Atrial Fibrillation? Don't Blame Caffeine, Says Women's Health Study

August 31, 2010

August 31, 2010 (Bethesda, Maryland) — It's a bad rap, blaming atrial fibrillation (AF) on caffeine intake, regardless of how much of the popular, potable stimulant one generally consumes, or so an analysis based on the Women's Health Study (WHS) population would suggest [1].

In a 14-year follow-up of >30 000 middle-aged women initially without heart disease or other serious health issues, incident AF was no more common at any level of caffeine intake than it was at the lowest level. The study appears in the September 2010 issue of the American Journal of Clinical Nutrition.

"Increased caffeine consumption is not associated with an increased risk of atrial fibrillation in our population, and this confirms one of the few prior prospective studies on this issue, a Danish study that showed very similar findings," lead author Dr David Conen (University Hospital, Basel, Switzerland) said to heartwire . He was referring to a 2005 analysis [2] based on about 48 000 people in the Danish Diet, Cancer, and Health Study, whose caffeine intake was markedly higher than that of the WHS participants. Still, the AF incidence was similar across intake quintiles.

"We can now say that there is definitely no large study out there showing that average long-term consumption of caffeine is associated with atrial fibrillation. I think when you drink moderate amounts, or even high amounts, of caffeine on a regular, stable basis, you can be reassured knowing that there is not an association with the arrhythmia."

As covered by heartwire , the WHS randomized 39 876 female healthcare professionals aged >45 to receive low-dose aspirin, vitamin E, both, or neither for primary prevention of cardiovascular and cancer events. The trial's primary results were reported in 2005.

Over a median of 14.4 years, there were 945 confirmed cases of incident AF among the 33 638 participants in the current analysis. Their average daily caffeine intake ranged from only 22 mg in the lowest-intake quintile to 656 mg in the highest quintile. Coffee accounted for about 81% of caffeine consumed, tea about 10%, and cola about 7%. The researchers assumed that the content of caffeine was 137 mg/cup of coffee, 47 mg/cup of tea, 46 mg/can or bottle of cola, and 7 mg/serving of chocolate candy.

In multivariate analysis, none of the adjusted hazard ratios for incident AF were significantly elevated across increasing quintiles of caffeine intake compared with the lowest-quintile reference.

Relative Risk (RR) for Atrial Fibrillation by Quintiles of Caffeine Intake in the Women's Health Study

Quintile Caffeine intake (mg/d) RR for AF (95% CI)*
2 135 0.88 (0.72–1.06)
3 285 0.78 (0.64–0.95)
4 402 0.96 (0.79–1.16)
5 656 0.89 (0.73–1.09)

*Relative to quintile 1, mean caffeine intake 22 mg/d; adjusted for age, systolic blood pressure, body-mass index, hypertension, diabetes, hypercholesterolemia, smoking, exercise, alcohol consumption, parental history of MI, treatment group, fish intake, and race/ethnicity.

There was something of a U-shaped association between intake and risk, however, in that the risk fell a significant 22% in the third quintile.

"That's really a secondary finding, and it needs confirmation before we can claim that [moderate] caffeine consumption is protective. But it's definitely interesting and worth further study," Conen said.

None of the vehicles for caffeine intake covered by the analysis, which included chocolate along with coffee, tea, and cola, showed a significant individual relationship with AF incidence.

Not in their final publication, Conen said, was a preliminary subgroup analysis showing risk of incident AF by different baseline levels of body-mass index and blood pressure, different age subgroups, and other criteria. "We did not find any indication that the subgroups differed in their response to caffeine in their risk of atrial fibrillation."

As the analysis looked at average caffeine consumption, it couldn't determine whether there might be a transient risk to ingesting an unusually large amount of caffeine in a short period, Conen acknowledged.

"One could imagine that drinking eight espressos one evening might increase your short-term risk of atrial fibrillation the next day, but we don't have this information," he said. "Our study cannot exclude the possibility that there might be strong short-term effect of caffeine."

The WHS was funded by the National Heart, Lung and Blood Institute and the National Cancer Institute. None of the current authors report a conflict of interest.