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Caffeine is a central nervous system and metabolic stimulant, 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 flow of thought, increased focus, and better general body coordination, and later at the spinal cord level at higher doses. Once inside the body, it has a complex chemistry, and acts through several mechanisms as described below.
Caffeine is completely absorbed by the stomach and small intestine within 45 minutes of ingestion. After ingestion it is distributed throughout all tissues of the body and is eliminated by first-order kinetics.
The half-life of caffeine — the time required for the body to eliminate one-half of the total amount of caffeine consumed at a given time — 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 3-4 hours. In women taking oral contraceptives this is increased to 5-10 hours, and in pregnant women the half-life is roughly 9-11 hours. Caffeine can accumulate in individuals with severe liver disease when its half-life can increase to 96 hours. 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.
Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system (specifically, the 1A2 isozyme) into three metabolic dimethylxanthines, which each have their own effects on the body:
* Paraxanthine (84%) – Has the effect of increasing lipolysis, leading to elevated glycerol and free fatty acid levels in the blood plasma.
* Theobromine (12%) – Dilates blood vessels and increases urine volume. Theobromine is also the principal alkaloid in cocoa, and therefore chocolate.
* Theophylline (4%) – Relaxes smooth muscles of the bronchi, and is used to treat asthma. The therapeutic dose of theophylline, however, is many times greater than the levels attained from caffeine metabolism.
Each of these metabolites is further metabolized and then excreted in the urine.
Mechanism of action
Caffeine acts through multiple mechanisms involving both action on receptors and channels on the cell membrane, as well as intracellular action on Calcium and cAMP pathways. By virtue of its purine structure it can act on some of the same targets as adenosine related nucleosides and nucleotides, like the cell surface P1 GPCRs for adenosine, as well as the intracellular Ryanodine receptor which is the physiological target of cADPR (cyclic ADP ribose), and cAMP-phosphodiesterase (cAMP-PDE). Although the action is agonistic in some cases, it is antagonistic in others. Physiologically, however, caffeine action is unlikely due to increased RyR opening, as it requires plasma concentration above lethal dosage. The action is most likely through adenosine receptors.
Like alcohol, nicotine, and antidepressants, caffeine readily crosses the blood brain barrier. Once in the brain, the principal mode of action of caffeine is as an antagonist of adenosine receptors found in the brain. 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. The reduction in adenosine activity results in increased activity of the neurotransmitter dopamine, largely accounting for the stimulatory effects of caffeine. Caffeine can also increase levels of epinephrine/adrenaline, possibly via a different mechanism. Acute usage of caffeine also increases levels of serotonin, causing positive changes in mood.
The inhibition of adenosine may be relevant in its diuretic properties. Because adenosine is known to constrict preferentially the afferent arterioles of the glomerulus, its inhibition may cause vasodilation, with an increase in renal blood flow (RBF) and glomerular filtration rate (GFR). This effect, called competitive inhibition, interrupts a pathway that normally serves to regulate nerve conduction by suppressing post-synaptic potentials. The result is an increase in the levels of epinephrine and norepinephrine/noradrenaline released via the hypothalamic-pituitary-adrenal axis. Epinephrine, the natural endocrine response to a perceived threat, stimulates the sympathetic nervous system, leading to an increased heart rate, blood pressure and blood flow to muscles, a decreased blood flow to the skin and inner organs. Biochemically, it stimulates glycogenolysis, inhibits glycolysis, and stimulates gluconeogenesis to produce more glucose in the muscles and release of glucose into the blood stream from the liver.
Caffeine is also a known competitive inhibitor of the enzyme cAMP-phosphodiesterase (cAMP-PDE), which converts cyclic AMP (cAMP) in cells to its noncyclic form, 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.
Caffeine (and theophylline) can freely diffuse into cells and causes intracellular calcium release (independent of extracellular calcium) from the calcium stores in the Endoplasmic Reticulum(ER). This release is only partially blocked by Ryanodine receptor blockade with ryanodine, dantrolene, ruthenium red, and procaine (thus may involve ryanodine receptor and probably some additional calcium channels), but completely abolished after calcium depletion of ER by SERCA inhibitors like Thapsigargin (TG) or cyclopiazonic acid (CPA). The action of caffeine on the ryanodine receptor may depend on both cytosolic and the luminal ER concentrations of Ca2+. At low millimolar concentration of caffeine, the RyR channel open probability (Po) is significantly increased mostly due to a shortening of the lifetime of the closed state. At concentrations >5 mM, caffeine opens RyRs even at picomolar cytosolic Ca2+ and dramatically increases the open time of the channel so that the calcium release is stronger than even an action potential can generate. This mode of action of caffeine is probably due to mimicking the action of the physiologic metabolite of NAD called cADPR (cyclic ADP ribose) which has a similar potentiating action on Ryanodine receptors.
