With coffee bars proliferating from Seattle to Baltimore, and the specialty coffees they feature promising to turn around a decades-long decline in American coffee consumption, the news about coffee's effects on health is surprisingly good.
A substantial amount of research, including several large studies done in the last few years, has turned up little solid scientific evidence to indict a moderate intake of coffee or caffeine as a serious or even minor health threat.
"Some of the most serious hazards that were linked to caffeine in the past have not panned out," said Dr. James L. Mills, who studies caffeine's effects on pregnancy at the National Institute of Child Health and Human Development in Bethesda.
After peaking in 1962 at 3.12 cups per person a day, coffee consumption endured a 30-year slide in popularity, finally stabilizing in the mid-1990s around 1.7 cups, according to a survey conducted last winter by the National Coffee Association.
Of course, caffeine is also present in black and green teas and in soft drinks. Eighty percent of Americans consume at least one beverage containing caffeine every day, and among Americans over 18, the per-capita consumption of caffeine is about 200 milligrams a day.
Dr. Mills noted that heavy consumers of caffeine -- those who drink eight or more 5-ounce cups of coffee a day -- tend to be "very overworked, driven people who are generally not the best health risks."
For the average healthy person, about the most serious charge science can levy against caffeine is that it may be addictive. After 18 to 24 hours, its absence, even in those who consume it moderately, sometimes results in withdrawal symptoms, including severe headaches, fatigue, depression and poor concentration.
Concerns about the effects of caffeine on pregnancy and fetal development also persist.
Despite a score of studies on the relationship between caffeine and a woman's ability to conceive and deliver a full-term, full-size, healthy infant, researchers are still arguing about the reproductive risks of even relatively high doses of caffeine before or during pregnancy. All relevant studies have suffered from one or more methodological limitations that might invalidate their findings, scientists say.
While moderate consumption -- usually defined as two to four 5-ounce cups of coffee daily -- has thus far received a relatively clean bill of health even in people at high risk for developing heart disease or cancer, new studies have linked heavier daily intakes to heart attacks and bone loss in women.
In men with mild high blood pressure, several recent studies have shown that a significant rise in blood pressure can occur after just two or three cups of coffee, especially if caffeine is consumed before exercising.
Along with these cautionary findings has come encouraging news about caffeine's potential role in weight control.
Caffeine raises the rate at which the body burns calories for three or more hours after it is consumed, according to studies of healthy volunteers of normal weight in Denmark. Just 100 milligrams of caffeine -- the amount in one cup of coffee or two cans of cola -- can raise the metabolic rate by 3 to 4 percent, and larger intakes raise it even higher. If the consumer also exercises, the caloric burn stimulated by caffeine is greater still. But caffeine is no free lunch for dieters because it also effects the release of insulin, causing blood sugar to fall, which induces hunger pangs.
Caffeine, known chemically as 1,3,7-trimethylxanthine, is one of a class of methylxanthine compounds found in 63 plant products, including tea leaves, cocoa beans and coffee beans.
Similar to amphetamines but milder in its effects, caffeine stimulates the sympathetic nervous system, which regulates the body's automatic functions. As a central nervous system stimulant, it makes people feel more alert, temporarily relieves fatigue and promotes quick thinking. Its contrary effects on blood vessels often render it medically useful: it dilates arteries feeding the heart, increasing blood flow, and constricts arteries in the head, helping to counter migraine headaches.
The other chemicals
But caffeine is only one, albeit the best known, of about 500 chemicals in coffee. Indeed, caffeine was recently absolved of at least one of coffee's reputed ill effects -- the ability to raise serum cholesterol levels. That drawback, researchers in the Netherlands say, stems not from caffeine but from the oils in coffee beans that are extracted when the grounds are boiled. Those oils are not a problem when coffee is brewed through a paper filter, because the oils are left behind.
Decaffeinated coffee has lately captured a growing body of devotees who spurn caffeine's stimulating effect but still covet coffee's flavor and social attributes. Yet, research sponsored by the National Institutes of Health and directed by Dr. H. Robert Superko while at Stanford University has suggested that decaffeinated coffee is more likely than its caffeine-rich counterpart to raise levels of artery-damaging LDL cholesterol.
