The caffeine dose used in the study was 100 milligrams. That's roughly the amount of caffeine in an 8-ounce cup of coffee, depending on how you brew it.

Participants were told to avoid caffeine for 12 hours and nicotine for four hours before the study. They were tested twice -- once with caffeine and once with a placebo. The tests were done a day or two apart.

The brain scans showed more activity in brain regions tied to attention and short-term activity with caffeine, compared with the placebo.

It's the first time caffeine has been scientifically shown to have that effect, the researchers write.

The study of nearly 10,000 people showed that those who drank more than two cups of coffee or tea per day developed chronic liver disease at half the rate of those who drank less than one cup each day.

The study, conducted by the National Institute of Diabetes and Digestive and Kidney Diseases and Social & Scientific Systems Inc., found that coffee provided no protection to people at risk of liver disease from other causes, such as viral infections.

"While it is too soon to encourage patients to increase their coffee and tea intake, the findings of our study potentially offer people at high-risk for developing chronic liver disease a practical way to decrease that risk," said Constance E. Ruhl, who helped lead the study.

"Americans get more of their antioxidants from coffee than any other dietary source. Nothing else comes close," says study leader Joe Vinson, Ph.D., a chemistry professor at the university. Although fruits and vegetables are generally promoted as good sources of antioxidants, the new finding is surprising because it represents the first time that coffee has been shown to be the primary source from which most Americans get their antioxidants, Vinson says. Both caffeinated and decaf versions appear to provide similar antioxidant levels, he adds.

He cautions that high antioxidant levels in foods and beverages don't necessarily translate into levels found in the body. The potential health benefits of these antioxidants ultimately depends on how they are absorbed and utilized in the body, a process that is still poorly understood, says Vinson, whose study was primarily funded by the American Cocoa Research Institute.

Besides keeping you alert and awake, coffee has been linked to an increasing number of potential health benefits, including protection against liver and colon cancer, type 2 diabetes, and Parkinson's disease, according to some recently published studies. But there's also a downside: Java can make you jittery and cause stomach pains, while some studies have tied it to elevated blood pressure and heart rates. More research is needed, particularly human studies, to firmly establish its health benefits, Vinson says.

While the findings would seem to encourage people to go out and drink more coffee, Vinson emphasizes moderation. "One to two cups a day appear to be beneficial," he says. If you don't like coffee, consider drinking black tea, which is the second most consumed antioxidant source in the U.S. diet, Vinson says. Bananas, dry beans and corn placed third, fourth and fifth, respectively.