Caffeine is a naturally occurring substance, commonly found in plant-based foods and beverages. Common sources of caffeine include medications, colas, coffee, tea and chocolate. Caffeine has been defined as a drug, due to its ability to alter one or more body functions. However, caffeine has also been recognized as a GRAS (Generally Recognized as Safe) substance by the Food and Drug Administration. Thus caffeine may be added to many beverages and medications.
Specifically, caffeine has been classified as a mild stimulant. It stimulates the digestive tract, promoting elimination, as well as increasing the functioning of the central nervous system. Another effect of caffeine on the body is its ability to increase the body's heat production - a process called thermo genesis. Caffeine has also been documented to raise blood pressure, stimulate urine output and lipid breakdown.
Once in the body, caffeine circulates all tissues within 15 to 45 minutes, depending on the amount and source, as well as the individual. Caffeine is broken down by the liver, and excreted by the kidneys. Clearance is hindered by liver disease, pregnancy, and the use of oral contraceptives, while clearance is increased by smoking.
Caffeine is an addictive substance. The body adapts to its presence, causing withdrawal symptoms when intake is ceased completely, or even decreased. These symptoms often include headaches, drowsiness, and fatigue. Many medications contain caffeine to eliminate headaches commonly associated with a low caffeine intake. It has been recommended caffeine intake be within a moderate range. This would include the equivalent of about 2-3 cups of coffee per day.
Excessive amounts of caffeine have been noted to produce such symptoms as headaches, drowsiness and fatigue. Large amounts of caffeine may also produce reactions that are very similar to anxiety attacks. Additional symptoms may include restlessness, dizziness, agitation, intestinal discomfort, and sleeping difficulties. It is for this reason that caffeine should not be consumed within four hours of bedtime.
In conclusion, caffeine has generally been proven safe for human consumption; however, to avoid unpleasant side affects, moderation is the key. Remember that many medications also include caffeine.
Source of information: Nutrition for Health and Health Care, Caffeine- Addictive drug?, Beneficial drug? Whitney et al., 1996