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Physical and psychological effects of caffeine

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Caffeine has widespread physical and physiological effects.


Global consumption of caffeine has been estimated at 120,000 tonnes per annum,[1] making it the world's most popular psychoactive substance. This number equates to one serving of a caffeine beverage for every person, per day. Caffeine is a central nervous system and metabolic stimulant,[2] and is used both recreationally and medically to reduce physical fatigue and restore mental alertness when unusual weakness or drowsiness occurs. Caffeine stimulates the central nervous system first at the higher levels, resulting in increased alertness and wakefulness, faster and clearer thinking, increased concentration, and better general motor coordination, and later at the spinal cord level at higher doses.[3] Once inside the body, it has a complex chemistry, and acts through several mechanisms as described below.

Metabolism and half-life Edit

File:Caffeine metabolites.svg

Caffeine from coffee or other beverages is absorbed by the stomach and small intestine within 1 hour of ingestion and then distributed throughout all tissues of the body.[4] It is eliminated by first-order kinetics.[5] Caffeine can also be ingested rectally, evidenced by the formulation of suppositories of ergotamine tartrate and caffeine (for the relief of migraine)[6] and chlorobutanol and caffeine (for the treatment of hyperemesis).[7]

The half-life of caffeine—the time required for the body to eliminate one-half of the total amount of caffeine—varies widely among individuals according to such factors as age, liver function, pregnancy, some concurrent medications, and the level of enzymes in the liver needed for caffeine metabolism. In healthy adults, caffeine's half-life is approximately 4.9 hours. In women taking oral contraceptives this is increased to 5–10 hours,[8] and in pregnant women the half-life is roughly 9–11 hours.[9] Caffeine can accumulate in individuals with severe liver disease, increasing its half-life up to 96 hours.[10] In infants and young children, the half-life may be longer than in adults; half-life in a newborn baby may be as long as 30 hours. Other factors such as smoking can shorten caffeine's half-life.[11] Fluvoxamine reduced the apparent oral clearance of caffeine by 91.3%, and prolonged its elimination half-life by 11.4-fold (from 4.9 hours to 56 hours).[12]

Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system (specifically, the 1A2 isozyme) into three metabolic dimethylxanthines,[13] which each have their own effects on the body:

Each of these metabolites is further metabolized and then excreted in the urine.

Mechanism of actionEdit

Caffeine and adenosine

Caffeine's principal mode of action is as an antagonist of adenosine receptors in the brain.

Like alcohol and nicotine, caffeine readily crosses the blood–brain barrier that separates the bloodstream from the interior of the brain. Once in the brain, the principal mode of action is as an antagonist of adenosine receptors.[14] The caffeine molecule is structurally similar to adenosine, and binds to adenosine receptors on the surface of cells without activating them (an "antagonist" mechanism of action). Therefore, caffeine acts as a competitive inhibitor.

Adenosine is found in every part of the body, because it plays a role in the fundamental ATP-related energy metabolism, but it has special functions in the brain. There is a great deal of evidence that concentrations of brain adenosine are increased by various types of metabolic stress including anoxia and ischemia. The evidence also indicates that brain adenosine acts to protect the brain by suppressing neural activity and by increasing blood flow.[15] Thus, caffeine, by counteracting adenosine, has a generally disinhibitory effect on brain activity. It has not been shown, however, how these effects cause increases in arousal and alertness.

Adenosine is released in the brain through a complex mechanism.[15] There is evidence that adenosine functions as a synaptically released neurotransmitter in some cases, but stress-related adenosine increases appear to be produced mainly by extracellular metabolism of ATP. It is not likely that adenosine is the primary neurotransmitter for any group of neurons, but rather that it is released together with other transmitters by a number of neuron types. Unlike most neurotransmitters, adenosine does not seem to be packaged into vesicles that are released in a voltage-controlled manner, but the possibility of such a mechanism has not been completely ruled out.

Several classes of adenosine receptors have been described, with different anatomical distributions. A1 receptors are widely distributed, and act to inhibit calcium uptake. A2A receptors are heavily concentrated in the basal ganglia, an area that plays a critical role in behavior control, but can be found in other parts of the brain as well, in lower densities. There is evidence that A 2A receptors interact with the dopamine system, which is involved in reward and arousal. (A2A receptors can also be found on arterial walls and blood cell membranes.)

