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DURHAM, N.C. -- A study by researchers at Duke University Medical Center shows that caffeine taken in the morning has effects on the body that persist until bedtime and amplifies stress consistently throughout the day. These results show for the first time that the effects of caffeine last considerably longer than originally thought, said the scientists, and that caffeine exaggerates stress in people who consume it every day.

The study, which was funded by the National Institutes of Health, appears in the July/August 2002 issue of Psychosomatic Medicine.

"The effects of coffee drinking are long-lasting and exaggerate the stress response both in terms of the body's physiological response in blood pressure elevations and stress hormone levels, but it also magnifies a person's perception of stress," said James D. Lane, Ph.D., associate research professor in the department of psychiatry and behavioral sciences at Duke and lead author of the study. "People haven't really accepted the fact that there could be a health downside to caffeine consumption, but our evidence – and that of other studies – shows that this downside exists and people should be aware of it in order to make the best possible health choices."

To determine the effects of caffeine on people as they go about their normal activities, the researchers enrolled 47 healthy, habitual coffee drinkers in a double blind, placebo-controlled study. That is, neither the participants nor the researchers knew when the participants were receiving caffeine or the placebo.

To qualify for the study, the coffee drinkers were asked to fill out a daily diary of caffeine intake for a period of one week. They kept the diary in order to determine the average amount of caffeine they consumed on a daily basis, the type of caffeinated beverages they drank, and the variation from day to day, all of which helped verify the participants as habitual coffee drinkers.

Once the coffee drinkers qualified for the study, their responses to caffeine were measured on two different, randomly chosen days. On one day, the coffee drinkers were given a 250-milligram dose of caffeine in the morning and again at lunchtime. On the other day, they were given identical capsules containing a placebo at the same time interval. The doses of caffeine -- equal to four cups of coffee -- and the dosage times were chosen to reflect normal patterns of coffee drinking in adults. Half of the study participants received caffeine on the first day of study and the others received it on the second day. The coffee drinkers were given, on average, two to three days off between study days during which they could consume as many caffeinated beverages as they normally desired.

On both study days, coffee drinkers wore a portable monitor that measured blood pressure and heart rate four times an hour from early morning until bedtime, while they went about their normal daily activities. They were asked to collect urine samples so that the researchers could measure the amount of stress hormones they had produced that day. They were also asked to keep a diary to record their perceived stress levels as well as their physical position – standing, sitting or lying down – each time the monitor was activated.

When the researchers compared the caffeine days to the placebo days they discovered that caffeine consumption significantly raised systolic and diastolic blood pressure consistently throughout the day and night, and adrenaline levels rose by 32 percent. The researchers found that the elevated levels persisted as the evening progressed to bedtime.

The study also showed that while caffeine increases blood pressure and heart rate, it also amplifies those effects at the times when participants report higher levels of stress during their day, said Lane. The caffeine appears to compound the effects of stress both psychologically in terms of perceived stress levels and physiologically in terms of elevated blood pressures and stress hormone levels -- as if the stressor is actually of greater magnitude, he said.

"The caffeine we drink enhances the effects of the stresses we experience, so if we have a stressful job, drinking coffee makes our body respond more to the ordinary stresses we experience," he said. "The combination of stress and caffeine has a multiplying, or synergistically negative effect.

"Everyone accepts that stress can be unhealthy. Our results suggest that drinking coffee or other caffeinated drinks can make stress even more unhealthy."

The researchers noted that while habitual coffee drinkers might be expected to demonstrate tolerance to the effects of caffeine, they still showed significant responses to the drug.

"Our findings indicate that eliminating coffee and other caffeinated beverages from the diet could be a helpful way to decrease blood pressure and other stress reactions," said Lane. "I think that people who feel 'stressed out' should at least consider quitting caffeine to see if they feel better. Quitting caffeine could be particularly beneficial for people suffering from high blood pressure, just as diet and exercise can help keep blood pressure under control."

The researchers said that despite the perceived safety of overwhelmingly popular caffeinated beverages such as coffee, the drug does show short-term negative health effects that, if continued over a period of years, could increase risk of heart attack and stroke.

"While today's cup of coffee might not, by itself, cause you much harm, the cumulative effects of drinking it day after day over a lifetime could really be unhealthy," Lane concluded.

Other authors on the study are Carl Pieper, DrPH, Barbara Phillips-Bute, Ph.D., John Bryant, Ph.D., and Cynthia Kuhn, Ph.D., all of Duke.