Coffee Does Have Harmful Short-Term Health Implications

Coffee Does Have Harmful Short-Term Health Implications


This is a review of a 14-day study that was written by the American Heart Association, but was conducted at the University of California, San Francisco, where they looked into caffeinated coffee’s negative effect on Premature Ventricular Contractions and positive effect on step count.

Good and Bad

According to the American Heart Association Drinking caffeinated coffee appears to have both beneficial and harmful short-term health effects. On the negative side effects such as increased abnormal heartbeats, and reduced sleep duration, but on the positive side effects such as increased physical activity, and a lower risk of cardiovascular disease.


The Study author Gregory Marcus, M.D., M.A.S., associate chief of cardiology for research and endowed professor of atrial fibrillation research at the University of California, San Francisco said, "Coffee is the most commonly consumed beverage in the world, yet its health effects remain uncertain." 

"While the majority of long-term observational studies have suggested multiple potential benefits of drinking coffee, this is the first randomized trial to investigate the real-time, physiologic consequences of coffee consumption."

The Study Devices

Dr. Marcus and colleagues enrolled 100 adult volunteers, and they were assigned to wear the following:

  • ECG devices to continuously track and record heart rhythms

  • Fitbits that were used to track their physical activity and their sleep, and

  • Glucose monitors to continuously track blood sugar levels for two weeks

The Cohort

The participants were an average age of 38 years, 51% were women and 48% were white. The researchers also obtained DNA saliva samples from the participants to assess genetic variants that may affect caffeine metabolism.

The Protocol

The cohorts were then randomly assigned to either avoid completely or consume coffee for no more than two consecutive days in a row, they all had to do this for 14 consecutive days. 

Coffee and espresso consumption were recorded in real time via a "time stamp button" on the ECG monitor. The researchers tracked their trips to coffee shops with geo-tracking. In addition, participants completed daily questionnaires to detail how much coffee they had consumed every morning.

The Results

Analysis of the data found that coffee consumption was associated with a 54% increase in Premature Ventricular Contractions; this is a type of abnormal heartbeat originating in the lower chambers of the heart, and often reported to feel like a skipped heartbeat.

In contrast, drinking more coffee was associated with fewer episodes of supra-ventricular tachycardia, an abnormally rapid heart rhythm arising from the upper heart chambers.

Need to Worry?

So, the 54% increase in Premature Ventricular Contractions does sound worrying, but in a previous study Dr. Gregory Marcus is on record as saying, with regard to coffee consumption and PVC, “PVCs are common and are usually regarded as harmless. We all have them once in a while, and generally they're considered benign. 

But we and others have shown that more PVCs are an independent risk factor for heart failure over time. Not everyone with more PVCs has heart failure, but it is a factor."

Sleep and Activity

Consuming coffee was consistently associated with more physical activity as well as less sleep. Specifically:

  • Participants who consumed coffee logged more than 1,000 additional steps per day compared to days when they did not drink coffee

  • On the days participants drank coffee, they had 36 fewer minutes of sleep per night according to their Fitbit devices

  • Drinking more than one coffee drink more than doubled the number of irregular heartbeats arising from the heart's lower chambers

  • Each additional cup of coffee consumed was associated with nearly 600 more steps per day, and 18 fewer minutes of sleep per night

  • There were no differences in continuously recorded glucose measured when the study participants consumed versus avoided coffee.

These findings were corroborated by analyses of adherence to their randomization assignment and amplified when more versus less coffee was consumed.


Dr. Marcus stated, "More physical activity, which appears to be prompted by coffee consumption, has numerous health benefits, such as reduced risks of Type 2 diabetes and several cancers, and is associated with greater longevity.

On the other hand, reduced sleep is associated with a variety of adverse psychiatric, neurologic and cardiovascular outcomes. 

More frequent abnormal heartbeats from the upper heart chambers influence risk of atrial fibrillation, and more frequent abnormal beats from the lower chambers, or ventricles, increase the risk of heart failure. 

These results highlight the complex relationship between coffee and health."

Genetic Variants

The study participants with genetic variants associated with faster caffeine metabolism exhibited more abnormal heart beats originating in the ventricles, or PVCs, when more caffeinated coffee was consumed.

The slower an individual metabolized caffeine based on their genetics, the more sleep they lost when they drank caffeinated coffee.

The investigators also sought to determine if changes in exercise or sleep influenced coffee's effects on abnormal heart rhythms, and no such association was identified.

Cause and Effect

Dr. Marcus noted that because coffee was randomly assigned to the study participants, cause-and-effect can be inferred. These observations were made during repeated assessments of days when coffee was consumed versus when it was not for each study participant, eliminating concerns regarding differences in individual-level characteristics as an explanation for these results.

Previous Studies

100 participants is a good sample size, but it was only over 2 weeks, a previous study I covered followed 382,535 for 10 years.

The results of that study were that having two to three cups of coffee a day was associated with the greatest benefit, translating to a 10%-15% lower risk of developing coronary heart disease, heart failure, a heart rhythm problem, or dying for any reason.

The researchers did observe a U-shaped relationship with coffee intake and new heart rhythm problems. The maximum benefit was seen among people drinking two to three cups of coffee a day.

Scientific Studies: 

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