Beyond its general neuroprotective effects, there are reasons to believe that adenosine may be more specifically involved in control of the sleep-wake cycle. Robert McCarley and his colleagues have argued that accumulation of adenosine may be a primary cause of the sensation of sleepiness that follows prolonged mental activity, and that the effects may be mediated both by inhibition of wake-promoting neurons via A1 receptors, and activation of sleep-promoting neurons via indirect effects on A2A receptors.[16] More recent studies have provided additional evidence for the importance of A2A, but not A1, receptors.[17]

Some of the secondary effects of caffeine are probably caused by actions unrelated to adenosine. Caffeine is known to be a competitive inhibitor of the enzyme cAMP-phosphodiesterase (cAMP-PDE), which converts cyclic AMP (cAMP) in cells to its noncyclic form, thus allowing cAMP to build up in cells. Cyclic AMP participates in activation of protein kinase A (PKA) to begin the phosphorylation of specific enzymes used in glucose synthesis. By blocking its removal caffeine intensifies and prolongs the effects of epinephrine and epinephrine-like drugs such as amphetamine, methamphetamine, or methylphenidate. Increased concentrations of cAMP in parietal cells causes an increased activation of protein kinase A (PKA) which in turn increases activation of H+/K+ ATPase, resulting finally in increased gastric acid secretion by the cell. Cyclic AMP also increases the activity of the funny current, which directly increases heart rate.

Metabolites of caffeine also contribute to caffeine's effects. Paraxanthine is responsible for an increase in the lipolysis process, which releases glycerol and fatty acids into the blood to be used as a source of fuel by the muscles. Theobromine is a vasodilator that increases the amount of oxygen and nutrient flow to the brain and muscles. Theophylline acts as a smooth muscle relaxant that chiefly affects bronchioles and acts as a chronotrope and inotrope that increases heart rate and efficiency.[18]

Caffeinated spiderwebs

Caffeine has a significant effect on spiders, which is reflected in the construction of their webs.

Effects when taken in moderationEdit

File:Main side effects of Caffeine.png

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.[3] Consumption of caffeine does not eliminate the need for sleep, it only temporarily reduces the sensation of being tired throughout the day.

With these effects, caffeine is an ergogenic, increasing the capacity 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 who consumed caffeine compared to control subjects.[20] 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.[21] 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.[22]

Caffeine citrate has proven to be of short and long term benefit in treating the breathing disorders of apnea of prematurity and bronchopulmonary dysplasia in premature infants.[19] The only short term risk associated with caffeine citrate treatment is a temporary reduction in weight gain during the therapy,[23] and longer term studies (18 to 21 months) have shown lasting benefits of treatment of premature infants with caffeine.[24][25]

While relatively safe for humans, caffeine is considerably more toxic to some other animals such as dogs, horses, and parrots due to a much poorer ability to metabolize this compound. Caffeine has a much more significant effect on spiders, for example, than most other drugs do.[26]

Caffeine relaxes the internal anal sphincter muscles and thus should be avoided by those with fecal incontinence.[27]

Tolerance and withdrawalEdit

Caffeine content of select common food and drugs.[28][29]
Product Serving size Caffeine per serving (mg) Caffeine per litre (mg)
Caffeine tablet (regular strength) 1 tablet 100
Caffeine tablet (extra strength) 1 tablet 200
Excedrin tablet 1 tablet 65
Hershey's Special Dark 1 bar (43 g; 1.5 oz) 31
Hershey's milk chocolate 1 bar (43 g; 1.5 oz) 10
Brewed coffee 207 mL (7 U.S. fl oz) 80–135 386–652
Drip coffee 207 mL (7 U.S. fl oz) 115–175 555–845
Coffee, decaffeinated 207 mL (7 U.S. fl oz) 5 24
Coffee, espresso 44–60 mL (1.5-2 U.S. fl oz) 100 1691–2254
Coffee, Starbucks (Tall 12 U.S. fl oz) 240 650-700
Tea, leaf or bag 177 mL (6 U.S. fl oz) 50 281
Green tea 177 mL (6 U.S. fl oz) 30 169
Coca-Cola Classic 355 mL (12 U.S. fl oz) 34 96
Mountain Dew 355 mL (12 U.S. fl oz) 54.5 154
Jolt Cola 695 mL (23.5 U.S. fl oz) 280 402
Red Bull 250 mL (8.2 U.S. fl oz) 80 320
XS Energy Drink 250ml (8.2 U.S. fl oz) 83 332
Monster Energy 473 mL (16 U.S. fl oz) 160 338
Wired X344 473 mL (16 U.S. fl oz) 344 727
Foosh Energy Mints 1 mint 100
Buzz Bites 1 chew 100
Buckfast Tonic Wine 750 mL (25.4 U.S. fl oz) 281 375
V Energy Drink 250 mL (8.5 U.S. fl oz) 77.5 310
NOS 650 mL (22 U.S. fl oz) 343 528

