HOW CAFFEINE MOVES THROUGH THE BODY

HOW CAFFEINE MOVES THROUGH THE BODY

Caffeine is almost always taken by mouth, and so it is absorbed into the blood primarily through the linings of the stomach, small intestine, and large intestine. It is only slowly absorbed through the stomach, and so most absorption occurs at the next step along the gastrointestinal tract, the small intestine. However, once it reaches the intestines, virtually all of the caffeine that was ingested is absorbed. A given oral dose of caffeine takes full effect within thirty to sixty minutes, depending upon how much food is in the stomach and intestines and how concentrated the caf­feine is in the substance that contains it.

Caffeine is evenly distributed throughout the body, metabolized by the liver, and its breakdown products are excreted through the kidneys. The body eliminates it rather slowly, with the half-life of a given dose of caf­feine being approximately three hours. Thus, some of the caffeine that one consumes in the morning is still around well into the afternoon. A person who drinks several cups of coffee or caffeinated sodas across a morning or afternoon is adding on to an existing load of caffeine with each subse­quent drink and may end up feeling rather jittery by the end of the day.

HOW CAFFEINE WORKS

Caffeine is the best known of a class of compounds called xanthines (pro­nounced "zan-theenez"). Theophylline, another xanthine found in tea, is prescribed for breathing problems because it relaxes and opens breathing passages. However, there is so little of it in brewed tea that it exerts no significant stimulant effects in that form. In addition to a small amount of caffeine, chocolate contains theobromine, another xanthine, but one with far less potency than caffeine.

All the xanthines, including caffeine, have multiple actions. The major action is to block the action of a neurotransmitter/neuromodulator called adenosine, which is in the brain (more on this in the following). There are also adenosine receptors throughout the body, including those in blood vessels, fat cells, the heart, the kidneys, and many types of smooth mus­cle. These multiple actions create a confusing picture because the direct effects of caffeine on a system can be enhanced or suppressed by indirect effects on other systems.

EFFECTS ON THE BRAIN

Adenosine receptors, the main site of caffeine action, cause sedation when adenosine binds to them. Adenosine, a by-product of cellular metabolism, leaks out of cells. So, as neurons become more active, they produce more adenosine, and this provides a "brake" on all the neural activity—an ingenious self-regulation by the brain. Caffeine thus produces activation of brain activity by reducing the ability of adenosine to do As job. This is a good example of how a drug can produce an effect (in this case, central nervous system iCNSI stimulation) by inhibiting the action of a neu­rotransmitter that produces an inhibiting effect (a positive coming from two negatives). At moderate doses of around 200 milligrams (about what you get from one to two cups of strong coffee), electroencephalograph (EEG) studies indicate that the brain is aroused. Higher doses, in the range of 500 milligrams, increase heart rate and breathing. Activation of these centers also causes a constriction, or narrowing, of blood vessels in the brain (though outside the brain caffeine has a direct effect on blood vessels that does just the opposite—dilating, or widening, them).

Caffeine also lowers the amount of blood flow within the brain. It seems strange at first that a drug with such strong stimulant effects in the brain would actually decrease blood flow within the brain. But studies have shown that a dose of 250 milligrams (about what you get from two to three cups of coffee) reduces blood flow by nearly one-fourth in the gray matter of the brain (made up mostly of nerve cells) and by about one-fifth in the white matter through which fibers connect groups of nerve cells

into functioning circuits. The fact that caffeine has such powerful stimulant effects despite its decrease of cerebral blood flow underscores how powerful its stimulant effects really are. Further, the effects of a single dose of caffeine on cerebral blood flow were the same in heavy caffeine users and in light users, indicating that the blood flow effect is not one to

which people become tolerant.

People may develop a mild tolerance to some of the effects of caffeine, but most tolerant people can achieve an arousing effect by increasing the dose. The tolerance that develops to the brain-arousing effects of caffeine is less severe than the tolerance that develops to some of its effects on other parts of the body (see the following).

Dependence on caffeine can develop as well, as indicated by the occur­rence of withdrawal symptoms when caffeine intake is abruptly stopped. Between twelve and twenty-four hours after the last dose of caffeine, users generally experience headaches and fatigue that may persist for several days to a week but that are usually strongest during the first two days after quitting. Nonprescription pain relievers such as acetaminophen (Tylenol) or ibuprofen relieve the headaches, and moderate doses can be taken throughout the withdrawal period—just be careful to avoid taking pain medications that include caffeine Many people have found that they enjoy, and indeed rely on, the psy­chological effects of caffeine. While this wouldn't meet our definition of addiction, most caffeine users find the effects pleasant enough to continue using this drug. Therefore, those who decide to quit should also be pre­pared to give up those caffeine-aided feelings of alertness and mild eupho­ria, which may have become a very regular and important part of each day. A related issue is that people who drink caffeinated beverages often do so at the same or similar times of day. In that way the drinking itself may become a part of important daily rituals. It is important to anticipate that changing those rituals may be difficult as well.