.75 and 1.5 percent by weight. The average cup of coffee thus contains about 100 mg (0.003 ounce) of caffeine. The caffeine content of tea varies greatly depending on the strength of the tea, but it averages about 40 mg. There are also about 40 mg (0.0014 ounce) of caffeine in a 12-ounce glass of carbonated cola beverage.

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Physiological effects

Caffeine has a stimulating effect particularly on the central nervous system, heart, blood vessels, and kidneys. Its effects result mainly from its inhibition of adenosine, a breakdown product of cell metabolism that acts as a sleep-regulating chemical in the body—when adenosine binds to its receptors, it generates sleep pressure (a biological mechanism signaling an increasing need for sleep). Caffeine is very close in structure to adenosine, allowing it to readily attach to adenosine receptors. However, unlike adenosine, caffeine does not induce sleep signaling and therefore delays sleep. Specifically in the kidneys, caffeine blockade of adenosine activity prevents sodium and water reabsorption, resulting in diuretic effects.

Because caffeine consumption is associated with improved motor performance, decreased fatigue, enhanced sensory activity, and increased alertness, many people consume coffee or other caffeine-containing beverages as part of the morning ritual of awakening. However, caffeine intake may also result in irritability, nervousness or anxiety, jitteriness, headaches, and insomnia. Decaffeinated coffee and caffeine-free soft drinks offer consumers the choice of regulating their caffeine intake while continuing to enjoy these beverages.