How Much Do You Know About Caffeine?

Caffeine in low to moderate amounts (one to two cups of coffee) acts as a mild stimulant, boosting heart rate and blood pressure. This results in a feeling of alertness and in increased concentration. Caffeine's effects kick in about 15 minutes after it is consumed and can last up to several hours. At higher doses (more than three cups of coffee a day) and in people sensitive to caffeine, however, it can cause anxiety, insomnia, headaches, stomach irritation, and heartbeat irregularities, according to the American Dietetic Association (ADA) and the American Heart Association (AHA). People who regularly consume caffeine may experience a "withdrawal" 12 to 24 hours afterward, the AHA says. The chief symptom is headache, but other symptoms can include anxiety, fatigue, and drowsiness. These symptoms usually ease within a day or two.

The Scandinavian countries, Austria, and the Netherlands have the highest coffee consumption in the world, at more than 22 pounds per person per year. The United States, as well as Italy and Algeria, has among the lowest coffee consumption, at less than 11 pounds per person per year. Coffee consumption in Brazil is more than 13 pounds per person per year. Ireland, the United Kingdom, Turkey, and Kuwait drink the most tea, with Ireland's per person consumption at nearly 7 pounds a year, the United Kingdom at nearly 6 pounds, and Turkey and Kuwait at nearly 5 pounds.

That's about equal to two 12-ounce cups of brewed coffee. But heavy caffeine users may consume nearly 500 mg a day. Coffee and tea account for about 20 percent of caffeine intake for U.S. adults; soft drinks account for about 5 percent of intake. For children and teens, tea and soft drinks provide the most caffeine in their diets, with the average child taking in about 21 mg of caffeine a day.

Studies so far have not established a link between moderate caffeine consumption (1 to 2 cups of coffee a day) and illnesses such as cancer or heart disease, the AHA says. Studies also have not demonstrated an effect of caffeine on fibrocystic breast disease. Recent research has found that at higher amounts (3 cups of coffee a day), caffeine can lead to spinal bone loss and thinning of other bones, particularly in older women. Caffeine also can interfere with bone development in children, because it interferes with calcium absorption. Another recent study found that moderate caffeine consumption may help prevent the development of type 2 diabetes.

Recent research has found that the polyphenols in green tea may help prevent certain cancers, such as stomach cancer.

According to the ADA, a cup of coffee prepared in an automatic drip maker contains 137 mg of caffeine; a cup prepared in an automatic percolator has only 117 mg. Ounce for ounce, coffee has much more caffeine than tea. A cup of black tea brewed for five minutes contains 65 mg of caffeine; a cup of green tea contains 31 mg. Soft drinks also contain caffeine. Colas and Mountain Dew contain 36 to 54 mg of caffeine per 12-ounce can. Cocoa, on the other hand, contains only 5 mg of caffeine in 6 ounces.

Check the product label to find out the amount of caffeine in OTC products.

Instant coffee generally contains less caffeine than brewed coffee, the ADA says. If you brew your tea for about a minute, instead of the usual three minutes, you can cut the amount of caffeine in half. Avoiding caffeine after noon can help if you have trouble sleeping at night because of too much caffeine. Other ideas from the ADA on reducing caffeine: Brew half caffeinated/half decaffeinated coffee; switch to decaffeinated or herbal teas; don't buy soft drinks that contain caffeine (most clear soft drinks are caffeine-free, as are flavors like orange and root beer, but check the label). Also keep in mind that some medications contain caffeine. Talk to your health care provider or pharmacist if you have questions about caffeine and medication.