"When researchers exposed mice to a chemical that causes a loss of dopaminergic neurons in a pattern similar to that observed in Parkinson's disease, those that had first been given caffeine equivalent to moderate amounts of coffee in humans lost fewer neurons than those not given caffeine," explains Ascherio.
Caffeine also seems to protect human brain cells.
In a meta-analysis that pooled 13 studies, drinkers of regular coffee—but not decaf—had a 30 percent lower risk of Parkinson's than non-drinkers.1
And it doesn't take much caffeine—just 100 to 200 milligrams a day. "Even a modest amount—the equivalent of between one and two cups of coffee per day—is associated with a lower risk," notes Ascherio.
Caffeine doesn't protect everybody, however.
"In our study, women who took postmenopausal hormones didn't benefit," says Ascherio. That's consistent with animal studies, in which caffeine doesn't protect female mice against Parkinson's if they're also given estrogen.
In the Nurses' Health Study, which tracked nearly 81,000 women for 20 years, and the Health Professionals Follow-Up Study, which tracked 46,000 men for 10 years, those who drank two to three cups of regular coffee a day had about a 20 percent lower risk of gallstones than non-drinkers.2, 3
"Tea, decaf coffee, and caffeinated soft drinks weren't protective, probably because they don't contain enough of what's making the difference— caffeine," says Michael Leitzmann of the National Cancer Institute in Bethesda, Maryland.
One theory: caffeine may stimulate the gallbladder to contract, which helps empty it of stone-forming cholesterol and bile pigments.
"Caffeine improves alertness and reaction time in people, whether they're habitual consumers of caffeine or not," says Harris Lieberman, a psychologist and caffeine investigator at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts.
But "the effect is clearly limited to the ability to maintain attention," he adds. "Things like memory or complex reasoning won't improve."
In the sleep-deprived, however, caffeine has a more striking impact. "It improves almost everything you can measure," says Lieberman. "It makes you more alert, it seems like you can perform complex tasks better, and your memory is better."
"People who are falling asleep on the job can't do much of anything," says Lieberman. "If you give them something that wakes them up and makes them focus, they're going to do better."
That's why the military is studying caffeine.
"When it's not possible for soldiers to get sufficient sleep," says Lieberman, "we recommend moderate doses of caffeine — 100 or 200 mg — to help maintain alertness and performance." In fact, the Army is about to put chewing gum with caffeine into some rations.
It's not just drowsy soldiers who can benefit from a caffeine jolt. Thousands of drivers are alive today thanks to a well-timed cup of coffee.
(Remarkably brave) French researchers accompanied young males as they drove 125 miles on a highway between 2 a.m. and 3 a.m. When the young men were given coffee with 200 mg of caffeine before getting behind the wheel, they inadvertently crossed into another lane an average of two times during their drive. When they were given decaf, they crossed over an average of six times. 4
It's no coincidence that people offer guests a cup of coffee.
After consuming anywhere from 20 mg to 200 mg of caffeine, "people report increased well-being, happiness, energy, alertness, and sociability," says caffeine expert Roland Griffiths of the Johns Hopkins School of Medicine in Baltimore.
That may be why a study that tracked more than 86,000 women for 10 years found that those who drank at least two cups of regular coffee a day were about 60 percent less likely to commit suicide than those who drank none. 5 (More than 200 mg of caffeine produces "increased anxiety, nervousness, jitteriness, and upset stomach" in some people, says Griffiths.)
Caffeine helps the body burn fat instead of carbohydrate, and it blunts the perception of pain. Both can boost endurance.
For example, endurance runners who ran to exhaustion on a treadmill lasted an average of 32 minutes without caffeine, but made it to 42 minutes after drinking coffee with around 250 mg of caffeine. 6
And you don't have to be a trained athlete to benefit.
"There's no question that caffeine will improve aerobic physical endurance innon-athletes as well," says Lieberman, adding that people who run, jog, swim, or cycle can last longer if they've had 200 mg to 600 mg of caffeine beforehand. "And new research suggests that caffeine can also improve anaerobic performance," he adds. That includes lifting heavy objects and sprinting short distances.
