Though some coffee drinkers develop a tolerance for caffeine, those who don’t may be sending up their blood pressure with each cup, according to new research.

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,” said Dr. Noha H. Farag, a researcher at the VA Medical Center and University of Oklahoma Health Sciences Center in Oklahoma City.

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.