There is no evidence that regularly drinking three or more cups of coffee a day is linked to high blood pressure, according to
a new analysis of data pooled from several published studies, although the researchers did find a slight link with lower levels of
You can read how Zhenzhen Zhang, from the Department of Epidemiology, Michigan State University, East Lansing, and colleagues arrived at these conclusions after conducting a systematic review and meta-analysis of prospective observational studies in a paper published online on 30 March in the American Journal of Clinical Nutrition.
Zhang and colleagues wrote in their background information that although two meta-analyses of randomized controlled trials did find that increased coffee consumption was linked to a slightly higher blood pressure, these trials did not last long (85 days or less).
So they decided to systematically review and meta-analyze (where you pool data from several studies and treat it as if it came from one large study) studies that followed people over a longer period of time while looking at the link between habitual coffee intake and risk of high blood pressure.
To find candidate studies that fit their criteria they searched electronic records (MEDLINE, EMBASE, Agricola, and Cochrane Library) up to and including August 2009 and found six prospective cohort trials that studied the association of coffee drinking with incident hypertension or blood pressure.
Altogether the six studies enrolled 172,567 participants, and over a mean follow-up ranging from 6.4 to 33.0 years, 37,135 of the participants had incident hypertension.
The results of their pooled analysis showed that:
- Compared with the lowest category of coffee consumption, under one cup a day (a cup being about 237 mL), the relative risk for hypertension for participants who drank 1 to 3 cups a day was 1.09 (95% confidence interval CI ranging from 1.01 to 1.18), for those drinking 3 to 5 cups a day it was 1.07 (95% CI 0.96-1.20), and for 6 a day or more, it was 1.08 (95% CI: 0.96, 1.21).
- There was a dose-response relationship that looked like an inverse J-shape: with high blood pressure risk rising up to three cups a day, and then decreasing at consumption rates above this (relative risk of 6 compared to 0 cups a day was 0.99, with 95% CI 0.89 to 1.10).
The results come as no surprise to some experts.
Professor Lawrence Krakoff, a cardiology specialist who studies high blood pressure at Mount Sinai Medical Center in New York, was not involved in the study. He told Reuters news agency that he does not think coffee consumption is a risk factor for high blood pressure.
But he said "if people are drinking 12 cups a day and aren't sleeping, I assume that's an important issue".
The researchers did not compare the effect of caffeinated versus decaffeinated coffee consumption because not all the studies they examined looked at this consistently.
The relationship between coffee drinking and blood pressure may be complicated for several reasons, including the possibility of different effects in different people, perhaps due to genetic differences that make it safe for some to drink a lot of coffee but not others, according to one of the authors quoted by Reuters Health.
"Habitual coffee consumption and risk of hypertension: a systematic review and meta-analysis of prospective observational studies."
Zhenzhen Zhang, Gang Hu, Benjamin Caballero, Lawrence Appel, and Liwei Chen
American Journal of Clinical Nutrition, first published online 30 March 2011.
Additional source: Reuters Health.
Written by: Catharine Paddock, PhD