Probably the most important effects of caffeine are its ability to enhance mood and mental and physical performance. At consumption levels up to 200 milligrams (the amount in about 16 ounces of ordinary brewed coffee), consumers report an improved sense of well-being, happiness, energy, alertness and sociability, Roland Griffiths of the Johns Hopkins School of Medicine reported, although higher amounts sometimes cause anxiety and stomach upset.

Millions of sleep-deprived Americans depend on caffeine to help them make it through their day and drive safely. The drug improves alertness and reaction time. In the sleep-deprived, it improves memory and the ability to perform complex tasks.

For the active, caffeine enhances endurance in aerobic activities and performance in anaerobic ones, perhaps because it blunts the perception of pain and aids the ability to burn fat for fuel instead of its carbohydrates.

Recent disease-related findings can only add to coffee’s popularity. A review of 13 studies found that people who drank caffeinated coffee, but not decaf, had a 30 percent lower risk of Parkinson’s disease.

Another review found that compared with noncoffee drinkers, people who drank four to six cups of coffee a day, with or without caffeine, had a 28 percent lower risk of Type 2 diabetes. This benefit probably comes from coffee’s antioxidants and chlorogenic acid.

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