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Caffeine, theophylline, and theobromine are three chemically related compounds that make up the methylxanthines.
They are all classified as mild central nervous system (CNS) stimulants
Products that contain caffeine may also contain different amounts of the other methylxanthines, i.e., tea contains theophylline and milk chocolate actually contains a higher proportion of theobromine than caffeine
Sources of caffeine include beverages such as coffee and tea, food such as chocolate, prescription medications, over-the-counter preparations, and illicit street drugs such as look-alike pills and filler for heroin and cocaine
See Table 8.1 for specific concentrations in foods, beverages, and medications)
It is thought plants that contain methylxanthines have been used to make popular beverages since the Stone Age
Early European explorers discovered caffeine containing products in various parts of the world, i.e., coffee in Ethiopia, tea in China, and the cacao plant (source of chocolate), in Mexico and Central and South America
Caffeine was not always well received initially, however, along with nicotine and alcohol, it has had a greater effect on human civilization than all other nonmedical psychoactive substances combined
Prevalence of Caffeine Consumption
Caffeine is considered a “cradle to grave drug” because it is used nonmedically by young children and adults alike, which is true of no other psychoactive substance
Good estimates of caffeine consumption around the world are difficult to obtain because it is contained in so many different products, and how the product is prepared can make a difference in the caffeine content
However, from estimates derived, the U.S. average is triple the world average, and well over half is from coffee consumption
It is estimated that in the U.S. the amount of caffeine consumed in soft drinks is equal to that in tea
From 1960 to 1982 caffeinated coffee consumption declined by about 40% while soft drink consumption more than tripled
Though caffeinated coffee consumption has been stable since the early 1980s, the contribution of soft drinks to total caffeine consumption is significant with surveys indicating two-thirds of the population in 1989 reporting soft drink consumption the day before compared to one-third in 1962
Caffeine consumption does not vary much by subgroup in the U.S. with the exception of age
Caffeine consumption increases with age until the elderly years (65 and older), and then declines
For “dose” of caffeine consumed, which takes into account body weight, children aged 1-5 have the highest exposure to caffeine after adults aged 18 and older
Estimates are that 90% of the world’s population regularly consumes products containing caffeine with coffee, tea, and soft drinks the most common sources
Pharmacology of Caffeine
Caffeine’s effects are due to its impact on the inhibitory neurotransmitter adenosine.
Adenosine leads to behavioral sedation, regulation of oxygen delivery to cells, dilation of cerebral and coronary blood vessels, and production of asthma. Caffeine and other methylxanthines occupy adenosine receptors and then block the action of that transmitter
Caffeine is rapidly absorbed from the gastrointestinal tract and quickly reaches the brain because it can pass the blood-brain barrier
Half-life in the blood varies among people and ranges from 2 ½ to 7 ½ hours
Peak levels occur 15-45 minutes after the drug is taken but depend on the source
It is equally distributed in total body water and freely crosses the placenta to the fetus, though typical doses of caffeine consumption by mothers appear to pose little health risk to the fetus
The liver does most of the metabolizing of caffeine and it is excreted almost entirely by the kidneys
There is variance among people in caffeine metabolism and excretion from the body, for example, rates of these processes are slower in people who have been using over a shorter period of time.