Because caffeine is primarily an antagonist of the central nervous system's receptors for the neurotransmitter adenosine, the bodies of individuals who regularly consume caffeine adapt to the continual presence of the drug by substantially increasing the number of adenosine receptors in the central nervous system. This increase in the number of the adenosine receptors makes the body much more sensitive to adenosine, with two primary consequences.[30] First, the stimulatory effects of caffeine are substantially reduced, a phenomenon known as a tolerance adaptation. Second, because these adaptive responses to caffeine make individuals much more sensitive to adenosine, a reduction in caffeine intake will effectively increase the normal physiological effects of adenosine, resulting in unwelcome withdrawal symptoms in tolerant users.[30]

Other research questions the idea that up-regulation of adenosine receptors is responsible for tolerance to the locomotor stimulant effects of caffeine, noting, among other things, that this tolerance is insurmountable by higher doses of caffeine (it should be surmountable if tolerance was due to an increase in receptors), and that the increase in adenosine receptor number is modest and does not explain the large tolerance which develops to caffeine.[31]

Caffeine tolerance develops very quickly, especially among heavy coffee and energy drink consumers. Complete tolerance to sleep disruption effects of caffeine develops after consuming 400 mg of caffeine 3 times a day for 7 days. Complete tolerance to subjective effects of caffeine was observed to develop after consuming 300 mg 3 times per day for 18 days, and possibly even earlier.[32] In another experiment, complete tolerance of caffeine was observed when the subject consumed 750–1200 mg per day while incomplete tolerance to caffeine has been observed in those that consume more average doses of caffeine.[33]

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. 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".[34]

Withdrawal symptoms—possibly including headache, irritability, an inability to concentrate, drowsiness, insomnia and pain in the stomach, upper body, and joints[35]—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, can relieve the pain symptoms, as can a small dose of caffeine.[36] Most effective is a combination of both an analgesic and a small amount of caffeine.

This is not the only case where caffeine increases the effectiveness of a drug. Caffeine makes pain relievers 40% more effective in relieving headaches and helps the body absorb headache medications more quickly, bringing faster relief.[37] For this reason, many over-the-counter headache drugs include caffeine in their formula. It is also used with ergotamine in the treatment of migraine and cluster headaches as well as to overcome the drowsiness caused by antihistamines.


In large amounts, and especially over extended periods of time, caffeine can lead to a condition known as caffeinism.[38][39] 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.[40][41] Furthermore, because caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis, and gastroesophageal reflux disease.[42]

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).

Caffeine intoxicationEdit

File:Main symptoms of Caffeine overdose.png

An acute overdose of caffeine, usually in excess of about 300 milligrams, dependent on body weight and level of caffeine tolerance, can result in a state of central nervous system over-stimulation called caffeine intoxication,[43] colloquially "caffeine jitters". The symptoms of caffeine intoxication are not unlike overdoses of other stimulants. It may include restlessness, nervousness, excitement, insomnia, flushing of the face, increased urination, gastrointestinal disturbance, muscle twitching, a rambling flow of thought and speech, irritability, irregular or rapid heart beat, and psychomotor agitation.[41] In cases of much larger overdoses mania, depression, lapses in judgment, disorientation, disinhibition, delusions, hallucinations and psychosis may occur, and rhabdomyolysis (breakdown of skeletal muscle tissue) can be provoked.[44][45]

In cases of extreme overdose, death can result. The median lethal dose (LD50) given orally, is 192 milligrams per kilogram in rats.[46] The LD50 of caffeine in humans is dependent on weight and individual sensitivity and estimated to be about 150 to 200 milligrams per kilogram of body mass, roughly 80 to 100 cups of coffee for an average adult taken within a limited time frame that is dependent on half-life. Though achieving lethal dose with caffeine would be exceptionally difficult with regular coffee, there have been reported deaths from overdosing on caffeine pills, with serious symptoms of overdose requiring hospitalization occurring from as little as 2 grams of caffeine. An exception to this would be taking a drug such as fluvoxamine which blocks the liver enzyme responsible for the metabolism of caffeine, thus increasing the central effects and blood concentrations of caffeine dramatically at 5-fold. It is not contraindicated, but highly advisable to minimize the intake of caffeinated beverages, as drinking one cup of coffee will have the same effect as drinking five.[47][48][49][50] Death typically occurs due to ventricular fibrillation brought about by effects of caffeine on the cardiovascular system.

