What's in caffeine anyway?
Many of us can’t imagine starting the day without a cup of coffee. One reason may be that it supplies us with a jolt of caffeine, a mild stimulant to the central nervous system that quickly boosts our alertness and energy levels.  Of course, coffee is not the only caffeine-containing beverage. Read on to learn more about sources of caffeine, and a review of the research on this stimulant and health.
Absorption and Metabolism of Caffeine
The chemical name for the bitter white powder known as caffeine is 1,3,7 trimethylxanthine. Caffeine is absorbed within about 45 minutes after consuming, and peaks in the blood anywhere from 15 minutes to 2 hours.  Caffeine in beverages such as coffee, tea, and soda is quickly absorbed in the gut and dissolves in both the body’s water and fat molecules. It is able to cross into the brain. Food or food components, such as fibers, in the gut can delay how quickly caffeine in the blood peaks. Therefore, drinking your morning coffee on an empty stomach might give you a quicker energy boost than if you drank it while eating breakfast.
Caffeine is broken down mainly in the liver. It can remain in the blood anywhere from 1.5 to 9.5 hours, depending on various factors.  Smoking speeds up the breakdown of caffeine, whereas pregnancy and oral contraceptives can slow the breakdown. During the third trimester of pregnancy, caffeine can remain in the body for up to 15 hours. 
People often develop a “caffeine tolerance” when taken regularly, which can reduce its stimulant effects unless a higher amount is consumed. When suddenly stopping all caffeine, withdrawal symptoms often follow such as irritability, headache, agitation, depressed mood, and fatigue. The symptoms are strongest within a few days after stopping caffeine, but tend to subside after about one week.  Tapering the amount gradually may help to reduce side effects.
Sources of Caffeine
Caffeine is naturally found in the fruit, leaves, and beans of coffee, cacao, and guarana plants. It is also added to beverages and supplements. There is a risk of drinking excess amounts of caffeinated beverages like soda and energy drinks because they are taken chilled and are easy to digest quickly in large quantities.
Coffee. 1 cup or 8 ounces of brewed coffee contains about 95 mg caffeine. The same amount of instant coffee contains about 60 mg caffeine. Decaffeinated coffee contains about 4 mg of caffeine. Learn more about coffee.
Espresso. 1 shot or 1.5 ounces contains about 65 mg caffeine.
Tea. 1 cup of black tea contains about 47 mg caffeine. Green tea contains about 28 mg. Decaffeinated tea contains 2 mg, and herbal tea contains none. Learn more about tea.
Soda. A 12-ounce can of regular or diet dark cola contains about 40 mg caffeine. The same amount of Mountain Dew contains 55 mg caffeine.
Chocolate (cacao). 1 ounce of dark chocolate contains about 24 mg caffeine, whereas milk chocolate contains one-quarter of that amount.
Guarana. This is a seed from a South American plant that is processed as an extract in foods, energy drinks, and energy supplements. Guarana seeds contain about four times the amount of caffeine as that found in coffee beans.  Some drinks containing extracts of these seeds can contain up to 125 mg caffeine per serving.
Energy drinks. 1 cup or 8 ounces of an energy drink contains about 85 mg caffeine. However the standard energy drink serving is 16 ounces, which doubles the caffeine to 170 mg. Energy shots are much more concentrated than the drinks; a small 2 ounce shot contains about 200 mg caffeine. Learn more about energy drinks.
Supplements. Caffeine supplements contain about 200 mg per tablet, or the amount in 2 cups of brewed coffee.
In the U.S., adults consume an average of 135 mg of caffeine daily, or the amount in 1.5 cups of coffee (1 cup = 8 ounces).  The U.S. Food and Drug Administration considers 400 milligrams (about 4 cups brewed coffee) a safe amount of caffeine for healthy adults to consume daily. However, pregnant women should limit their caffeine intake to 200 mg a day (about 2 cups brewed coffee), according to the American College of Obstetricians and Gynecologists.
The American Academy of Pediatrics suggests that children under age 12 should not consume any food or beverages with caffeine. For adolescents 12 and older, caffeine intake should be limited to no more than 100 mg daily. This is the amount in two or three 12-ounce cans of cola soda.
Caffeine and Health
Caffeine is associated with several health conditions. People have different tolerances and responses to caffeine, partly due to genetic differences. Consuming caffeine regularly, such as drinking a cup of coffee every day, can promote caffeine tolerance in some people so that the side effects from caffeine may decrease over time. Although we tend to associate caffeine most often with coffee or tea, the research below focuses mainly on the health effects of caffeine itself. Visit our features on coffee, tea, and energy drinks for more health information related to those beverages.
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