A study published in European Journal of Preventative Cardiology uses data from UK Biobank to look at impact of coffee subtypes on cardiovascular disease, arrhythmias, and mortality.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This is an interesting piece of research. Its main stated aim is not so much to look for associations between coffee consumption and health, as to see whether there’s evidence that those associations differ between different types of coffee that people might choose to consume. There have been previous studies looking generally at correlations between coffee consumption and cardiovascular disease (CVD; that is, heart attacks, strokes etc.), and the more recent ones have generally found an association between consuming moderate amounts of coffee and a reduced risk of CVD.
“One issue in interpreting the results of those previous studies, which applies just as much to this one, is that they are observational. That is, generally the researchers did not tell the participants what to drink – the participants just did what they would have done anyway, and the researchers recorded what they drank, as well as other characteristics, and also recorded whether they were diagnosed with CVD, or died. The problem that always arises with observational studies is that there are always differences between groups that (say) drink different amounts of coffee, apart from the differences in their coffee consumption. It could be that some combination of these other differences might be what determines any differences in CVD risk or death rate, and not necessarily the differences in coffee consumption at all. The researchers on these studies are aware of that, and make statistical adjustments to allow, as far as they can, for these other differences between groups. The researchers on this new study certainly did that, for a wide range of potential confounders, as these other differences are known. There’s a list of the variables that they used in these adjustments in the press release (and, of course, the research paper).
“Adjusting in this way does make it rather more likely that any association between coffee consumption and health or mortality is caused by the differences in coffee consumption, rather than by something else. But one can never be sure that all possible potential confounders have been dealt with, and you can’t adjust for factors on which you have insufficient data. For instance, in this study, the researchers could not adjust for the participants’ levels of physical activity, because that had not been recorded for many of the participants. Yet levels of physical activity are known to be associated with CVD risk, and may possibly also be associated with coffee consumption, so it’s possible that they are part of the cause of the differences in risk between different coffee consumption levels. Because you can never be sure that everything relevant has been dealt with by statistical adjustment, a single study like this can’t on its own establish what causes what. The observed differences in mortality rates and CVD risk, between people who drank different amounts and types of coffee, might well be caused (at least in part) by the differences in coffee consumption, but one just can’t be sure of that.
“This was a large study, with almost 450,000 participants followed up for more than twelve years (on average). The researchers made several different comparisons of health outcomes (CVD, arrhythmias (problems with the rhythm of heart beats), and deaths from CVD or from any cause). In almost every case they found a U-shaped pattern of association between the health outcome and coffee consumption, with some evidence that those who drank only small amounts of coffee daily having (on average) slightly better health outcomes than those who drank none at all, drinkers of two or three daily cups having better outcomes still, but those who drank but those who drank a lot of coffee (more than 5 cups a day) generally having on average worse outcomes than the moderate coffee drinkers, and on some measures, outcomes as bad as or even worse than those who didn’t drink coffee.
“Though in most cases the associations do follow this general shape, there were some differences in risk depending on the type of coffee that people said they drank. Often ground coffee had the strongest association with the risks, and decaffeinated and instant coffee had weaker associations, but there were differences in this pattern between different health risks. Interestingly, one pretty clear pattern was that there was essentially no evidence of an association between the risk of heart rhythm problems (arrhythmias) and the consumption of decaffeinated coffee, while both ground and instant coffee consumption were associated with arrhythmia risk in the typical U-shaped way (lower risk, for moderate consumption, than in non-coffee drinkers). But consumption of decaf was clearly associated with some of the other health outcomes. As one of the researchers points out in the press release, coffee has more than 100 biologically active components, not just caffeine. This particular study couldn’t assess which of those components might be causing the differences in risk, if indeed the cause is something to do with the coffee, because it couldn’t examine what is going on within the participants’ bodies.
“A large study like this can sometimes find statistically robust evidence of associations that are actually quite small and unimportant in real-world terms, simply because they put together data from such a lot of individuals. But, for several comparisons in this study, the size of the effect is reasonably substantial – so, if coffee really is playing a part in causing the risk differences, that part could be important. For instance, consider two groups of people, one of which is just like the non-coffee drinkers in this study (on average) in terms of age, sex, ethnicity, tea and alcohol consumption, and all the other quantities which were used for adjustment in this study, and the other of which is like the first group in terms of these factors, except that each of the people drinks 2-3 cups of coffee each day. The top line of the press release implies that the second, coffee-drinking group will (on average) live longer than the first group, though whether in fact that happens does depend on whether the coffee is causing the differences in death rates. But how much longer? One way is to look at what’s sometimes called the effective age of the people involved.
“If we assume for now that coffee is the cause of the risk differences, then it’s possible to estimate that, for ground coffee drinkers, the lifespans of the coffee-drinking group will be roughly the same as the lifespans of people aged between 2.5 and 3.5 years younger than them in the non-coffee group. Putting it another way, on average the bodies of the coffee-drinking group are, in terms of mortality, between 2.5 and 3.5 years younger than their true age, compared to the non-coffee drinkers of the same chronological age. Their effective age is 2.5 to 3.5 years less than the effective age of the non-coffee drinkers. This isn’t quite the same as saying that they will live between two and a half and three and a half years longer, but it’s reasonably close to that (on average). So it’s a fairly substantial difference in effective age, and that could matter if the association is indeed one of cause and effect.
