Diabetes is a perfect example. At least six large-scale, long-term studies have shown that men who drink 7 to 10 cups of coffee a day are 50% to 80% less likely to develop Type 2 diabetes than non-drinkers. In one of the largest such studies, which included more than 125,000 healthy people, women who drank at least 6 cups of coffee a day were 29% less likely to develop the disease than women who didn't drink coffee. Results were published in 2004 in the journal Annals of Internal Medicine. More recent analyses show that both regular coffee and decaf are equally protective, Graham says.
Pure caffeine, in fact, can exacerbate the symptoms of diabetes. Graham and colleagues have done studies in which they controlled levels of sugar and insulin in their patients' blood. Insulin is the hormone that allows cells to remove sugar from the bloodstream, and diabetics can't make enough of it. These experiments show that a given amount of insulin can process 30% to 40% less sugar when caffeine is around, Graham says. In other words, when you drink caffeine and then eat some carbs, 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. In some of his studies, participants abstain from caffeine for two days. Then, on the morning of the study, they take a pill containing 300 to 350 milligrams of caffeine (the amount in three standard cups of coffee) before eating or drinking a carbohydrate-rich meal or drink.
If people are at a healthy weight, results show, they can usually maintain stable sugar levels -- by making more insulin to compensate for caffeine's sugar-spiking effects. People who are overweight or diabetic, on the other hand, often can't produce more insulin, and their blood sugar rises. Even when they eat a meal three hours later, the effect of the caffeine is the same.
"It has an extended response," Graham says, "and it's a response that one would interpret as far from optimal for health."
A boost for athletes
Caffeine is a common ingredient in diet pills, because it forces fat cells to release fat into the blood, but that does not translate into fat burning or weight loss -- common myths with no evidence to back them up.
In serious athletes, though, caffeine can enhance both speed and endurance by helping muscles contract with slightly more force. In a 1998 study, for example, Graham and colleagues challenged nine runners to push themselves to exhaustion on a treadmill at a strenuous pace. Before each of five efforts and without knowing what they were swallowing, runners took a caffeine capsule, a placebo capsule, caffeinated coffee, decaf coffee or decaf with caffeine added (to see if decaffeination was altering the coffee in ways other than simply removing caffeine).
On average, the athletes were able to run for about 32 minutes in both non-caffeinated trials. With a boost from caffeine pills and caffeinated decaf, they ran 7 to 10 minutes longer, Graham reported in the Journal of Applied Physiology. Oddly enough, the regular coffee had no effect, even though blood tests showed that there was just as much caffeine in the runners' bodies in all caffeinated trials.
"We kept testing more subjects because we didn't believe the data," Graham says.
Caffeine in the form of sports drinks, soda or pills can improve endurance, he says, but coffee doesn't work. "Our only conclusion is that there must be factors in coffee that work in the opposite fashion." Some studies suggest that a class of antioxidants called chlorogenic acids might have something to do with it, he adds.
Still despite that research, caffeine probably won't help the average Joe Treadmiller. A 1991 study in France found that even though highly trained swimmers sprinted faster in the 100-meter freestyle after taking caffeine, the stimulant didn't increase the speed of untrained swimmers. Only when muscles are pushed past the point of exhaustion, Graham says, does caffeine kick in to give them extra oomph.
No verdict yet
Despite evidence that caffeine causes bones to leach calcium and fears that caffeine can cause early contractions or make it hard to conceive, 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."
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How much punch they pack
Healthful, unhealthful or somewhere in between, caffeine is likely safe at 300 milligrams or less per day. Here's how it adds up in a variety of common drinks and pills.