The mechanisms behind the bioactive compounds in coffee, tea, and chocolate are more complex than they first appear.
Counterintuitive Effects
Caffeine is a methylxanthine that works in the brain by blocking adenosine receptors and is structurally similar to adenosine. Research on volunteers has shown that as caffeine concentration rises, it can stop functioning like a stimulant. At an amount roughly equivalent to five cups of espresso, caffeine not only fails to stimulate the central nervous system but can produce a calming effect.
The Habit Factor
Science gives mixed assessments of caffeine’s addictive potential. While the body can become accustomed to caffeine, habituation is not the same as addiction. The mild dependence many people develop usually does not cause severe withdrawal symptoms when they quit coffee. With high consumption (daily doses of 300 milligrams of caffeine or more), adenosine activity increases, and stopping coffee is often followed only by a temporary calming effect. Longtime coffee drinkers commonly find their well-being returns to normal surprisingly quickly.
The Heart’s Limits
Chronic hypertension is not common among regular coffee drinkers: even consuming up to a liter of coffee a day tends to raise systolic blood pressure by at most about ten points. Only extreme overdoses can be fatal. Toxicologists warn that very large doses of caffeine can leave little chance of survival. When consumed prudently, caffeine poses little harm to the heart. In fact, arrhythmia is diagnosed less often in habitual coffee drinkers than in people who abstain entirely.
Questioning the Risks
Despite caffeine’s reputation as a potentially harmful substance, coffee drinkers appear to have lower rates of some malignant tumors. Some clinicians even use caffeine to help deliver anti-tumor agents to organs. Doctors do express concern about high caffeine intake during pregnancy: a commonly cited safe limit is 200 milligrams of caffeine per day.
A Dangerous Duo
Combining coffee with alcohol is strongly discouraged. Caffeine counteracts alcohol’s depressant effects, creating a false sense of sobriety that can lead to loss of self-control and increased intoxication. That combination raises the heart’s susceptibility to arrhythmia, which in some cases can lead to malfunction even with a small amount of caffeine. After a series of deaths among young people linked to arrhythmias and heart attacks, some countries banned caffeinated alcoholic energy drinks.
A Toxic Relative
Theobromine is a close chemical relative of caffeine. While it is absent from coffee, it is abundant in cocoa and its derivatives (chocolate). Theobromine can be more toxic than caffeine because cocoa contains it in much larger amounts than coffee contains caffeine (roughly 2–10% in cocoa products). Smaller amounts of theobromine (about 1–2%) occur in tea leaves, guarana berries, and kola nuts. Theobromine can speed the heart rate, improve coronary blood flow, dilate bronchi, and relax smooth muscle. By increasing the heart’s oxygen demand, a theobromine overdose can alter myocardial function. Theobromine is a dangerous poison for dogs, cats, horses, and rodents: the lethal dose for these animals is about 200–300 milligrams of the alkaloid per kilogram of body weight.
The Secret of Tea
Tea’s antioxidant properties come from polyphenols—particularly catechins—and from methylxanthines such as theophylline. Like caffeine, theophylline has diuretic effects. For that reason, avoid using tea to wash down medications. Theophylline can weaken the action of beta-blockers used in cardiology and increase the diuretic effect of furosemide.
