Is it really safe to drink 25 cups of coffee a day?

Cup of black coffee
4th June 2019

A study has found no link between drinking coffee and having stiffer arteries - but that doesn’t mean that unlimited coffee has no effect on your cardiovascular health. We look behind the headlines.  

People who drink more than three cups of coffee a day had similar measures of arterial stiffness as those drinking one cup or less, a study part-funded by the BHF has found.  
 
This research has been widely reported as “25 cups of coffee a day won’t harm your heart”, but that isn’t demonstrated by the research.  

The research

The researchers divided the study participants into three categories based on how much coffee they drink each day: one cup or less, more than one cup up to and including three (moderate), and more than three cups (heavy). People with heart and circulatory disease, or those who drank more than 25 cups of coffee a day (a very small number) were excluded. 
 
The researchers found no difference in blood pressure, heart rate and arterial stiffness between the three groups once they took account of other factors which might affect arterial stiffness, including age, gender, ethnic background, height, weight, blood pressure, diet, alcohol consumption and whether they smoke.  

Stiffer arteries can put extra pressure on the heart, and increases the risk of having a heart attack or stroke

 
The study analysed information collected from 8,412 men and women, with an average age of 61, in the UK Biobank Imaging Study, which is part-funded by the BHF. MRI scans and infrared pulse wave tests were used to measure the stiffness of their arteries. Stiffer arteries can put extra pressure on the heart, and increases the risk of having a heart attack or stroke.  
 
The research was led by scientists at Queen Mary University of London, and was presented at the British Cardiovascular Society (BCS) Conference in Manchester.  

How the research was reported 

Newspapers were eager to declare that coffee has no harmful effects on the heart, which didn’t reflect the fact that the study only measured one aspect of cardiovascular health: arterial stiffness. It didn’t look at other risks such as abnormal heart rhythms or cholesterol levels – which have both been linked to high coffee consumption in the past. 
 
The majority of media coverage also focussed on people drinking up to 25 cups of coffee a day. The Times reported ‘Perk up, that 25th cup of coffee won’t harm your heart’, and The Guardian’s headline read ‘Up to 25 cups of coffee a day safe for heart health, study finds’. The Sun claimed ‘Drinking 25 cups of coffee a day is ‘like drinking one and may not harm heart’, scientists say’.  

 

According to the researchers, people in this group drank an average of five cups a day.

These misleading claims were based on the fact that the researchers excluded participants who drank more than 25 cups a day. However only two people in the study actually drank 25 cups of coffee a day. These people would have been included in the category of people drinking more than three cups a day.
 
According to the researchers, people in this group drank an average of five cups a day. We don’t know whether a separate analysis of people drinking 25 cups of coffee would have found an increase in arterial stiffness – and two people certainly wouldn’t be enough to draw any conclusions from.  
 
Despite having celebrated that drinking 25 cups of coffee a day isn’t bad for the heart, The Times published a second article titled “Why it’s not OK to drink 25 cups of coffee a day”. This warned that the European Food Safety Authority recommends limiting caffeine consumption to 400mg — about four or five cups of coffee — a day, and the NHS advises pregnant women to have no more than 200mg, because excessive amounts are linked to miscarriages. 
 
In a follow up piece in The Guardian, journalist Sam Wollaston documented his attempt to drink 25 cups of coffee in one morning. He said he did this because ‘Science says it's safe to drink 25 cups of coffee today’. This is not what the study showed, and it could be dangerous to consume such a high quantity of caffeine. Thankfully he stopped his experiment after seven and a half cups, due to feeling nauseous, jittery and shaky – and having coffee breath. 

How good was the research?  

A strength of the research is that it studied a lot of people, which reduces the likelihood that the results were due to chance.
 
Previous studies looking at the effects of coffee on cardiovascular health have produced conflicting results, which may be because they studied smaller groups of people. Importantly, each of the three categories of coffee consumption included large numbers of people: 3,892 drinking one cup or less, 2,978 drinking more than one cup up to three, and 1,542 drinking more than three.  
 
One drawback of the study is that the group with the lowest coffee consumption still included people drinking one cup of coffee per day. We therefore don’t know whether people drinking no coffee at all had lower arterial stiffness than coffee drinkers. 
 
Finally, this was an observational study – so while it might have revealed an association between coffee consumption and arterial stiffness, if it existed, it could not have established a “cause and effect” relationship.

The BHF view 

Professor Metin Avkiran, BHF Associate Medical Director, said:   
 
“Understanding the impact that coffee has on our heart and circulatory system is something that researchers and the media have had brewing for some time.   
 
“There are several conflicting studies saying different things about coffee, and it can be difficult to filter what we should believe and what we shouldn’t. This research will hopefully put some of the media reports in perspective, as it rules out one of the potential detrimental effects of coffee on our arteries when drunk in moderation.”   

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