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Caffeine is a chemical compound found naturally in such foods as coffee beans, tea, cacao beans (chocolate), kola nuts, and guarana. It is well known as a stimulant of the central nervous system, heart, and respiration. It is also a diuretic. It is added to some soft drinks such as colas and Mountain Dew.

Caffeine is an alkaloid of the methylxanthine family, which also includes the similar compounds theophylline and theobromine. In its pure state it is an intensely bitter white powder. Its chemical formula is C8H10N4O2, its systematic name is 1,3,7-trimethylxanthine or 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione, and its structure is shown below.

In this diagram, black is carbon, blue is nitrogen, red is oxygen, and white is hydrogen.

Note: some carbon and hydrogen atoms are omitted in this diagram.

The amount of caffeine in some common consumables is approximately as follows:

  • Chocolate, milk - 3-6mg per ounce
  • Chocolate, bittersweet - 25mg per ounce
  • Cocoa - 0.5mg per ounce
  • Coffee, brewed - 5-20mg per ounce
  • Coffee, instant - 4-12mg per ounce
  • Coffee, decaffeinated - 0.4-0.6mg per ounce
  • Energy drink - 10mg per ounce
  • Espresso - 100mg per ounce
  • Soft drink (caffeinated) - 3-8mg per ounce
  • Stimulant Pill - 200mg
  • Tea, brewed (USA)- 2.5-11mg per ounce
  • Tea, brewed (other) - 3-14mg per ounce
  • Tea, instant - 3.5mg per ounce
  • Tea, canned iced - 2-3mg per ounce
  • Yerba Mate, cured herb - 280-425mg per ounce

Caffeine is thought to act on the brain by blocking adenosine receptors. Adenosine, when bound to receptors of nerve cells, slows down nerve cell activity; this happens, among other times, during sleep. The caffeine molecule, being similar to adenosine, binds to the same receptors but doesn't cause the cells to slow down; instead, the caffeine blocks the receptors and thereby the adenosine action. The resulting increased nerve activity causes the release of the hormone epinephrine, which in turn leads to several effects such as higher heart rate, increased blood pressure, increased blood flow to muscles, decreased blood flow to the skin and inner organs, and release of glucose by the liver. In addition, caffeine, similar to amphetamines, increases the levels of the neurotransmitter dopamine in the brain.

Caffeine is quickly and completely removed from the brain and, unlike alcohol or other CNS stimulants, its effects are short lived. In addition, caffeine does not affect concentration or higher mental functions, and hence caffeinated drinks are often consumed in the course of work.

Continued consumption of caffeine can lead to tolerance. Upon withdrawal, the body becomes oversensitive to adenosine, causing the blood pressure to drop dramatically, leading to headache and other symptoms. Recent studies suggest that caffeine intake (in coffee) may decrease the risk of developing Parkinson's disease, but additional study is needed.

Too much caffeine can lead to caffeine intoxication. The symptoms of this disorder are restlessness, nervousness, excitement, insomnia, flushed face, diuresis, and gastrointestial complaints. They can occur in some people after as little as 250 mg/d. More than 1 g/d may result in muscle twitching, rambling flow of thought and speech, cardiac arrythmia[?], and psychomotor agitation[?]. Caffeine Intoxication can lead to symptoms similar to panic disorder[?] and generalized anxiety disorder[?]. The LD50 is estimated to be about 10 grams, the equivalent of about 50 cups of coffee.

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