Though some coffee drinkers develop a tolerance for caffeine,
those who don't may be sending up their blood pressure with
each cup, new research suggests.
In a study of regular caffeine consumers, researchers found
that many persistently showed small blood pressure spikes
shortly after a large dose of caffeine -- even when that
dose came after several days of high caffeine intake.
The findings, according to the study authors, add to evidence
that many people who regularly down caffeinated beverages
do not develop a tolerance to caffeine's effects. If such
a "low tolerance" person also has high blood pressure, it
may be best to pass on that second cup of coffee, they say.
"My advice would be for individuals who have high blood
pressure -- and particularly if their blood pressure is
not easily controlled with medication -- to limit their
caffeine intake," Dr. Noha H. Farag, a researcher at the
VA Medical Center and University of Oklahoma Health Sciences
Center in Oklahoma City, stated.
He and his colleagues report the findings in the American
Journal of Hypertension.
The role of caffeine in high blood pressure is not entirely
clear. A key reason is that while lab experiments have found
caffeine to trigger a short-term rise in blood pressure,
studies of the general population have often failed to link
caffeine intake with the risk of high blood pressure.
One explanation that has been given for this discrepancy
is that in real life, people who regularly consume caffeine
develop a tolerance for the stimulant and eventually fail
to have a blood pressure response to their morning coffee.
However, Farag noted, a number of studies have suggested
that while some people may indeed build up a tolerance to
caffeine, others continue to have a blood pressure rise
after their daily shot of java.
The current study is a continuation of an earlier lab experiment
in which Farag's colleagues found that about half of regular
caffeine consumers showed a small blood pressure spike after
ingesting caffeine -- even after spending several days downing
the equivalent of six cups of coffee per day.
In this latest study, the researchers used portable blood
pressure monitors to follow blood pressure changes throughout
the day in the same group of people. Such "ambulatory" blood
pressure measurements, Farag noted, give a better idea of
the effect caffeine may have in everyday life.
The study was conducted over 4 weeks. For one week, participants
took placebo capsules, which contained no caffeine, for
5 days; on the sixth day, they were given a large dose of
caffeine -- equivalent to about seven cups of coffee --
and had their blood pressure monitored for 24 hours. On
other weeks, they consumed a moderate or high amount of
caffeine for 5 days before having their large caffeine dose
on test day.
Based on the previous study, the men and women were divided
into "low" and "high" tolerance groups.
Farag's team found that participants with a high caffeine
tolerance showed a blood pressure spike only during the
week in which they consumed no caffeine for 5 days then
had a big dose on test day.
In contrast, the low tolerance group still showed blood
pressure elevations during the weeks in which they had caffeine
for several days before testing -- indicating that they
had not built up a tolerance to the stimulant's blood pressure
effects.
For healthy people with normal blood pressure, the impact
of caffeine may not pose a health risk, according to Farag.
But for those with high blood pressure or risk factors for
it, he said, caffeine intake could be a "significant factor"
-- and stress could compound the blood pressure effect.
There is no easy way for people to tell whether they have
high or low caffeine tolerance; systematic monitoring of
a person's blood pressure response to various caffeine doses
is the only route, according to Farag.
Given that, he advised that people with high blood pressure
moderate their caffeine intake -- along with getting regular
exercise and eating a healthful diet.
SOURCE: American Journal of Hypertension, May 2005.
