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,
Caffeine is found in many plant species, where it acts as a natural pesticide, with high caffeine levels being observed in seedlings that are still developing foliage, but are lacking mechanical protection; Caffeine paralyzes and kills certain insects feeding upon the plant. High caffeine levels have also been found in the surrounding soil of coffee bean seedlings. Therefore, it is understood that caffeine has a natural function as both a natural pesticide and an inhibitor of seed germination of other nearby coffee seedlings, thus giving it a better chance of survival
The precise amount of caffeine necessary to produce effects varies from person to person depending on body size and degree of tolerance to caffeine. It takes less than an hour for caffeine to begin affecting the body and a mild dose wears off in three to four hours. Consumption of caffeine does not eliminate the need for sleep; it only temporarily reduces the sensation of being tired. Caffeine leads to fewer mistakes caused by tiredness in shift workers.
With these effects, caffeine is an ergogenic, increasing a person's capability for mental or physical labor. A study conducted in 1979 showed a 7% increase in distance cycled over a period of two hours in subjects that consumed caffeine compared to control subjects. Other studies attained much more dramatic results; one particular study of trained runners showed a 44% increase in "race-pace" endurance, as well as a 51% increase in cycling endurance, after a dosage of 9 milligrams of caffeine per kilogram of body weight.Additional studies have reported similar effects. Another study found 5.5 milligrams of caffeine per kilogram of body mass resulted in subjects cycling 29% longer during high-intensity circuits.
In large amounts, and especially over extended periods of time, caffeine can lead to a condition known as caffeinism. Caffeinism usually combines caffeine dependency with a wide range of unpleasant physical and mental conditions including nervousness, irritability, anxiety, tremulousness, muscle twitching (hyperreflexia), insomnia, headaches, respiratory alkalosis, and heart palpitations. Furthermore, because caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis, and gastroesophageal reflux disease.] Caffeine may also increase the toxicity of certain other drugs, such as paracetamol.
There are four caffeine-induced psychiatric disorders recognized by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition: caffeine intoxication, caffeine-induced anxiety disorder, caffeine-induced sleep disorder, and caffeine-related disorder not otherwise specified (NOS).
