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Coffee drinkers, even those genetically predisposed to slower caffeine metabolism, were not at heightened risk of developing cardiac arrhythmias, a large cohort study found.

Instead, each cup of coffee was associated with a 3% lower risk of incident arrhythmia (adjusted HR 0.97, 95% CI 0.96-0.98) among U.K. Biobank participants with a mean follow-up of 4.5 years. Small reductions were observed for atrial fibrillation and/or flutter (HR 0.97, 95% CI 0.96-0.98) and supraventricular tachycardia (HR 0.96, 95% CI 0.94-0.99) in particular.

Additionally, a mendelian randomization study did not show that caffeine metabolism-related genes modify the relationship between coffee consumption and arrhythmias, reported Gregory Marcus, MD, MAS, of University of California San Francisco, and colleagues in .

"These data suggest that common prohibitions against caffeine to reduce arrhythmia risk are likely unwarranted," the authors said. They noted that their results are in line with those of other recent studies showing no link between coffee consumption and increased tachyarrhythmias.

Marcus' group cited some potential mechanisms for coffee's observed antiarrhythmic effects in the study, namely its prolongation of left atrial effective refractory periods, blocking of adenosine receptors, antioxidant and anti-inflammatory properties, and catecholaminergic properties.

"Overall, the results ... strengthen the evidence that caffeine is not proarrhythmic, but they should not be taken as proving that coffee is an antiarrhythmic -- this distinction is of paramount importance," cautioned Zachary Goldberger, MD, MS, of University of Wisconsin-Madison, and Rodney Hayward, MD, of University of Michigan and the VA Ann Arbor Healthcare System.

"Health care professionals can reassure patients that there is no evidence that drinking coffee increases the risk for developing arrhythmias. This is particularly important for the many patients with benign palpitations who are devastated when they think, or are told, that they have to stop drinking coffee," according to their .

The U.K. Biobank included more than 500,000 participants. Median coffee consumption was 2 cups per day, though 22.1% did not drink coffee.

Marcus and colleagues counted 386,258 people (mean age 56; 52.3% women; over 90% white) without a prior diagnosis of arrhythmia in their analysis.

People who drank more coffee were more likely to be older, white, and male. As a group, they also tended to report more peripheral artery disease, cancer, smoking, and alcohol drinking.

Study authors acknowledged that residual or unmeasured confounding could not be excluded given the observational nature of their study.

Another major caveat was their reliance on self-reported coffee intake at a single point in time. "Not only can this lead to recall bias, but subsequent and substantial changes in coffee consumption are also possible, including reductions due to new signs or symptoms (i.e., patients with palpitations may avoid coffee)," Goldberger and Hayward warned.

"Finally, it is important to recognize the distinction between coffee and caffeine. Caffeine is only one element of coffee, which contains other bioactive compounds such as diterpene alcohols and chlorogenic acids. As such, the mendelian randomization analysis is truly centered on caffeine, not coffee exposure," the editorialists added.

"The current study suggests that we can tell patients that waking up to a cup of coffee is not a dangerous ritual," they nevertheless maintained." However, it will be more important to listen to patients about their symptoms in association with coffee or caffeine exposure and engage in shared decision-making on an individual level."

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