“The reductions in effective age are rather smaller for different types of coffee – between one and two years, for decaf, and between three quarters of a year and one and a half years for instant coffee. In this case, it looks as if instant coffee drinking has a weaker association with the risk than does decaffeinated coffee, but on some other measures, decaf has a weaker association with the risk than does instant coffee. I can’t emphasise enough that this study hasn’t shown that the association is definitely one of cause and effect. There’s already some statistical uncertainty in those estimates, and if we could actually know the extent to which the coffee consumption was the cause of the risk differences, that could change things a lot. But the size of the changes in effective age does at least make it clear that coffee consumption is worthy of further research, to find out more precisely what’s causing what.”
Dr Annette Creedon, Nutrition Scientist and Manager at the British Nutrition Foundation, said:
“This study aimed to provide insights into the role of caffeine on cardiovascular outcomes by comparing caffeinated and decaffeinated coffee consumers. Data from just under 450,000 participants from the UK Biobank data set were included and the sample identified for inclusion in the study were followed to determine whether a cardiovascular outcome was diagnosed during the follow-up period (12.5 year).
“The findings may suggest that drinking 2-3 cups of instant, ground or decaffeinated coffee each day was associated with a significant reduction in cardiovascular disease and mortality, and arrhythmia (an irregular heartbeat) reduction was seen with caffeinated but not decaffeinated coffee. However, coffee consumption was self-reported by the study’s participants and did not consider that participants may consume more than one type of coffee. This study had a median follow-up period of 12.5 years during which many aspects of the participants diet and lifestyle may have changed. It is also possible that respondents over or under-estimated the amount of coffee that they were consuming at the start of the study when they self-reported their intake. It is therefore difficult to determine whether the outcomes can be directly associated with the behaviours in coffee consumption reported at the start of the study.
“Non-coffee drinkers served as controls in this study, but it is unclear whether they consumed caffeine from other dietary sources (e.g. tea or other drinks) and if so, whether they consumed a comparable quantity of caffeine. Hence, it is possible that differences observed between the control group and the test group may be due to factors other than the coffee subtype.
“The authors do acknowledge that drinking caffeinated coffee can result in some negative side effects for some individuals and this can be particularly relevant to individuals who are sensitive to the effects of caffeine. Hence, the findings of this study do not indicate that people should start drinking coffee if they do not already drink it or that they should increase their consumption.”
Dr Charlotte Mills, Hugh Sinclair Lecturer in Nutritional Sciences, University of Reading, said:
“This manuscript adds to the body of evidence from observational trials associating moderate coffee consumption with cardioprotection, which looks promising. However, with observational research like this, you can’t be sure what direction the relationship goes e.g. does coffee make you healthy or do inherently healthier people consume coffee? Randomised controlled trials are needed to prove the relationship between coffee and cardiovascular health. A key limitation of this work is that not all confounders linked to cardiovascular health have been considered, e.g. socioeconomic status, habitual diet and physical activity.
“A merit of this work is the attempt to separate the type of coffee i.e. ground, instant and decaf coffee. However, coffee is chemically complex, it contains a huge number of bioactive components and the levels of these differ depending on how the coffee is made. E.g. boiled ground coffee contains bioactives called diterpenes which are associated with unfavourable vascular outcomes whereas filtered ground coffee does not- so the full picture cannot be understood from this work. Randomised controlled trials are needed to fully understand the relationship between coffee and health before recommendations can be made.”
Dr Duane Mellor, Registered Dietitian and Senior Teaching Fellow, Aston Medical School, Aston University, said:
“This is yet another observational study which has found that moderate coffee consumption (2-3 cups per day) is associated with a lower risk of cardiovascular disease and related risk of mortality. This study reviewed data from the UK Biobank study which followed up nearly 1/2 million people for over a decade. Using responses to questions about the type of coffee people drank using a touch screen survey, it found those who drank 2-3 cups of coffee per day, whether it was instant, decaffeinated or ground had a lower risk of cardiovascular disease. This might suggest that it is not simply the caffeine which could potentially explain any associated reduction in risk. This study follows a number of studies suggesting moderate coffee may drinking reduce risk of cardiovascular related mortality, but as recently we have seen similar effects reported associated with tea drinking, perhaps it is behaviours linked to drinking coffee (or as in the recent tea drinking study – drinking tea) which could also help reduce risk of heart disease? It is important to consider what is meant by a cup of coffee, as data from studies where coffee grounds are boiled this can have a negative effect on blood fats including cholesterol, which is not common in the UK where the data from this current paper was collected. Also, it is important that a simple cup of coffee perhaps with a little milk is very different to a large latte flavoured with a syrup and added cream. So, moderate coffee consumption might be associated with lower risk of heart disease, but it is how it is consumed which is important, so ideally without added sugar and other calorie rich extras and think carefully about any snacks such as cakes and biscuits which you might be tempted to include with your coffee.”
‘The impact of coffee subtypes on incident cardiovascular disease, arrhythmias, and mortality: long-term outcomes from the UK Biobank’ by David Chieng et al. was published in the European Journal of Preventative Cardiology at 00:05 A.M. UK time Tuesday 27 September 2022.
Dr Duane Mellor: “No conflicts of interest.”
Dr Charlotte Mills: “I have previously worked in collaboration with Nestle on research relating to coffee and health funded by UKRI. I am also working with Paulig on another coffee and health related project, again funded by UKRI.”
Dr Annette Creedon: “Funding to support the British Nutrition Foundation’s charitable aims and objectives comes from a range of sources including membership, donations and project grants from food producers and manufacturers, retailers and food service companies, contracts with government departments; conferences, publications and training; overseas projects; funding from grant providing bodies, trusts and other charities. Further information about the British Nutrition Foundation’s activities and funding can be found at http://www.nutrition.org.uk/aboutbnf/.”
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. My quote above is in my capacity as an independent professional statistician.”