"Don’t talk to me before I have had my first coffee!"Many people can hardly imagine starting the day without a cup of this hot beverage. The scent, the warmth, the short breath – all this is somehow part of it for them. A look at the figures also shows how popular coffee is: Worldwide, more than 160 million bags of coffee are currently consumed each year. The European Union is one of the largest sales markets for coffee. According to the German Coffee Association, every German citizen drank an average of 168 liters of coffee in 2020 – the equivalent of 1344 cups of 125 milliliters each, or about three and a half cups per day.
At the same time, however, the morning coffee ritual is often accompanied by a guilty conscience. Finally, the drink can damage the cardiovascular system, deprive you of sleep and make you shaky. Or maybe not?
In fact, many things we think we know about coffee belong to the realm of myths. More recent studies suggest that the hot drink could even have a positive effect on health. So what is really true?
The problem with coffee studies
Nutrition studies often take place at the population level. This means that many thousands of people are questioned about their behavior and their health, and then the data are statistically analyzed. For example, numerous studies starting in 1960 concluded that coffee drinkers have a higher than average incidence of cancer. But let’s imagine what an office day used to look like: Next to empty coffee cups were ashtrays filled to the brim. And this is often the problem with such studies, reports Giuseppe Grosso of the Università di Catania in Italy: "The people who drank coffee usually smoked as well. Researchers often did not consider this at the time." In a 2017 review study, Grosso and his team went back through the data from previous meta-analyses. If they looked only at non-smokers, they even showed a positive effect: coffee apparently reduced mortality from cancer.
The example illustrates how difficult it is to study what effects a particular food or diet has on health. There are many factors that can distort the picture: besides whether someone smokes or not, for example, a person’s physical activity level, their level of education, their age or their gender. In addition, different people have different habits. Some drink coffee for breakfast, while for others it is a substitute for a meal. That alone can make a difference, in part because the ingredients in different foods can affect one another. For example, caffeine inhibits the absorption of iron from food, studies show.
More conclusive results are usually provided by randomized controlled trials (RCTs), which are at best double-blinded. They are, so to speak, the gold standard in science. The test subjects are randomly divided into two groups. One group participates in a particular intervention, for example, consuming a certain amount of a food every day; the other group – the control group – omits it. The effect of drugs can be studied quite well in this way. In this case, the control group can even be given a placebo, which looks like the real drug but contains no active ingredient. In nutrition studies, however, such a design is much more difficult to implement. It’s true that with dietary supplements, there are RCTs to some extent. "But unlike drugs, which introduce something completely new into our bodies, we eat all kinds of foods every day," writes Giuseppe Grosso.
Coffee itself also contains many different ingredients. While the focus of many studies is on caffeine, this is joined by hundreds of other biologically active phytochemicals such as polyphenols, trigonelline, melanoidins, magnesium, and potassium, whose composition can also vary by type and preparation. This makes it more complicated to understand the mechanisms behind any effects. And: So far, there is no uniform standard as to what "a cup of coffee" actually is.
Coffee does not increase the risk of cancer – it rather lowers it
More recent studies therefore try to statistically include as many factors as possible that could falsify the results, and take a close look at various components in the mechanisms of action. Caffeine, however, remains a key issue.
There is now quite a lot of evidence that coffee can actually reduce the risk of cancer. For example, this was the conclusion of a meta-analysis of 105 prospective observational studies in 2016. The hypothesis to be investigated is determined before the start of the study. The data is then collected specifically for this purpose with a suitable group of test persons – even if this is also only observational data, i.e. there is no intervention in the dietary behavior of the participants. One advantage of this study design, however, is that at least you get data that are best tailored to your research question.
However, the health-promoting effect of coffee does not apply equally to all types of cancer. In the study, the hot beverage reduced the risk of cancer of the mouth, throat, liver, colon, prostate, and uterus. The risk of lung cancer, on the other hand, was statistically increased in coffee drinkers, and no correlation was found for other types of cancer. Meta-analyses are only as good as the individual studies they examine. Some studies may therefore have been inaccurate or may not have taken various factors into account. Overall, however, the results suggest that regular coffee consumption at least does not increase the risk of cancer. Other studies that did not find a link between coffee and cancer point in this direction.
The positive effects that some studies find could be due to various mechanisms. It is possible that coffee components such as polyphenols, diterpenes and melanoidins reduce oxidative stress and prevent associated cell damage. There is also evidence that coffee supports processes such as DNA repair and apoptosis – programmed cell death – as well as counteracting the spread of metastases. Diterpenes also help metabolize carcinogenic compounds.
Research shows that where you conduct studies on the health benefits of coffee could make a difference. For example, in an analysis of coffee’s impact on several diseases and overall mortality, people in Europe and Asia benefited more from drinking the hot beverage than residents of the United States. The authors explain this by the fact that coffee is prepared differently in some cases. In addition, the parameters used in the individual studies may have varied. And genetic differences may also play a role.
No danger for the cardiovascular system
The concern that coffee could promote cardiovascular diseases or be dangerous for people with such diseases has also not been substantiated in studies. Sometimes drinking the hot beverage can even have a preventive effect, which is probably due to the antioxidant properties of various ingredients, as in the case of cancer.
However, opinions differ on exactly how much coffee should be consumed to achieve this positive effect, depending on the study. Two Australian researchers wrote in a 2021 review study that the clearest effects are seen with two to three cups per day. Others see a health-promoting effect at one to two cups or three to five cups daily. Of course, it also depends on the study conditions and what exactly was being compared. In any case, more than six cups does not seem to be recommended, though the study authors note: "It is difficult to determine at what level coffee consumption becomes a concern because the cup size measured can vary by study and because there is little data on very high levels of regular consumption.
Whether coffee prevents Alzheimer’s has not been proven
On the brain, coffee has both a direct and indirect effect, A 2020 study suggests that caffeine is responsible, as decaffeinated coffee had no effect on thinking performance. The hot drink may even reduce the risk of developing Alzheimer’s dementia. Australian researchers looked at beta-amyloid deposits in the brains of older people over ten years and found fewer accumulations in those who drank more coffee. But overall, the evidence for a preventive effect of coffee against Alzheimer’s is still quite thin, experts note.
Things look more robust for depression. Several studies conclude that moderate coffee consumption is associated with a lower risk of depression. A role is probably played by the fact that caffeine modulates the receptors for the neurotransmitter dopamine, which is popularly known as a happiness messenger. The effects were more pronounced in women than in men.