Study linking coffee consumption to improved health outcomes leaves some experts skeptical

11 Oct 2022 --- A new study has correlated consuming two to three cups of caffeinated or decaffeinated coffee a day with a decreased risk of developing cardiovascular disease (CVD) and reduced mortality rates. It also links caffeinated coffee consumption with a reduced risk of developing arrhythmia. However, some experts say it may be impossible to rule out other correlative factors.

The study investigated the associations between the types and amounts of coffee consumed and the incidence of arrhythmias and ischemic stroke, coronary heart disease and congestive heart failure in adults – 55.3% of which were women – aged 40 to 69 using information from the UK Biobank.

“In this large, observational study, ground, instant and decaffeinated coffee were associated with equivalent reductions in the incidence of cardiovascular disease and death from cardiovascular disease or any cause,” states Dr. Peter Kistler from the Baker Heart and Diabetes Research Institute in Melbourne, Australia, and co-author of the study. “The results suggest that mild to moderate intake of ground, instant and decaffeinated coffee should be considered part of a healthy lifestyle.”

“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,” retorts Dr. Kevin McConway, emeritus professor of applied statistics at The Open University in Milton Keynes, UK. 

Researchers state that 2-3 cups of ground coffee per day had the most positive effect on health.“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.”

Percolating curiosity
There have been quite a few studies stating that consuming coffee, and even tea, could lower mortality rates and reduce the risk of stroke and other CVDs. Other studies have linked coffee drinking to improved microbiomes, reduced rates of some cancers and reduced risks of developing Alzheimer’s disease. 

The hype around the beany beverage has caused an increase in consumer curiosity surrounding its health benefits, with global online searches on the topic increasing by 650% since 2021.

Not all of the press has been good. A study published earlier this year found that pollutants associated with caffeine and caffeine production are on the rise, with high levels being found in 258 European rivers and 13 European marine and coastal areas. In some places, it was the second highest pollutant besides paracetamol, likely due to the estimated 3,244 million tons of coffee drank in Europe over the course of just one year.

Grinding out the details
The study, published in the European Journal of Preventive Cardiology, bolsters the findings of former studies. It included 449,563 participants with no history of CVD or arrhythmia. Each completed a questionnaire about how much coffee they drank and whether they drank ground or instant coffee, caffeinated or decaffeinated.

They were then grouped into six categories based on daily intake: no cups, less than one cup, one cup, two to three cups, four to five cups and more than five cups. The majority of participants, 198,062 (44.1%), drank instant coffee, 82,575 (18.4%) drank ground coffee and 68,416 (15.2%) drank decaffeinated coffee. The control group consisted of 100,510 (22.4%) participants who drank no coffee.Some experts highlighted that other possible factors, like physical activity, were not taken into consideration.

The coffee drinkers were then compared to the control group (non-coffee drinkers) for incidence of CVD, arrhythmia and death and followed up with for a median period of 12.5 years with outcome information obtained from medical records and death certificates.

“Non-coffee drinkers served as controls in this study, but it is unclear whether they consumed caffeine from other dietary sources (for example, tea or other drinks) and if so, whether they consumed a comparable quantity of caffeine,” explains Dr. Annette Creedon, a nutrition scientist and manager at the British Nutrition Foundation. “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.”

Brewing up a controversy
All in all, 27,809 (6.2%) of the participants died during the follow-up period. Deaths occurred in all categories, with the lowest amount coming from those who drank two to three cups a day. As for types of coffee, those who drank ground coffee died 27% less, those who drank decaffeinated died 14% less, and those who drank instant coffee died 11% less.

Furthermore, 43,173 (9.6%) of the participants were diagnosed with CVD, with the lowest risk again being found among those who drank two to three cups a day with a 20%, 9% and 6% lower risk for ground, instant and decaffeinated, respectively. Arrhythmia was diagnosed in 30,100 (6.7%), with the lowest risk being among those who drank four to five cups of ground coffee a day (17% lower) and two to three cups of instant coffee per day (12% lower).

“Caffeine is the most well-known constituent in coffee, but the beverage contains more than 100 biologically active components,” Kistler elucidates. “It is likely that the non-caffeinated compounds were responsible for the positive relationships observed between coffee drinking, cardiovascular disease and survival.” 

“Our findings indicate that drinking modest amounts of coffee of all types should not be discouraged but can be enjoyed as a heart-healthy behavior.”

Since some people may have caffeine sensitivities, the study does not recommend starting to drink coffee.Yet, this is the very point with which some experts take issue. Dr. Duane Mellor, registered dietitian and senior teaching fellow at Aston Medical School, Aston University in Birmingham, UK, holds that there are too many unaccounted for variables to make a clear distinction of correlation, much less causation.

“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,” Mellor affirms. 

He continues: “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.”

Reaching the boiling point?
McConway clarifies that, with observational studies, it is almost impossible to rule out all of the possible cofactors, nor is it possible to adjust for all of the factors for which the researchers 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,” he states. “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,” McConway concludes.

Creedon, however, does applaud the authors for acknowledging that caffeine can have negative effects on some people. “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,” she says.

By William Bradford Nichols