An early “study” of the relative safety of coffee and tea was conducted in the 18th century by Sweden’s King Gustaf III. The king commuted the death sentence of a pair of identical twins on the condition that one drink tea and the other drink coffee three times a day. The imprisoned tea-drinker died first at age 83.

While that experiment didn’t prove much, recent and more sophisticated studies have cast light on the relative health effects of coffee and tea.

• Diabetes. Coffee contains substantial amounts of magnesium and other substances that may help the body regulate sugar metabolism and prevent diabetes. Heavy coffee drinkers – those averaging at least four cups a day – were less likely to develop diabetes, according to two large studies reported in 2004. While the news is reassuring for coffee drinkers, it’s premature to pick up the habit for this still-unproven benefit.

Tea drinking was not found to be protective against diabetes in those studies. But some research suggests that green tea may reduce insulin resistance, which might help fight type 2 diabetes.

• Cancer. Both coffee and tea are rich in a class of disease-fighting antioxidants called polyphenols. But in coffee, roasting the beans destroys most of these compounds. Green, black, and oolong teas retain more polyphenols after processing than coffee, and studies of tea drinkers have suggested a possible link between tea and reduced risks of various cancers, including stomach, skin, and breast.
• Heart disease. While studies have found that heavy coffee drinkers have a greater risk of heart attack, habitual tea drinkers appear to gain protection. A combined analysis of 13 studies estimated an 11 percent lower rate of heart attack among people who drank three cups of tea a day compared with those who drank no tea. In a 2003 clinical trial, people who took an extract of green tea in capsule form reduced their high cholesterol level.

Coffee, with its greater caffeine kick, tends to raise blood pressure and other ingredients in unfiltered coffee can sharply increase cholesterol levels. In a 2004 study in Finland, which tracked a large group of men for 14 years, those who drank more than three 8-ounce cups of coffee a day were 43 percent more likely to have a heart attack or die from heart disease than those who kept daily consumption down to one and one-half to three cups. But overall, people who drink one or two cups a day don’t seem to face a greater risk of heart problems or chronic high blood pressure.

A 2003 study even suggests a possible protective effect at moderate levels. Those who drank a daily cup or two of coffee were less likely to suffer heart problems. But those who drank two and one-half cups or more per day were three times as likely to have heart trouble as noncoffee drinkers.

• Parkinson’s disease. Caffeine, whether it’s from coffee, tea, or some other source, appears to be linked to a lower risk of Parkinson’s disease. Caffeine may protect the brain by boosting the supply of dopamine, a chemical messenger of nerve cells. In one long study, a 30-year effort involving 8,000 men, those who consumed the caffeine equivalent of three to five cups of coffee a day had one-fifth the risk of those who abstained from coffee. Women, however, appear to get no such benefit from caffeine. Even in men, direct evidence for protection from Parkinson’s is still lacking, so count those results as preliminary.
• Gallstones. Lab studies show that substances in coffee stimulate gallbladder function, and they also prevent the crystallization of bile into “stones.” A 1999 study of about 46,000 men found that daily drinkers of two to three cups of coffee were 40 percent less likely to develop gallstones. A more recent study of 81,000 women found that female coffee drinkers also appear to gain protection from gallstones. Tea and decaffeinated coffee didn’t offer any apparent protection.
• Healthy bones. Two studies since 2000 have reported higher bone-density measures among tea drinkers in China and Britain. But the caffeine in two to three cups of coffee a day has been shown to accelerate bone loss in postmenopausal women.
• Caffeine concerns. At high doses, coffee can result in lower birth weight in babies and may trigger miscarriage in pregnant women. In one study, drinking more than five cups a day appeared to double the risk of spontaneous abortion.

To be on the safe side, women who are pregnant or likely to become pregnant should drink no more than two cups of coffee a day. Coffee can stimulate heartburn in some people, and some evidence suggests it may worsen anxiety problems and contribute to the development of fibrous (benign) breast lumps. People with heartburn, breast lumps, or anxiety problems may want to try cutting back on coffee to see if it helps with the condition.