SAN DIEGO - People who drink four or five cups of coffee throughout the morning have slightly elevated blood pressure and higher levels of stress hormones all day and into the evening, creating a scenario in which the body acts like it is continually under stress, according to a group of Duke University Medical Center scientists.
In a study of 72 habitual coffee drinkers, the researchers found that subjects produced more adrenaline and noradrenalin and had higher blood pressure on days when they drank caffeine compared with days they abstained. The two stress hormones are vital to helping the body react quickly in times of danger or stress, but they can damage the heart over a lifetime of heightened production, said James Lane, associate research professor of psychiatry at Duke.
Lane prepared results of his study, funded by the National Heart, Lung and Blood Institute, for presentation Thursday to a meeting of the 1999 Society of Behavioral Medicine.
"Moderate caffeine consumption makes a person react like he or she is having a very stressful day," Lane said in an interview before the meeting. "If you combine the effects of real stress with the artificial boost in stress hormones that comes from caffeine, then you have compounded the effects considerably."
During the two-week study, the subjects experienced, on average, a 32 percent increase in adrenalin and a 14 percent increase in noradrenaline on days when they consumed caffeine. Their blood pressure rose an average of 3 points.
Lane's study builds on smaller ones in which he found that caffeine boosted blood pressure, heart rate and stress hormones in subjects who drank 4 to 5 cups of caffeine per day. In the current study, Lane replicated those findings and added to them by showing that subjects' blood pressures and stress hormone levels stayed elevated until bedtime, even though they last consumed caffeine between noon and 1 p.m.
Occasional surges of stress hormones temporarily raise heart rate, blood pressure and mental acuity - long enough to accomplish the task at hand. But an excess of stress hormones has been shown to compromise health in a variety of ways, from damaging blood vessels to weakening the immune system.
In addition, even the small boost in blood pressure seen in this study - an average of 3 points during the day and evening - can have clinical significance, Lane said. A review of nine major studies of blood pressure and heart-disease risk showed that a 5-point difference in diastolic blood-pressure - the lower number used to assess health risk - was associated with at least a 34 percent increase in stroke and a 21 percent increase in the incidence of coronary heart disease
While researchers have long known that caffeine can boost stress hormones and blood pressure, Lane said most studies have been conducted in a laboratory setting under tightly controlled circumstances where a single dose of caffeine is compared to none in a short time span. Lane said his body of research is unique because it measures blood pressure, heart rate and stress hormone levels at timed intervals during normal working conditions, while subjects are exposed to a range of moods and activities.
"You can measure how caffeine affects people in the laboratory, but that doesn't tell you what effects the drug has in the real world when people are exposed to normal stressors and activities," he said.
In the current study, Lane also studied the effects of caffeine on women taking oral contraceptives, since previous research suggested that this population might be more responsive to the negative effects of caffeine. But Lane found no such effect. In fact, women taking oral contraceptives showed slightly less of a stress response to caffeine than a control group of women.
Lane's next study will measure the effects of eliminating caffeine from the diets of people with high blood pressure. The goal is to see if stopping caffeine use can be a useful therapy in reducing hypertension, along with diet, exercise and salt reduction.
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