Caffeine is a stimulant of the methylxanthine class, and is the most commonly consumed psychoactive drug globally; it is nearly universally legal to produce and sell, usually in the form of caffeinated drinks. It is mainly used for its eugeroic (wakefulness promoting), ergogenic (physical performance-enhancing), or nootropic (cognitive-enhancing) properties (particularly in workplace settings); it is also widely used recreationally, often in social settings. Caffeine acts by blocking the binding of adenosine at a number of adenosine receptor types, inhibiting the centrally depressant effects of adenosine and enhancing the release of acetylcholine. Caffeine has a three-dimensional structure similar to that of adenosine, which allows it to bind and block its receptors. Caffeine also increases cyclic AMP levels through nonselective inhibition of phosphodiesterase, increases calcium release from intracellular stores, and antagonizes GABA receptors, although these mechanisms typically occur at concentrations beyond usual human consumption. Caffeine is a bitter, white crystalline purine, a methylxanthine alkaloid, and is chemically related to the adenine and guanine bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is found in the seeds, fruits, nuts, or leaves of a number of plants native to Africa, East Asia, and South America and helps to protect them against herbivores and from competition by preventing the germination of nearby seeds, as well as encouraging consumption by select animals such as honey bees. The most common sources of caffeine for human consumption are the tea leaves of the Camellia sinensis plant and the coffee bean, the seed of the Coffea plant. Some people drink beverages containing caffeine to relieve or prevent drowsiness and to improve cognitive performance. To make these drinks, caffeine is extracted by steeping the plant product in water, a process called infusion. Caffeine-containing drinks, such as tea, coffee, and cola, are consumed globally in high volumes. In 2020, almost 10 million tonnes of coffee beans were consumed globally. Caffeine is the world's most widely consumed psychoactive drug. Unlike most other psychoactive substances, caffeine remains largely unregulated and legal in nearly all parts of the world. Caffeine is seen as socially acceptable in most cultures and is encouraged in some. Caffeine has both positive and negative health effects. It can treat and prevent the premature infant breathing disorders bronchopulmonary dysplasia of prematurity and apnea of prematurity. Caffeine citrate is on the WHO Model List of Essential Medicines. It may confer a modest protective effect against some diseases, including Parkinson's disease. Caffeine can acutely improve reaction time and accuracy for cognitive tasks. Some people experience sleep disruption or anxiety if they consume caffeine, but others show little disturbance. Evidence of a risk during pregnancy is equivocal; some authorities recommend that pregnant women limit caffeine to the equivalent of two cups of coffee per day or less. Caffeine can produce a mild form of drug dependence – associated with withdrawal symptoms such as sleepiness, headache, and irritability – when an individual stops using caffeine after repeated daily intake. Tolerance develops to autonomic effects (blood pressure, heart rate, nervousness) and to alerting/vigilance benefits with regular use. Chronic exposure upregulates adenosine receptors, raising the effort threshold for allocating attention—even to mundane or previously enjoyable tasks—because habitual users operate from a lowered arousal baseline. Caffeine is classified by the US Food and Drug Administration (FDA) as generally recognized as safe. Lethal doses, over 10 grams per day for an adult, greatly exceed the typical dose of under 500 milligrams per day. The European Food Safety Authority reported that up to 400 mg of caffeine per day (around 5.7 mg/kg of body mass per day) does not raise safety concerns for non-pregnant adults, while intakes up to 200 mg per day for pregnant and lactating women do not raise safety concerns for the fetus or the breast-fed infants. A 6 ounce cup of coffee typically contains 50–175 mg of caffeine, depending on what "bean" (seed) is used (and how much), how it is roasted, and how it is prepared (e.g., drip, percolation, or espresso). Thus roughly 50–100 ordinary cups of coffee would be required to reach a lethal dose. However, pure powdered caffeine, which is available as a dietary supplement, can be lethal in tablespoon-sized amounts.
Uses
Medical
Caffeine is used for both prevention and treatment of bronchopulmonary dysplasia in premature infants. It may improve weight gain during therapy and reduce the incidence of cerebral palsy as well as reduce language and cognitive delay. On the other hand, subtle long-term side effects are possible. Caffeine is used as a primary treatment for apnea of prematurity, but not prevention. It is also used for orthostatic hypotension treatment. Some people use caffeine-containing beverages such as coffee or tea to try to treat their asthma. Evidence to support this practice is poor. It appears that caffeine in low doses improves airway function in people with asthma, increasing forced expiratory volume (FEV1) by 5% to 18% for up to four hours. The addition of caffeine (100–130 mg) to commonly prescribed pain relievers such as paracetamol or ibuprofen modestly improves the proportion of people who achieve pain relief. Consumption of caffeine after abdominal surgery shortens the time to recovery of normal bowel function and shortens length of hospital stay. Caffeine was formerly used as a second-line treatment for attention deficit hyperactivity disorder (ADHD). It is considered less effective than methylphenidate or amphetamine but more so than placebo for children with ADHD. Children, adolescents, and adults with ADHD are more likely to consume caffeine, perhaps as a form of self-medication.
Enhancing performance
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