 Caffeine is best known for its stimulant, or "wake-up," effect. Once a person consumes caffeine, it is readily absorbed by the body and carried around in the bloodstream, where its level peaks about one hour after consumption. Caffeine mildly stimulates the nervous and cardiovascular systems. It affects the brain and results in elevated mood, decreased fatigue, and increased attentiveness, so a person can think more clearly and work harder. It also increases the heart rate, blood flow, respiratory rate, and metabolic rate for several hours. When taken before bedtime, caffeine can interfere with getting to sleep or staying asleep.
Exactly how caffeine will affect an individual, and for how long, depends on many factors, including the amount of caffeine ingested, whether one is male or female, one's height and weight, one's age, and whether one is pregnant or smokes. Caffeine is converted by the liver into substances that are excreted in the urine.
Some people are more sensitive to the effects of caffeine than others. With frequent use, tolerance to many of the effects of caffeine will develop. At doses of 600 milligrams (about six cups of coffee) or more daily, caffeine can cause nervousness, sweating, tenseness, upset stomach, anxiety, and insomnia. It can also prevent clear thinking and increase the side effects of certain medications. This level of caffeine intake represents a significant health risk.
Current research on how caffeine affects a variety of health issues is summarized below. Keep in mind that most experts agree that moderate use of caffeine is not likely to cause any health problems.
Studies have looked at the effects of caffeine on heart health. Moderate caffeine consumption does not appear to adversely affect cardiovascular health.
Caffeine appears to increase the excretion of calcium, a mineral needed for healthy bones. Calcium is particularly important to prevent osteoporosis, a bone disease characterized by loss of bone strength and seen especially in older women (although men get it too). Moderate caffeine intake does not seem to cause a problem with calcium, as long as one is consuming the recommended amount (adult men and women should be taking between 1,000 and 1,200 milligrams of calcium, depending on age and gender).
In the past there have been concerns that the caffeine in coffee may cause cancer. Research has shown that caffeine in coffee does not cause breast or intestinal cancer. However, not enough research has been done to determine if caffeine in coffee is involved in urinary bladder or pancreatic cancer. Taken in moderation, it is unlikely that caffeine will cause cancer.
Evidence suggests that, at levels over 500 milligrams per day, caffeine may delay conception. Moderate caffeine consumption does not appear to be of concern to women trying to get pregnant. Moderate consumption is also important for a healthy pregnancy. Excessive caffeine intake has been associated with miscarriages and low birth weight babies.
Because children have developing nervous systems, it is important to moderate their caffeine consumption. For children, major sources of caffeine include soft drinks and chocolate.
Caffeine may be useful as part of a weight control program because it increases the rate at which the body burns calories for three or more hours after being consumed.
Caffeine's ability to improve physical performance is well known among well-trained athletes. Through a mechanism that is not completely understood, caffeine seems to increase endurance and speed in some situations.
Excessive use of caffeine is restricted in international competitions.
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