Caffeine may also directly inhibit delayed rectifier and A-type K+ currents and activate plasmalemmal Ca2+ influx in certain vertebrate and invertebrate neurons.
The metabolites of caffeine contribute to caffeine's effects. Theobromine is a vasodilator that increases the amount of oxygen and nutrient flow to the brain and muscles. Theophylline, the second of the three primary metabolites, acts as a smooth muscle relaxant that chiefly affects bronchioles and acts as a chronotrope and inotrope that increases heart rate and efficiency. The third metabolic derivative, 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.
Effects when taken in moderation
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.
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 tests. 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. The extensive boost shown in the runners is not an isolated case; 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.
Breathing problems in premature infants, apnea of prematurity, are sometimes treated with citrated caffeine, which is available only by prescription in many countries. A reduction in bronchopulmonary dysplasia has been exhibited in premature infants treated with caffeine citrate therapy regimens. The only short term risk associated with this treatment is a temporary reduction in weight gain during the therapy. It is speculated that this reduction in bronchopulmonary dysplasia is tied to a reduction in exposure to positive airway pressure.
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.
Tolerance and withdrawal
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. 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.
Caffeine tolerance develops very quickly, especially among heavy coffee drinkers. 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 per day for 18 days, and possibly even earlier. Partial tolerance to caffeine has been observed in all other areas, studies with mice indicate that after a long period of caffeine exposure the learning benefits of caffeine observed earlier cannot be found to any significant level. Considering that 80% to 90% of American adults consume caffeine daily, and their mean daily caffeine intake exceeds 200 mg/day, it can be surmised that a large fraction of the U.S. adult population is completely tolerant to most of the effects of caffeine.
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".
Withdrawal symptoms — possibly including headache, irritability, an inability to concentrate, and stomach aches — 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. Caffeine causes excess release of stomach acids during ingestion. When in withdrawal the stomach acid levels decrease substantially and can cause some stomach aches in certain people. The aches normally last between 24-48 hours and can be confused with constipation. Analgesics, such as aspirin, can relieve the pain symptoms, as can a small dose of caffeine. 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. 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." 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. However, since both "regular" and decaffeinated coffees have been shown to stimulate the gastric mucosa and increase stomach acid secretion, caffeine is probably not the sole component of coffee responsible.
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).
Other side effects of caffeine overuse include: dizziness, tachycardia, blurred vision, drowsiness, dry mouth, flushed dry skin, diuresis, loss of appetite, nausea and stomachaches.
An acute overdose of caffeine, usually in excess of 400 milligrams (more than 3-4 cups of brewed coffee), can result in a state of central nervous system overstimulation called caffeine intoxication. Some people seeking caffeine intoxication resort to insufflation (snorting) of caffeine powder, usually finely crushed caffeine tablets. This induces a faster and more intense reaction. The symptoms of caffeine intoxication 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.
In cases of extreme overdose, death can result. The median lethal dose (LD50) of caffeine is 192 milligrams per kilogram in rats. 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 timeframe 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. 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 disorders
Long-term overuse of caffeine can elicit a number of psychiatric disturbances. Two such disorders recognized by the American Psychiatric Association (APA) are caffeine-induced sleep disorder and caffeine-induced anxiety disorder.
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.
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. 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 withhold further caffeine. 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.
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. 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 PD, but researchers still do not know how this occurs.
Effects on memory and learning
An array of studies found that caffeine could induce certain changes in memory and learning. In one 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.
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 cingulum, a part of the brain that controls attention. The caffeinated subjects also performed better on the memory tasks.
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. In essence, focused thought coupled with Caffeine consumption increases mental performance.
Effects on baldness
A study has found that caffeine blocks the effects of dihydrotestosterone (DHT), known to damage hair follicles. Scientists estimate up to 60 cups of coffee a day would be needed for significant amounts to reach follicles in the scalp, prompting German cosmetics firm Alpecin to develop caffeine-rich solutions for topical application to the head.
Effects on the heart
Caffeine increases the levels of cAMP in the heart cells, mimicking the effects of epinephrine. cAMP diffuses through the cell and acts as a "secondary messenger," activating protein kinase A (PKA; cAMP- dependent Protein Kinase). This increased PKA activity increases the responsiveness of cardiomyocytes to the calcium currents that control beating.
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.
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