Coffee and blood pressure

Does coffee increase blood pressure?

It is difficult to give definitive advice on coffee drinking, as we have seen in previous Bandolier reports ( Bandolier 58 ). It is safe to say that pregnant women should reduce their caffeine to reduce the chance of a spontaneous abortion and low birthweight babies. Hypertensive men will also reduce the chance of a stroke. However, reducing coffee consumption increases the risk of colorectal cancer. Here, we review a meta-analysis, looking at the effect of coffee consumption on blood pressure.

Message

This meta-analysis does not answer our question satisfactorily. Results were pooled from small, mostly poor quality studies. It clearly shows the need for a high quality study examining the effect of coffee on blood pressure. In the meantime, the evidence to date suggests that for normotensive people, drinking coffee in moderation (less than 8 cups a day) should not affect their blood pressure.

Reference

S Jee et al. The effect of chronic coffee drinking on blood pressure: a meta-analysis of controlled clinical trials. Hypertension 1999 33: 647-652.

Search

Studies were searched for in MEDLINE, up to December 1997, and bibliographies of papers. To be included, studies had to have compared coffee drinkers with non-coffee drinkers (and not caffeine tablets versus decaffeinated coffee for example); coffee consumption had to have been the only difference between the two groups (eg. smoking had to apply equally to both); mean blood pressure changes had to have been reported for each group; the investigation had to have lasted for more than 24 hours.

Abstracted from each study was information on sample size, study design, participant characteristics (eg. gender, age, baseline blood pressure, change in blood pressure), type and preparation of coffee and amount consumed.

Results

Eleven studies, with 522 participants, met the criteria. The studies were small (participant numbers ranged from 8 to 99) and had different designs. One was not randomised and three were not single or doubled blinded. Participants were mostly men with average ages ranging from 26 to 56 years. The effect of coffee was looked at from between 14 to 79 days. Blood pressure was measured in different ways, the details of which are not given.

The type and preparation of coffee differed between studies (instant/boiled/drip; filtered/not filtered). The amount of coffee consumed and the caffeine content also differed, ranging from three to eight cups and 435 mg to 887 mg (with data missing from four studies).

On average, coffee drinkers systolic and diastolic blood pressure increased by 2.4 mmHg (95% confidence interval 1.0 to 3.7) and 1.2 mmHg (95% confidence interval 0.4 to 2.1) respectively, compared with non-coffee drinkers. On average, for every cup of coffee consumed, systolic blood pressure increased by 0.8 mmHg and diastolic blood pressure increased by 0.5 mmHg.

Comment

This meta-analysis pooled results from small studies with methodological flaws and differences in type, preparation and amount of coffee consumed. The studies did not take smoking into account which is often associated with coffee drinking (and has well known effects on blood pressure). The participants in the studies had normal blood pressure and so we are unable to tell what the effects of coffee drinking are on people who are at risk for hypertension or who already have hypertension.

This meta-analysis tells us that there is a need for a large, well-designed study looking at the effects of coffee on blood pressure. In the meantime, it suggests that the effect of drinking 8 cups of coffee a day (75 mg caffeine per cup) on people with normal blood pressure is small.