Caffeine is the world's best-known stimulant. Popular beverages such as coffee, tea, and cola-based soft drinks all contain significant amounts of caffeine, as do the cacao bean, the chief ingredient in chocolate products, and guarana, the South American root extract used in many forms. When the use of caffeine as a component of many popular analgesics is factored, or when consumed in a pure-form stimulant such as a pill, there is no question that caffeine is also the most-consumed stimulant in the world. It is believed that more than 95% of the adult population of the United States consumes caffeinated products of one form or another. Caffeine is generally believed to be mildly addictive.

Caffeine is classified as a stimulant because when consumed in even small amounts, it will stimulate the central nervous system, the cardiac muscle (causing an increased heart rate), and the respiratory system. Caffeine will delay the onset of fatigue and it is a strong diuretic.

Caffeine is chemically defined in the class of substances known as alkaloids, nitrogen-based compounds that are typically found in plants. Alkaloids produce a pharmacological effect on the human body when consumed; cocaine, nicotine, and ephedrine are well-known examples of alkaloids.

The impact of caffeine on the body's metabolism first occurs with respect to the substance important to the transfer of energy within the body: adenosine. When consumed, caffeine blocks the operation of adenosine, and as a result the body produces epinephrine (adrenaline), which quickly causes a higher heart rate, increased blood pressure, and greater blood flow.

Athletes who are training or competing in anaerobic disciplines such as ice hockey or track and field events often consume caffeine (usually in the form of coffee or a sports supplement) to acquire a feeling of alertness and an ability to respond more quickly to the circumstances of competition. The actual effect of caffeine on performance for these sports may be more a psychological dependence on the belief that the caffeine stimulates more effective performance.

Caffeine has a proven positive effect on the function of the body in endurance sports, where the it relies primarily on its aerobic system to provide the requisite energy to compete. The ingestion of caffeine will increase the level of circulating fatty acids in the bloodstream, which permits these fat stores to be oxidized, or burned, as fuel. Accessing these fatty acids as fuel means that the body's stores of energy (known as glycogen), created through the processes that convert it from food carbohydrates, will not be exhausted in long competition. Marathon runners, long-distance cyclists, and cross-country skiers have been established as prime beneficiaries of caffeine when it is consumed approximately one hour before competition.

The effect of caffeine on the conversion of fatty acids to energy is further enhanced if the athlete is not a regular coffee drinker or does not consume caffeine products. In an alternative to the pre-competition consumption of caffeine by endurance athletes, further research has demonstrated the efficacy of caffeine in sports drinks. The carbohydrates in the caffeine-enhanced sport drinks were found to be absorbed by the body at a rate 26% faster than the rate achieved without caffeine. The ability of the body to utilize this portable energy source effectively saved bodily stores of energy.

Caffeine is considered an illegal, performance-enhancing substance in Olympic competition only if found in an athlete's system in levels in excess of 12 mcg/ml (micrograms/milliliter)—an amount that is the rough equivalent to the caffeine contained in eight to ten 6-oz (175 ml) cups of coffee—consumed within two hours of testing. In 2004, the World Anti-Doping Agency (WADA) removed caffeine from its list of prohibited substances. It is unlikely that an athlete could ingest caffeine in excess of the Olympic standard without causing an adverse impact upon the body's renal (kidney) and urinary systems, as the diuretic impact of high levels of caffeine will reduce hydration and stimulate urine production.

Caffeine withdrawal is a general description of the physical symptoms noted in persons who reduce the amount of caffeine that they consume. Excessive caffeine consumption reduces the system's resistance to the effects of caffeine, meaning more will be required to achieve the desired effect. Typical withdrawal will cause significant headaches, irritability, reduced sleep or insomnia, stomach upset, and cardiac arrhythmia.

A toxic level of caffeine is the equivalent to the consumption of between 50 and 150 6-oz (175 ml) cups of coffee. There is a well-established connection in caffeine research between the ingestion of caffeine and a loss of calcium density in bones, which can lead to the onset of osteoporosis, a condition whereby the bones become brittle and more prone to fracture.

SEE ALSO Ephedra; Free fatty acids in the blood; Glycogen depletion; Stimulants.