Because it is one of the most commonly consumed drinks around the world, health impacts of coffee have been investigated many times.
The latest study the researchers conducted a systematic review of all previous recorded summaries of coffee and health research, to get an overall picture of the state of knowledge on the subject. The majority of the research reviewed found that coffee was linked to a lower risk of disease or death.
On the other hand, most of the research is observational, which means we don’t know whether coffee is the cause of the health benefits. And some studies, including those looking at lung cancer and women’s risk of fracture, found coffee was linked to a higher health risks.
The research also identified that coffee drinking during pregnancy was associated with a raised risk of miscarriage, pre-term birth, low birth weight and leukaemia in children.
Unlikely To Be Harmful
Eliseo Guallar, Johns Hopkins professor of epidemiology and medicine, in an accompanying editorial, writes:
“Should doctors recommend drinking coffee to prevent disease? Should people start drinking coffee for health reasons? The answer to both questions is ‘no’.”
Three to four cups of coffee a day are unlikely to be harmful (if you are not pregnant or at high risk of fracture) but there is still no compelling evidence that coffee brings significant health benefits.
The study was carried out by researchers at the University of Southampton and the University of Edinburgh. It was an umbrella review, meaning it was a systematic review of previously-published systematic reviews, in this case only ones that included a meta-analysis.
The researchers needed to get a high-level overview of the state of research on the topic, because they intend to carry out a randomised controlled trial into the effects of coffee as a treatment for liver disease.
They needed to see whether coffee was likely to be beneficial or harmful overall, before they started their liver disease research. Systematic reviews are a good way to see the overall state of research into a topic, but they are only as reliable as the studies that go into them.
After summarising and analysing the research, the study showed high coffee consumption was linked to beneficial results in 19 health outcomes and harmful results in six health outcomes.
For the other 34 outcomes investigated, high versus low coffee consumption made no difference to the results, or too small a difference to be sure it was not down to chance.
Beneficial outcomes included:
10% lower risk of death during the study period (relative risk (RR) 0.9, 95% confidence interval (CI) 0.85 to 0.96) for people who drank more versus less coffee
19% lower risk of death from cardiovascular disease (such as heart attack or stroke) for people who drank coffee regularly versus no coffee (RR 0.81, 95% CI 0.72 to 0.9)
18% lower risk of getting cancer (RR 0.82, 95% CI 0.74 to 0.89) for people who drank more versus less coffee (although this did not apply to every cancer)
29% lower risk of non-alcoholic fatty liver disease (RR 0.71, 95% CI 0.60 to 0.85) for people who drank any versus no coffee
30% lower risk of type 2 diabetes (RR 0.70, 95% CI 0.65 to 0.75) for people who drank more versus less coffee
The links between high coffee consumption and beneficial outcomes were strongest for liver disease, liver cancer, gout, death after heart attack and leukaemia.
Harmful outcomes were seen for leukaemia in childhood (where the mother had consumed more versus less coffee during pregnancy), lung cancer, pregnancy loss, rheumatoid arthritis, low birth weight and fracture (for women only, not men).
The researchers said the results for lung cancer only applied for smokers, and that this result was likely to be a result of people who drink more coffee being more likely to smoke. They say they found no evidence that non-smokers who drank coffee had an increased risk of lung cancer.
However, the researchers said their GRADE analysis showed that almost all the studies included in the review were of low or very low quality. They also said they found some evidence of publication bias – where only studies with positive results get published.
“many of the associations between coffee consumption and health outcomes … could be affected by residual confounding,”
meaning the results are affected by something else not accounted for in the study.
However, they conclude that coffee consumption seems “generally safe” and “more likely to benefit health than harm”. They say that future randomised control trials might exclude pregnant women, because of the risk of harm, and perhaps women at risk of fracture.
This big overview of research summarises the state of our knowledge about the effects of coffee on health. The results are mostly positive – people who drink more coffee tend to be less likely to have a range of diseases.
But the type of studies done so far means we can’t be sure whether coffee is the cause of this good health. It could be that the people who have low or no coffee do so because they already have an illness.
The limitations of the summary are mainly the quality of the studies that went into it. The researchers themselves say that they found the quality to be low or very low for almost all studies.
That means we can’t say that coffee is good for you. We don’t know what, if any, effect starting to drink coffee would have on a person’s health.
The research was carried out to see whether it was worth doing a better-quality study on the effects of coffee on people with liver disease. When that study has happened, we’ll have a better idea.
Poole Robin, Kennedy Oliver J, Roderick Paul, Fallowfield Jonathan A, Hayes Peter C, Parkes Julie et al. Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes BMJ 2017; 359 :j5024