Liver disease, pregnancy, and use of oral contraceptives slows the processes while cigarette smoking tends to quicken them
Other therapeutic drugs interact with caffeine to increase or decrease its metabolism and excretion
Tolerance and Dependence
Evidence of a distinct caffeine withdrawal syndrome has been available for some time and an individual’s pattern of using caffeine can meet the DSM-IV criteria for drug dependence
Withdrawal symptoms most consistently reported are headache and fatigue yet subjects also reported depression, decreased alertness, less contentment and relaxed mood, decreased activity and energy, greater sleepiness and drowsiness, and increased irritability
Symptoms range from mild to severe, begin within 12 to 24 hours of cessation of use, and may last about a week
Evidence for tolerance to caffeine is less clear than for withdrawal, however, rarely does dependence occur in drugs without tolerance
Tolerance develops to caffeine’s effects on renal function, sleep, and other physiological functions but little tolerance seems to develop to caffeine’s stimulant effects
People tend to be heavier or lighter users of caffeine due to their individual ability to tolerate caffeine more so than their tolerance
Acute Effects of Caffeine
Caffeine elevates mood and this accounts for the popularity of coffee and tea as morning wake-up beverages. It is also speculated depressed people “medicate” themselves by using caffeine products
Effects on human task performance are complicated. For example, caffeine improves task performance by decreasing fatigue and increasing vigilance, so that over time performance does not drop below what is typical for a person rather than pushing it above what is normal
However, caffeine impairs the decision-making part of the task but improves the motor component. Thus it can be seen that the drug’s effects are different even for different components of the same task
See Table 8.3 on page 177 for Caffeine’s Acute Effects on Human Performance
In addition to the pharmacological action, caffeine’s effects depend on the dose of the drug, the setting which it is used, and the personality of the user
Some interactive effects noted among caffeine, nicotine, and alcohol include smokers smoking less cigarettes after they drink coffee, and the excretion of caffeine 50% faster in smokers
There also appear to be state-dependent effects of caffeine, nicotine, and alcohol in combination
Effects were primarily due to alcohol when it was paired with nicotine, but when caffeine was substituted for nicotine and paired with alcohol, the state-dependent effect was mostly due to caffeine
What is the significance of this finding ?
CONTEMPORARY ISSUE BOX 8.2 “Combining Energy Drinks and Alcohol”
Mixing energy drinks such as Red Bull and alcohol (Red Bull and vodka is popular in some clubs) has become popular among young people trying to increase their stamina or physical performance
Studies showed the combination did reduce the perception of headache, weakness, dry mouth, and motor coordination impairment but did not reduce the alcohol-related deficits on measures of motor coordination and visual reaction time.
Thus, combining these drugs can be especially harmful as some drinkers may believe they are less impaired than they actually are.
Caffeinism is the term for caffeine intoxication
It has been reported following consumption of as little as 250mg of caffeine a day, the average for adults in the U.S.
Generally, ingesting 600mg of caffeine a day greatly increases the chances of developing it
Consuming over 1,000mg a day increases the risk of experiencing symptoms such as muscle twitching, rambling flow of thought and speech, cardiac arrhythmia, periods of inexhaustibility, and psychomotor agitation
Other symptoms that have been reported include ringing in the ears and seeing flashes of light
The lethal dose of caffeine when taken orally is 10 grams for adults and 100mgs for children
The adult lethal dose is equivalent to 75 cups of coffee, 125 cups of tea, 200 colas, or 100 NoDoz tablets
Death due to caffeine overdose is rare, though, acute overexposure to caffeine has occurred
Chronic Effects of Caffeine Use
Research attempting to link caffeine use to medical problems has produced inconsistent findings
One study linked coffee consumption to osteoporosis yet more recent research suggests that this relationship can easily be negated with small amounts of milk in the diet
Though caffeine consumption has been commonly associated with indigestion, palpitations, tremor, headache, and insomnia, it is still considered a relatively safe drug
One study found an ingredient in coffee that appears to protect against liver cirrhosis, especially alcoholic cirrhosis. It is speculated caffeine plays a role though this protective effect was not found in tea drinkers.
The exception would be pregnant women who consume more than low to moderate doses and people with anxiety disorders who appear to be hypersensitive to caffeine’s effects
Earlier research found no relationship between caffeine and cancer or myocardial infarction, however a 2006 study established that persons with slow caffeine metabolism might be an exception.
Therapeutic Uses of Caffeine
Caffeine and other methylxanthines are in many medications used to treat a range of ailments
Caffeine is part of many remedies for headaches and colds because its stimulant properties help to counter some of the side effects of those medications
Caffeine is in appetite-suppressant medications because of its diuretic effects
Aminophylline, a cardiac and bronchial dilator that contains theophylline, is used to treat both cardiac and bronchial asthma
Future research needs to address the following issues/concerns:
Developing better ways to obtain accurate measures of caffeine consumption
Long-term effects of caffeine use in children
Development of tolerance to caffeine
How caffeine affects special populations such as those who are medically or psychiatrically ill
Caffeine’s effects in combination with other commonly used drugs