Sorting out the health effects of drinking coffee, with or without caffeine, is complicated by the fact that coffee drinkers are, on the whole, a different breed from drinkers of other beverages, including caffeine-containing tea. At least three large studies conducted here and in Europe have shown that coffee drinking, especially at high levels, is associated with behavior that is linked to serious illnesses, like cigarette smoking.
Decaf is different
Furthermore, decaf drinkers are different from those who drink coffee with caffeine.
In a study of 2,677 adults by Alan Leviton and Elizabeth N. Allred of Harvard Medical School and Boston Children's Hospital, which was published last year, women who drank only decaffeinated coffee behaved more like women who drank no coffee at all than like women who consumed coffee with caffeine.
Decaf drinkers in the study were more likely than other women to take vitamin supplements, eat vegetables in the cabbage family, use seat belts routinely and exercise regularly. The men who drank decaf exclusively typically weighed less and were more likely to consume a low-fat diet and to eat such vegetables.
To determine whether drinking coffee increases the risk of disease, researchers must take into account many "lifestyle" factors, since these factors and not coffee or caffeine could be responsible.
In studies that have taken other risk factors into account, people who consume fewer than four or five cups of coffee a day seem to incur no added cardiac risk, even if they already have clogged coronary arteries or irregular heart rhythms. Several studies found no drop in blood cholesterol levels when people with or without high cholesterol switched to decaf. But cardiac trouble begins to accrue at five or more five-ounce cups of coffee a day among both smokers and nonsmokers and especially in people with high blood pressure.
In 1981, coffee aficionados were momentarily dismayed by a report from Harvard researchers linking coffee (with and without caffeine) to pancreatic cancer, a disease with a very poor prognosis. But at least seven major studies, including one conducted last year in a large retirement community, have failed to find such a link, and the Harvard researchers, after further analysis of their data, retracted their findings.
And in several large studies, including a continuing Harvard-based study of 121,700 nurses, coffee drinkers have developed fewer breast cancers than abstainers.
A similar inverse relationship was found between heavy coffee consumption and cancers of the colon and rectum in a study of 1,255 cancer cases and 3,883 matched patients with unrelated conditions. Those who consumed five or more cups of coffee a day had a 40 percent lower risk of developing colon cancer, according to a research team headed by Dr. Lynn Rosenberg of the Boston University School of Public Health.
The first link reported between coffee and cancer concerned the bladder. But after analyzing 35 studies of the coffee-bladder cancer relationship, Yale University researchers concluded in 1993 that regular coffee consumption was not a "clinically important" risk factor for bladder cancer in men or women. Soon after, however, another study linked heavy coffee consumption to bladder cancer. And so the debate continues.
Caffeine has an undisputed negative effect on calcium metabolism. When exercise levels and smoking are taken into account, women who consume caffeine lose more calcium in their urine and have less dense bones than do nonconsumers, and thus may be more prone to fractures. But among more moderate coffee consumers, drinking just one glass of milk a day can offset the calcium loss induced by the caffeine in two cups of coffee, a recent study of nearly 1,000 postmenopausal women in southern California showed.
In 1980, following a study in which pregnant rats that were force-fed the human equivalent of 56 to 87 cups of strong coffee gave birth to pups with missing toes, the Food and Drug Administration warned pregnant women to avoid or at least moderate their consumption of caffeine. Two years later, the agency's concerns were reinforced by a study of 12,000 pregnant women that linked drinking four or more cups of coffee daily to premature birth and low birth weight. But while smoking turned out to be the chief culprit, the agency has yet to ease its warning.
A study of more than 7,000 Canadian women linked increasing doses of caffeine to a rising risk of fetal growth retardation, but there was no rise in premature births or low birth weight. The newest study, published this year by researchers at the University of North Carolina, also failed to document a link between caffeine consumption during pregnancy and pre-term births.
Still, enough research has suggested that caffeine may delay conception, increase the risk of miscarriage and slow fetal growth to prompt many public-health specialists to advise women planning pregnancy, as well as those who are already pregnant, to eliminate caffeine or restrict its consumption to the amount in one or two cups of brewed coffee a day.