Treatment of severe caffeine intoxication is generally supportive, providing treatment of the immediate symptoms, but if the patient has very high serum levels of caffeine then peritoneal dialysis, hemodialysis, or hemofiltration may be required.

Anxiety and sleep disordersEdit

Two infrequently diagnosed caffeine-induced disorders that are recognized by the American Psychiatric Association (APA) are caffeine-induced sleep disorder and caffeine-induced anxiety disorder, which can result from long-term excessive caffeine intake.

In the case of caffeine-induced sleep disorder, an individual regularly ingests high doses of caffeine sufficient to induce a significant disturbance in his or her sleep, sufficiently severe to warrant clinical attention.[43]

In some individuals, the large amounts of caffeine can induce anxiety severe enough to necessitate clinical attention. This caffeine-induced anxiety disorder can take many forms, from generalized anxiety to panic attacks, obsessive-compulsive symptoms, or even phobic symptoms.[43] Because this condition can mimic organic mental disorders, such as panic disorder, generalized anxiety disorder, bipolar disorder, or even schizophrenia, a number of medical professionals believe caffeine-intoxicated people are routinely misdiagnosed and unnecessarily medicated when the treatment for caffeine-induced psychosis would simply be to stop further caffeine intake.[51] A study in the British Journal of Addiction concluded that caffeinism, although infrequently diagnosed, may afflict as many as one person in ten of the population.[39]

Parkinson's diseaseEdit

Several large studies have shown that caffeine intake is associated with a reduced risk of developing Parkinson's disease (PD) in men, but studies in women have been inconclusive.[52] The mechanism by which caffeine affects PD remains a mystery. In animal models, researchers have shown that caffeine can prevent the loss of dopamine-producing nerve cells seen in Parkinson's Disease, but researchers still do not know how this occurs.[53]

Effects on memory and learningEdit

An array of studies found that caffeine could have nootropic effects, inducing certain changes in memory and learning. However, the tests performed contradict one another and the results have proven inconsistent and inconclusive.

Researchers have found that long-term consumption of low dose caffeine slowed hippocampus-dependent learning and impaired long-term memory in mice. Caffeine consumption for 4 weeks also significantly reduced hippocampal neurogenesis compared to controls during the experiment. The conclusion was that long-term consumption of caffeine could inhibit hippocampus-dependent learning and memory partially through inhibition of hippocampal neurogenesis.[54].

In another study, caffeine was added to rat neurons in vitro. The dendritic spines (a part of the brain cell used in forming connections between neurons) taken from the hippocampus (a part of the brain associated with memory) grew by 33% and new spines formed. After an hour or two, however, these cells returned to their original shape.[55]

Another study showed that subjects—after receiving 100 milligrams of caffeine—had increased activity in brain regions located in the frontal lobe, where a part of the working memory network is located, and the anterior cingulate cortex, a part of the brain that controls attention. The caffeinated subjects also performed better on the memory tasks.[56]

However, a different study showed that caffeine could impair short term memory and increase the likelihood of the tip of the tongue phenomenon. The study allowed the researchers to suggest that caffeine could aid short-term memory when the information to be recalled is related to the current train of thought, but also to hypothesize that caffeine hinders short-term memory when the train of thought is unrelated.[57] In essence, caffeine consumption increases mental performance related to focused thought while it may decrease broad-range thinking abilities.

Effects on the heartEdit

Caffeine binds to receptors on the surface of heart muscle cells which leads to an increase in the level of cAMP inside the cells (by blocking the enzyme that degrades cAMP), mimicking the effects of epinephrine (which binds to receptors on the cell that activate cAMP production). cAMP acts as a "second messenger," and activates a large number of protein kinase A (PKA; cAMP-dependent protein kinase). This has the overall effect of increasing the rate of glycolysis and increases the amount of ATP available for muscle contraction and relaxation. According to one study, caffeine in the form of coffee, significantly reduces the risk of heart disease in epidemiological studies. However, the protective effect was found only in participants who were not severely hypertensive (i.e. patients that are not suffering from a very high blood pressure). Furthermore, no significant protective effect was found in participants aged less than 65 years or in cerebrovascular disease mortality for those aged equal or more than 65 years.[58]

