A triple nonfat mocha may taste good, but it's likely the jolt that drives millions of people to fork over three bucks or more for the steaming cup of liquid. Between 80 percent and 90 percent of North Americans consume caffeine regularly, according to a 2004 review, with an average daily consumption equal to about two mugs of coffee or four 16-ounce bottles of soda.
The habit has become less guilt-inducing recently, with growing evidence that both coffee and tea can fight cancer, heart disease, diabetes, Parkinson's disease and more. Because most people equate these beverages with the caffeine in them, it's tempting to conclude that the stimulant is what gives these wonder drinks their powers.
That may not be the case. Caffeine's effects on health appear to be considerably more nuanced.
On the plus side, the drug does appear to help protect the brain from degenerative disease and, for many, keep the brain's gears churning, which is what drives most people to drink it in the first place. But habitual and large doses of caffeine can also stress the heart and interfere with insulin's ability to process sugar. And many of the benefits ascribed to caffeine may be due, in fact, to other chemicals that outweigh caffeine's negative effects.
"As soon as you say coffee, people think caffeine; as soon as you say caffeine, people think coffee," says Terry Graham, a metabolic physiologist at the University of Guelph in Canada, a longtime caffeine researcher who recently organized an international symposium on caffeine and health. Scientists have isolated antioxidants, polyphenols and micronutrients from coffee and tea, but there have been no long-term studies of how each ingredient, including caffeine, affects the body on its own or within a beverage.
"There are health benefits of coffee that have nothing to do with caffeine," Graham says. In fact, he adds: "There are no health benefits I've ever seen documented for caffeine."
Anyone who has used a latte to get through an after-lunch meeting - or ended up with stomach-churning jitters after one too many shots of espresso - knows that ingesting caffeine has physical consequences. The molecule, which is identical in every beverage that contains it, blocks the action of a naturally produced chemical called adenosine.
Produced in the brain, adenosine normally accumulates during activity and declines during sleep. By the end of the day, it dulls the nervous system, causing drowsiness and a sense of calm and helping the body fall asleep. When caffeine gets in its way, by binding to adenosine receptors throughout the body, the body reacts as if a tiger were about to attack. Blood vessels constrict, causing a rise in blood pressure. The liver releases extra sugar into the bloodstream. A rush of excited energy creates a feeling of alertness, even invincibility.
That feeling, to many people, is a good thing. When asked to rate their alertness, many dozens of studies published since the 1960s show that caffeine users report feeling more awake than do people who have been given a caffeine-free pill or beverage, says biological psychologist Peter Rogers, lead researcher of the Dietary Caffeine and Health Study at the University of Bristol in Britain.
Studies that link caffeine with mental skills and quick reflexes, however, may simply prove how addictive the drug is, Rogers says.
"What we find is that if you take caffeine away from caffeine drinkers, they perform worse than when they get caffeine and they perform worse than people who don't normally get caffeine," Rogers says. "The traditional interpretation is that people do better on caffeine and that caffeine does make you more alert."
It may be withdrawal that makes people falter, he says, not caffeine that makes them sharper. If they're not already coffee drinkers, taking up the habit won't help.
It could, however, hurt their hearts. Studies show an average blood pressure increase of 4/2 millimeters of mercury (mmHg) after taking caffeine, according to a 2004 review of research published in the journal Psychosomatic Medicine. That small spike won't matter much for most people, but it could exacerbate the condition of someone on the edge of heart problems, says Jim Lane, a psychophysiologist at Duke University Medical Center in Durham, N.C., who has extensively studied the relationship between caffeine, stress and health.
A number of long-term studies have compared the health of people who choose to guzzle coffee and tea with those who avoid the stuff. And those studies are turning up perplexing results that contradict the isolated effects of caffeine.
Some data, for example, suggest that drinking coffee might help prevent heart disease. In a study published in February in the American Journal of Clinical Nutrition, scientists from the City University of New York and State University of New York Health Science Center at Brooklyn followed more than 6,500 healthy adults for nearly nine years. In people older than 65, results showed, those who drank four or more cups of coffee a day were about half as likely to die from cardiovascular disease as their coffee-free peers.
Coffee contains thousands of compounds that might be responsible for such benefits, but polyphenols, similar to the healthful compounds found in chocolate and red wine, as well as antioxidants are the most likely candidates.
Several large studies have linked large daily doses of coffee with lower rates of Parkinson's disease, as well. One theory is that regular caffeine consumption makes the brain more sensitive to adenosine, which has been shown to help protect the brain from ministrokes caused by oxygen and sugar deprivation, Rogers says.
And coffee drinking has also been linked to lower rates of liver cancer, Type 2 diabetes and gout, among other ills. However, scientists warn, correlations such as these don't prove that the drink itself (or any ingredient in it) is responsible for the protective effect. In some cases, the benefits probably come from other ingredients.
Diabetes is a perfect example. At least six large-scale, long-term studies have shown that men who drink seven to 10 cups of coffee a day are 50 percent to 80 percent less likely to develop Type 2 diabetes than nondrinkers. More recent analyses show that both regular coffee and decaf are equally protective, Graham says.
However, pure caffeine can exacerbate the symptoms of diabetes. Graham and colleagues have done studies that show when you drink caffeine and then eat carbohydrates, the level of sugar in your blood goes up, forcing the production of more insulin to compensate for the extra sugar. And once sugar levels are up, they tend to stay up, Graham says, particularly in people who are obese or diabetic.
"It has an extended response," Graham says, "and it's a response that one would interpret as far from optimal for health."
Still, most scientists agree that a daily dose of 300 milligrams or less is safe for most people, including pregnant women. And as research continues, scientists may yet find more evidence to make our morning lattes seem as good for our bodies as they are for our brains.
"If you're not drinking coffee or tea, don't start because of the health benefits," says Michele Tuttle, a registered dietitian in Columbia, Md. "But if you're already drinking them, enjoy in moderation."
Emily Sohn wrote this article for the Los Angeles Times.
Here are some everyday products and their caffeine content:
Regular brewed coffee (8 ounces): 135 milligrams
Starbucks coffee (8 ounces): 130-327 milligrams *
Instant coffee (8 ounces): 95 milligrams
Espresso (1 ounce): 30-50 milligrams
Excedrin (two tablets): 130 milligrams
Red Bull (8.5 ounces): 80 milligrams
Black tea (8 ounces): 40-70 milligrams
Mountain Dew and Diet Mountain Dew (12 ounces): 55 milligrams
Diet Coke (8 ounces): 34 milligrams
*depends on brewing process