Is Caffeine and How Does It Work?
Caffeine (C8H10N4O2) is the common name for trimethylxanthine (systematic name is 1,3,7-trimethylxanthine
or 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione). The chemical is also known as coffeine, theine, mateine, guaranine, or
methyltheobromine. Caffeine is naturally produced by several plants, including coffee beans, guarana, yerba maté, cacao beans,
and tea. For the plants, caffeine acts as a natural pesticide. It paralyzes and kills insects that attempt to feed on the
plants. The molecule was first isolated by the German chemist Friedrich Ferdinand Runge in 1819.
When purified, caffeine is an intensely bitter white powder.
It is added to colas and other soft drinks to impart a pleasing bitter note. However, caffeine is also an addictive stimulant.
In humans, it stimulates the central nervous system, heart rate, and respiration, has psychotropic (mood altering) properties,
and acts as a mild diuretic.
A normal dose of caffeine is generally considered to be
100 mg, which is roughly the amount found in a cup of coffee. However, more than half of all American adults consume more
than 300 mg of caffeine every day, which makes it America's most popular drug. Caffeine is generally consumed in coffee, cola,
chocolate, and tea, although it is also available over-the-counter as a stimulant.
Caffeine is believed to work by blocking adenosine receptors
in the brain and other organs. This reduces the ability of adenosine to bind to the receptors, which would slow down cellular
activity. The stimulated nerve cells release the hormone epinephrine (adrenaline), which increases heart rate, blood pressure,
and blood flow to muscles, decreases blood flow to the skin and organs, and causes the liver to release glucose. Caffeine
also increases levels of the neurotransmitter dopamine.
Caffeine is quickly and completely removed from the brain.
Its effects are short-lived and it tends not to negatively affect concentration or higher brain functions. However, continued
exposure to caffeine leads to developing a tolerance to it. Tolerance causes the body to become sensitized to to adenosine,
so withdrawal causes blood pressure to drop, which can result in a headache and other symptoms. Too much caffeine can result
in caffeine intoxication, which is characterized by nervousness, excitement, increased urination, insomnia, flushed face,
cold hands/feet, intestinal complaints, and sometimes hallucinations. Some people experience the symptoms of caffeine intoxication
after ingesting as little as 250 mg per day. The lethal ingested dose, for an adult person, is estimated to be 13-19 grams.
While generally considered safe for people, caffeine can be very toxic to household pets, such as dogs, horses, or parrots.
Caffeine intake has been demonstrated to reduce the risk of type II diabetes mellitus. In addition to use as a stimulant and
flavoring agent, caffeine is included in many over-the-counter headache remedies.
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other uses, see Caffeine (disambiguation).
is a xanthine alkaloid compound that acts as a stimulant
in humans. Caffeine is sometimes called guaranine when found
in guarana, mateine when found in
mate, and theine when found in tea.
It is found in the leaves and beans of the coffee
plant, in tea, yerba mate, and guarana berries,
and in small quantities in cocoa, the kola nut and the Yaupon
Holly. Overall, caffeine is found in the beans, leaves, and
fruit of over 60 plants, where it acts as a natural pesticide
that paralyzes and kills certain insects feeding upon them.
Caffeine is a central nervous system
(CNS) stimulant, having the effect of temporarily warding off drowsiness
and restoring alertness. Beverages containing caffeine, such as coffee, tea,
soft drinks and energy drinks enjoy great popularity: caffeine is the
world's most widely consumed psychoactive substance. In North America, 90% of adults consume caffeine
Many natural sources of caffeine also contain widely varying
mixtures of other xanthine alkaloids, including the cardiac stimulants theophylline and theobromine and other substances such
Caffeine has a significant effect on spiders,
which is reflected in their web construction
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. 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.
Caffeine is sometimes administered in combination with medicines
to increase their effectiveness. 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.
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. It is speculated
that this reduction in bronchopulmonary dysplasia is tied to a reduction in exposure to positive airway pressure.
 The only short
term risk associated with this treatment is a temporary reduction in weight gain during the therapy.
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.
Caffeine is metabolized in the liver into three primary metabolites:
paraxanthine (84%), theobromine (12%), and theophylline
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 about 3-4 hours.
In women taking oral contraceptives this is increased to around 13 hours, and in pregnant women the half-life is 18-20 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.
Caffeine's principal mode of action is as an antagonist
of adenosine receptors in the brain. They are presented here side by side for comparison.
The principal mode of action of caffeine is as an antagonist
of adenosine receptors in the brain. The caffeine molecule is structurally similar to adenosine, and binds
to adenosine receptors on the surface of cells without activating them (a "false transmitter" method of antagonism). 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 and a release of glucose
by 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 the messaging cascade produced
by cells in response to stimulation by epinephrine, so 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.
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.
From top to bottom:
Effect caffeine has on spiders
Metabolism breakdown of