Over the years coffee has been blamed for causing everything from high blood pressure and high cholesterol (and thus heart disease) to pancreatic cancer, fibrocystic breast disease, and bone loss. The main focus has been on caffeine, one of the most extensively studied substances in food. But in nearly every instance early research linking coffee or caffeine to health problems has been refuted by better subsequent studies. "Not guilty" has repeatedly been the verdict. The pendulum has swung so far that some researchers now suggest that coffee may actually have health benefits.

Coffee usually contains anywhere from 60 to 120 milligrams of caffeine in six ounces. Caffeine's benefits are well known. It is a mild psychoactive substance--it stimulates the central nervous system. Thus, it improves reaction time, mental acuity, alertness, and mood, wards off drowsiness, and helps millions wake up and feel better in the morning. It also has an analgesic effect, which is why it is added to some pain relievers.

Like all plant foods, coffee (derived from a bean) contains many naturally occurring chemicals--more than 1,000 have been identified so far--some potentially harmful, some potentially healthful. As in tea, many of the beneficial substances are antioxidants, which help protect against cell-damaging free radicals, and thus may reduce the risk of heart disease and cancer. In fact, a recent study found that coffee is the No.1 source of antioxidants in the U.S., largely because Americans drink so much of it.

If that's not enough, a handful of recent studies have linked coffee or caffeine to a reduced risk of several diseases:

• Type 2 diabetes. In February researchers looking at 88,000 women in the Nurses' Health Study at Harvard found that those who drank at least two cups of coffee a day--regular or decaf--have a lower risk of diabetes. Several previous studies of men and women have had similar findings, though in men it may take at least four cups a day to have this effect.

• Parkinson's disease. In a well-designed study from Honolulu, men who drank no coffee were two to three times more likely to develop the disease than those who drank one to four cups a day. Decaf was not included in the study.

• Symptomatic gallstone disease. In 2002 another part of the Nurses' Health Study suggested there's a protective effect, but it seemed to take four cups a day. Caffeine is thought to be the primary protective ingredient. Three years earlier the same researchers found a reduced risk in men.

• Liver damage. Regular coffee and tea may help prevent liver disease in people at high risk (due to alcoholism, obesity, or diabetes, for instance), according to a recent study in Gastroenterology.

Nervous about coffee

It's easy to see why some people would worry about coffee. First of all, because caffeine is a stimulant, it can cause jitters and insomnia. It can also boost heart rate, which is why people with certain heart problems are sometimes advised to avoid it. Coffee can also cause stomach upset and heartburn.

Moreover, the effects of coffee can be confusing to evaluate. Here are some examples:

• Caffeine can indeed raise blood pressure and heart rate briefly in those not used to it, though this effect varies greatly from person to person. The key question is whether habitual coffee drinking leads to hypertension. And the answer is no, according to most research, including a recent study of 155,000 women.

• A few studies have found that large quantities of unfiltered, European-style coffee (regular or decaf) can boost blood cholesterol slightly. However, paper filters seem to trap whatever culprits may be in the grounds. Most research has found no increase in cholesterol or cardiovascular risk from coffee drinking in general.

• While caffeine was a suspected risk factor for weak bones, that may be because people who drink lots of coffee tend not to consume milk, thus missing out on calcium and vitamin D.

• Overall, people who drink lots of coffee are more likely to smoke, eat poorly, and drink too much alcohol. Researchers take such factors into consideration, but they can't adjust for all of them.

Note for pregnant women: Some studies have indicated that high doses of caffeine may raise the risk of miscarriage and birth defects and possibly reduce fertility. Even though the evidence for this is not clear-cut, to be safe, pregnant women should drink no more than two cups a day.

Words to the wise: There's no health reason to deprive yourself of coffee if you like the lift it gives and the sociability it affords, unless you suffer adverse effects. On the other hand, though it won't be surprising if coffee producers and servers soon start promoting coffee as a health drink, the potential benefits are still too uncertain to lead anyone to start drinking coffee. Drink it only if you enjoy it.