Effects on childrenEdit

Scientific studies contradict the common belief that caffeine consumption causes stunted growth in children.[59] Children also can experience the same effects from caffeine as adults. Most energy drinks (containing extremely high amounts of caffeine) have been banned in many schools throughout the world.[60]

Caffeine intake during pregnancyEdit

Despite its widespread use and the conventional view that it is a safe substance, a 2008 study suggested that pregnant women who consume 200 milligrams or more of caffeine per day have about twice the miscarriage risk as women who consume none. However, another 2008 study found no correlation between miscarriage and caffeine consumption.[61] The UK Food Standards Agency has recommended that pregnant women should limit their caffeine intake to less than 200 mg of caffeine a day – the equivalent of two cups of coffee. The recommendation was based on findings of increased levels of miscarriage in women who consume more caffeine than this.[62][63] The FSA noted that the design of the studies made it impossible to be certain that the differences were due to caffeine per se, instead of other lifestyle differences possibly associated with high levels of caffeine consumption, but judged the advice to be prudent.

Dr De-Kun Li of Kaiser Permanente Division of Research, writing in the American Journal of Obstetrics and Gynecology, concluded that an intake of 200 milligrams or more per day, representing two or more cups, "significantly increases the risk of miscarriage".[64] However, Dr. David Savitz, a professor in community and preventive medicine at New York's Mount Sinai School of Medicine and lead author of the other new study on the subject published in the January issue of Epidemiology, found no link between miscarriage and caffeine consumption.[61]

Genetics and effects on healthEdit

A 2006 study by Dr. Ahmed El-Sohemy at the University of Toronto discovered a link between a gene affecting caffeine metabolism and the effects of coffee on health. [65] [66] Some people have a gene to metabolize caffeine more slowly, and for them drinking large quantities of coffee was found to increase the risk of myocardial infarction. For rapid metabolizers, however, coffee seemed to have a preventative effect. Slow and fast metabolizers are comparably common in the general population, and this has been blamed for the wide variation in studies of the health effects of caffeine.


Main article: Decaffeination

Caffeine extraction is an important industrial process and can be performed using a number of different solvents. Benzene, chloroform, trichloroethylene and dichloromethane have all been used over the years but for reasons of safety, environmental impact, cost and flavor, they have been superseded by the following main methods:

Water extractionEdit

Coffee beans are soaked in water. The water, which contains many other compounds in addition to caffeine and contributes to the flavor of coffee, is then passed through activated charcoal, which removes the caffeine. The water can then be put back with the beans and evaporated dry, leaving decaffeinated coffee with a good flavor.[67] Coffee manufacturers recover the caffeine and resell it for use in soft drinks and over-the-counter caffeine tablets.

Supercritical carbon dioxide extractionEdit

Supercritical carbon dioxide is an excellent nonpolar solvent for caffeine, and is safer than the organic solvents that are otherwise used. The extraction process is simple: CO2 is forced through the green coffee beans at temperatures above 31.1 °C and pressures above 73 atm. Under these conditions, CO2 is in a "supercritical" state: it has gaslike properties which allow it to penetrate deep into the beans but also liquid-like properties which dissolve 97–99% of the caffeine. The caffeine-laden CO2 is then sprayed with high pressure water to remove the caffeine. The caffeine can then be isolated by charcoal adsorption (as above) or by distillation, recrystallization, or reverse osmosis.[67]

Extraction by organic solventsEdit

Organic solvents such as ethyl acetate present much less health and environmental hazard than previously used chlorinated and aromatic solvents. Another method is to use triglyceride oils obtained from spent coffee grounds.


Some Latter-day Saints (Mormons), Seventh-day Adventists, Church of God (Restoration) adherents, and Christian Scientists[68] do not consume caffeine. A few followers from these religions believe that God is opposed to the use of all non-medical psychoactive substances.

The Church of Jesus Christ of Latter-day Saints has said the following with regard to caffeinated beverages: “With reference to cola drinks, the Church has never officially taken a position on this matter, but the leaders of the Church have advised, and we do now specifically advise, against the use of any drink containing harmful habit-forming drugs under circumstances that would result in acquiring the habit. Any beverage that contains ingredients harmful to the body should be avoided.” (Priesthood Bulletin, Feb. 1972, p. 4.) See also Word of Wisdom.

Gaudiya Vaishnava Hindus generally also abstain from caffeine, as it is alleged to cloud the mind and over-stimulate the senses. To be initiated under a guru, one must have had no caffeine (along with alcohol, nicotine and other drugs) for at least a year.

See also Edit



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