When you get a headache, the blood vessels in your brain dilate, or become wider. Caffeine causes blood vessels to constrict, which may explain why it can help relieve headache pain.
"It's also a mild analgesic, or painkiller, and it has the ability to increase the availability of other analgesics that it's combined with," says Robert Shapiro, a headache expert at the University of Vermont College of Medicine in Burlington.
That's why caffeine is included in prescription headache medications like Fiorinal and over-the-counter ones like Anacin and Excedrin.
"Consuming caffeine within three to five hours of bedtime will disturb the sleep of most people," says Tim Roehrs, director of research at the Sleep Disorders Center of the Henry Ford Hospital in Detroit. The caffeine interferes with adenosine, which many scientists believe is the brain's natural sleep regulator (see illustration "How Caffeine Works" above).
They may have trouble falling asleep. And once they do, "they'll experience frequent, brief awakenings during the night that they won't be aware of but that will diminish the restorative effect of their sleep," says Roehrs.
And that can start a cycle. "The next day they'll feel tired, so they'll consume more caffeine to stay alert. And that may disrupt their sleep the next night."
A 1988 National Institutes of Health study reported that as little as one cup of coffee a day could slash in half the odds of becoming pregnant. But subsequent studies have found that if caffeine affects fertility, it takes at least 300 milligrams a day. "Low to moderate caffeine consumption doesn't seem to reduce a woman's chance of becoming pregnant," says the Organization of Teratology Information Specialists.
A new study suggests that too much caffeine may increase the risk of miscarriage. Among 1,063 pregnant women interviewed by the researchers, 24 percent of those who consumed at least 200 mg of caffeine a day suffered miscarriages, compared with 10 percent of those who consumed less than 200 mg. 1 In response to the study, the March of Dimes recommends that "women who are trying to conceive or who are pregnant limit their caffeine intake from all sources to 200 mg or less every day."
Doses of caffeine more than 15 times greater than the amounts people normally consume can cause birth defects in animals. What about in humans?
According to two new reviews, there is no evidence that consuming the equivalent of three or more cups of coffee a day increases the risk of heart malformations and cleft palates.2, 3
However, the data aren't strong enough to say that there is absolutely no increase in risk. For one thing, the studies relied on the memories of women who had given birth to infants with birth defects. They might subconsciously have inflated estimates of their past coffee consumption if they believed that coffee increased risk.
The Food and Drug Administration's advice: "Pregnant women should avoid caffeine-containing foods and drugs, if possible, or consume them only sparingly."
Trying to sober up quickly?
"The research suggests that caffeine does reverse some of the effects of alcohol," says Johns Hopkins's Roland Griffiths. "But it's more likely to reverse the subjective effects than the performance effects."
In other words, people who are both inebriated and caffeinated will think they're okay, but their reaction time and judgment will still be impaired. And that could make them even more dangerous to themselves and to others, says Griffiths.
"College kids who are using combinations of caffeine and alcohol are more likely to be involved in accidents than those consuming just alcohol."
While caffeine can help relieve headache pain, "daily exposure appears to lower the threshold for provoking migraines in people who are genetically susceptible to them," warns the University of Vermont's Robert Shapiro.
And that might increase their risk of chronic daily headache syndrome—headaches 20 to 25 days a month.
"Caffeine may provide temporary relief," says Shapiro, "but the overall quality of life will be diminished by more frequent headaches and the symptoms of migraines."
Shapiro advises his patients with a history of occasional migraines to consume caffeine no more than two days a week.
Heart Disease. In a meta-analysis that pooled 10 studies that tracked more than 400,000 men and women for three years to 44 years, those who drank coffee every day — whether regular or decaf — were no more likely to suffer from heart disease than non-coffee drinkers. 1 If caffeine were a threat to the heart, coffee drinkers would have had a higher risk.
