Coffee and blood flow

It said a study has found that one cup is enough to reduce blood flow to the heart by 22% within an hour of being drunk. Though this research found that a group of volunteers had raised blood pressure and slightly constricted arteries an hour after drinking caffeinated coffee, the actual effect was modest and unlikely to have any adverse health effects. It is normal for arteries to dilate and constrict throughout the day, with exercise for example.

The researchers did not assess whether these changes persisted beyond an hour or how long it took for blood flow to return to its starting point. In addition, the study was in only 20 people, and with such a small number of participants, there is an increased likelihood that the results are due to chance alone.

Like many things, coffee should be drunk in moderation. Consuming high amounts of caffeine every day is likely to have some effect on wellbeing, regardless of whether this includes an effect on the heart and blood vessels. Where did the story come from?

This research was carried out by Dr S Buscemi and colleagues from the University of Palermo. The study was funded by The Italian Ministry of Education and the Associazione Onlus Nutrizione e salute, Italy. The paper was published in the peer-reviewed medical journal The European Journal of Clinical Nutrition.

The newspaper headlines tended to overemphasise the health consequences of coffee consumption based on the small changes in blood pressure and arterial dilation observed in this study. However, the Daily Mail did emphasis that this was a small study.

This study does not provide any evidence for the long-term effects of coffee on health. The press also tended to refer to other studies that have reported health benefits of coffee for diabetes, cancer and Alzheimer’s disease. The stories that have tended to hit the headlines on the effect of coffee on health have typically been relatively small and difficult to interpret. Much of this prior research has been covered by Behind the Headlines.


What kind of research was this?

This was a non-randomised, crossover double blind study looking at the effect of coffee on artery function in healthy individuals. The rate of blood flow is determined by dilation of the arteries, which are controlled by the cells (endothelial cells) that line the artery walls. The researchers wanted to assess whether caffeine would have an effect on these cells and affect artery function.

This was a very small study. It is not possible to say that the differences that the researchers observed were not down to chance.


What did the research involve?

The study recruited 10 male and 10 female hospital employees between the ages of 25 and 50, who all drank less than two cups of coffee per day. The participants were healthy, non-obese, non-smokers, who did not have heart disease or diabetes. To avoid the possibility that women’s menstrual cycle may affect their blood flow measurements, they were tested between the 7th and 21st day of their cycle.

The participants were asked to abstain from chocolate in the time leading up to the study and to have fasted overnight. Testing took place the following morning. The participants were randomised to receiving either a cup of caffeinated or decaffeinated Italian espresso coffee. Using ultrasound, a blinded researcher measured their blood flow in the brachial artery (a major blood vessel of the upper arm) before they drank the coffee, then 30 and 60 minutes afterwards. A blood sample was also taken before the coffee was drunk, and an hour later. Blood pressure and heart activity were monitored throughout the test.
Five to seven days later the experiment was repeated, with each participant receiving the opposite drink to the one they had drunk on the previous test.


What were the basic results?

After drinking caffeinated coffee, the participants’ systolic and diastolic blood pressure increased. At both 30 and 60 minutes, systolic blood pressure had increased from 113 to 116 (2.7% increase) and diastolic blood pressure increased from 68 to 72 (5.9% increase) (p<0.05).

Arterial flow decreased after drinking caffeinated coffee, to an average maximum of 22.1% at 60 minutes (p<0.05).


How did the researchers interpret the results?

The researchers conclude that: “caffeinated coffee induces significant endothelial dysfunction”. They suggest that coffee may have unfavourable acute cardiovascular and metabolic effects on endothelial function.


Conclusion

This study looked at how drinking a cup of caffeinated espresso affected arterial blood pressure and blood flow up to an hour afterwards. Although caffeinated espresso did appear to alter measurements of these compared to decaffeinated espresso, the researchers did not assess whether these changes persisted beyond an hour or how long it would take for blood pressure and blood flow to return to normal. The changes seen in systolic and diastolic blood pressure were only minimal, and it is unlikely that these small changes alone would have any health effects.

In addition, the study was in only 20 people. With such a small number of participants, there is an increased likelihood that the results are due to chance alone. Also, the participants were not randomly recruited, but were hospital employees. As such, it is possible that this small sample of people may not reflect the general population.

Taken together, these limitations make up a strong argument against the Telegraph’s headline that a “Single espresso a day 'can damage heart’”. Larger studies and further research is needed to assess the effect of coffee on circulation, and the long-term effects of coffee consumption on health.