Cancer. While earlier studies hinted that coffee increases the risk of pancreatic and kidney cancer, "it is unlikely that coffee has a substantial effect on risk," concluded an international panel of scientists last year after reviewing 66 studies of coffee and pancreatic cancer and 25 studies of coffee and kidney cancer.
According to two recent meta-analyses that looked at 10 studies, people who drink coffee have about half the risk of liver cancer compared to non-coffee drinkers. 2, 3 But the evidence is too limited to conclude that coffee is responsible, said the international panel. If further studies conclude that coffee protects against liver cancer, researchers will have to figure out whether it's the caffeine — or something else in the coffee — that's responsible.
Diabetes. Coffee drinkers have a lower risk of diabetes, but it's probably due to something other than the caffeine, says diabetes researcher Rob van Dam of the Harvard School of Public Health. In nine studies that tracked some 200,000 men and women for six years to 20 years, those who drank four to six cups of coffee a day were 28 percent less likely to be diagnosed with diabetes than those who drank no more than two cups a day. But in the two studies that looked at regular and decaf separately, people who drank more decaf also had a lower risk. 4
Why would coffee — but not caffeine — have an impact on blood sugar?
One theory, according to van Dam: "Chiorogenic acid, a major component of coffee, can delay glucose from being absorbed from the intestines, which would reduce spikes in blood sugar after a meal."
High Blood Pressure. Consuming caffeine every day for one week to several months can raise blood pressure by an average of 4 points (systolic) over 2 points (diastolic). But the rise seems temporary. In a study that followed more than 155,000 women for 10 years, those who drank regular or decaf coffee had no higher risk of hypertension than non-coffee drinkers. 5
Osteoporosis. You lose up to 5 milligrams of calcium for every six ounces of regular coffee (or two cans of cola) you drink. But you can offset the loss by adding one or two tablespoons of milk to your coffee or increasing your milk intake by that much, notes osteoporosis expert Robert Heaney of Creighton University in Omaha.
Dehydration. Contrary to what many believe, caffeine isn't a diuretic, at least not in the amounts most people consume. In eight studies, those who drank beverages containing 45 mg to 225 mg of caffeine produced no more urine than they did when they drank caffeine-free beverages. Ditto for nine of 13 studies in which people consumed 240 mg to 550 mg of caffeine.6
"Caffeinated fluids contribute to the daily human water requirement in a manner that is similar to pure water," says hydration expert Lawrence Armstrong of the University of Connecticut.
Caffeine was a diuretic, however, in the two studies that looked at more than 575 mg.
PMS. In the 1980s, surveys of Oregon college women suggested that caffeine consumption was "strongly related" to premenstrual syndrome. 7 That's one reason why women who are suffering from PMS are often told to avoid caffeine. Since then, more detailed studies by other researchers have failed to find a link. Among 3,300 middle-aged women enrolled in the Study of Women's Health Across the Nation, for example, there was no evidence that caffeine played a role in PMS. 8
Weight Loss. Consuming 100 mg of caffeine raises the metabolic rate by about 5 percent over the course of a day. 9 That's equivalent to burning 75 to 110 extra calories. Double the caffeine and you double the increase in metabolic rate.
Yet there is little evidence that consuming caffeine leads to weight loss over the long term. For example, when Harvard researchers tracked the weight of more than 58,000 nurses, dentists, pharmacists, and other health professionals for 12 years, women who increased their caffeine intake by about 140 mg a day — typically by drinking more coffee — gained only about four ounces less than women who didn't change their caffeine consumption. And men who increased their caffeine intake by about 200 mg a day gained an average of five ounces more.
If more caffeine means more calories burned, why doesn't it seem to help people shed pounds?"The long-term metabolic effects of caffeine are often weaker than its short-term effects because people develop a tolerance to caffeine," says Harvard's Rob van Dam.
And, of course, people may unconsciously compensate for burning extra calories by eating extra food during the day.
Growth. There is no good research on whether coffee or caffeine stunts growth in children.« back to “Interesting Ideas”