"Drinking three to five cups of coffee a day could help people live longer, new research has found," The Independent reports.
Research suggests a link between regular coffee consumption and reduced risk of chronic diseases, such as heart disease – whether people drank the normal or decaffeinated variety.
The results come from three studies with a total of 208,501 health professionals, followed up for more than 20 years. Overall, people who drank one to five cups of coffee a day were slightly less likely to have died by the end of the study, compared to people who did not drink coffee at all.
People who drank more than five cups a day were no more or less likely to have died. However, the results changed, depending on whether the researchers included people who smoked. This may be because heavy coffee drinking and smoking often go together, so the unhealthy effects of smoking may cancel out any minimal effects from coffee.
The results suggest that regular coffee drinking may have some benefits. However, the differences in the chance of death between coffee drinkers and non-coffee drinkers, while statistically significant, are modest, ranging from a 5% to a 9% reduction in risk.
The study cannot prove cause and effect, and even if it could, the results suggest that daily coffee consumption will do little for your long-term health if your general lifestyle is unhealthy.
Where did the story come from?
The study was carried out by researchers from the Harvard School of Public Health, Brigham and Women's Hospital, Harvard Medical School, Indiana University, the Universidad Autonoma de Madrid and the National University of Singapore.
It was funded by the US National Institutes of Health. There were no reported conflicts of interest.
The Independent and The Daily Telegraph reviewed the study in light of other recent research on coffee, giving a cautious welcome to positive findings and balancing this with warnings of the health risks (such as disrupted sleep) associated with caffeine.
The Metro was less cautious, asserting that the research means people who don't drink coffee are "missing out" and should "drink more of the black stuff".
The news reports did not include the actual figures about the differences in risk of death between coffee drinkers and non-coffee drinkers.
Some UK media sources carried the eminently sensible advice from Emily Reeve, Senior Cardiac Nurse at the British Heart Foundation, who said: "It is important to remember that maintaining a healthy lifestyle is what really matters if you want to keep your heart healthy, not how much coffee you drink."
What kind of research was this?
This was a prospective cohort study, based on three big groups (called cohorts) of health professionals, which aimed to see whether drinking caffeinated or decaffeinated coffee was associated with risk of death.
Cohort studies are observational, which means they watch to see what happens to people. This type of study can find links between factors (in this case, coffee drinking and length of life) but cannot show that one factor is the cause of another.
What did the research involve?
Researchers used information from three big cohort studies of healthcare workers in the US, which started in the 1970s and 1980s, and ran until December 2012. They looked at whether people drank coffee, and if so how much, and then followed them up to see whether they died during the course of the study. They adjusted their figures to take account of other factors that could affect the results, such as people’s age and lifestyle.
They were particularly interested in whether people smoked, and how that affected both coffee drinking and the results, because coffee drinking and smoking often go together. They also wanted to see whether decaffeinated and caffeinated coffee had different effects, and whether coffee drinking had an effect on deaths from specific diseases. They carried out different calculations, using data from the cohort studies to answer these questions.
The analysis of the data included tests to see whether people's coffee consumption changed over time, whether the results were affected by medical conditions people had at the start of the study, and people's diet, body mass index, smoking status and how often they exercised. The researchers analysed the data separately for each cohort, and then pooled it together.
What were the basic results?
Overall, the study found that 31,956 of the 208,501 people studied had died during the 21 to 28 years they were followed up. There was an association between coffee drinking and risk of death. Compared to people who drank no coffee:
- People who drank one cup of coffee a day or less were 5% less likely to have died (hazard ratio [HR] 0.95, confidence interval [CI] 0.91 to 0.99).
- People who drank one to three cups a day had a 9% lower chance of having died (HR 0.91, 95% CI 0.88 to 0.95).
- People who drank more than three to less than five cups had a 7% lower chance of having died (HR 0.93, 95% CI 0.89 to 0.97).
- People who drank five or more cups a day had no significantly different risk of death (HR 1.02, 95% CI 0.96 to 1.07).
It made little difference whether people drank caffeinated or decaffeinated coffee. However, when divided into these two subgroups, the risk reductions were only significant up to three cups a day. The separate analyses found that drinking more than three cups of either caffeinated or decaffeinated was not associated with mortality risk.
The researchers also found that non-smokers were less likely to drink coffee, and that only about a third of people who drank more than five cups a day were non-smokers.
They ran the figures again, this time including only people who never smoked. This time they found that drinking more than five cups a day did reduce the chance of death compared to people who didn't drink coffee at all, meaning that any amount of coffee seemed to reduce the risk of death, so long as people didn't smoke.
However, this could also be down to the smaller number of people in the >5 cup group when restricted to non-smokers, making the accuracy of this risk estimate slightly less reliable.
Looking at specific diseases, the study found people who drank coffee were less likely to have died from cardiovascular disease and diabetes, but more likely to have died from lung cancer or respiratory disease.
The researchers suspected that smokers were behind this result, so ran the figures again with non-smokers only, and found that the increased risk disappeared. Overall, there was no increase or decrease in risk of death from cancer, linked to drinking coffee.
How did the researchers interpret the results?
The researchers say that coffee consumption is linked to a lower risk of death, and that their finding of no reduced risk for those drinking over five cups a day was probably down to confounding by the numbers of heavy coffee drinkers who smoked.
They say there are "several plausible biological mechanisms" by which coffee might benefit health, including substances in coffee which reduce resistance to insulin and calm inflammation in the body.
This large study found that people who drink coffee have a slightly reduced risk of death compared to non-coffee-drinkers, up to the point of five cups a day. Beyond five cups, the picture is more complicated – it may be, as the researchers say, because of the link between heavy coffee drinking and smoking. However, we can't be sure that's the case.
The results for moderate coffee drinking are more consistent, but they still do not prove that coffee alone is the reason that coffee-drinkers were less likely to die during the study. The study has several strengths, including its large collective sample size, long duration of follow-up, and attempting to take into account various potential confounding factors, particularly smoking. However, the analyses may not have been able to account for the full effect of all of these or other, unmeasured health and lifestyle factors that could be influencing the results.
Other limitations include the possibility for inaccurate estimation of coffee intake. Although the study has separated into caffeinated or decaffeinated, it is not able to inform on all the nuances of coffee drinking today – such as instant, freshly ground, espresso, latte, cappuccino, etc. Also, though a large sample size, it includes only US health professionals, who may have distinct characteristics from other populations.
It is also important to note that the reduction in risk of death from drinking coffee, at less than 10% relative risk, is fairly small. There are other reasons why some people might want to avoid caffeine. It's a stimulant, and can interfere with sleep, especially if you drink it in the evening. It can raise blood pressure for a short time, which might be a problem for people with heart disease. It has also been linked to miscarriage, so pregnant women might want to avoid it.
If you want to increase your chances of living longer, coffee is unlikely to make a big difference. You'd be better off quitting smoking (if you smoke), eating a healthy diet, taking plenty of exercise and achieving or maintaining a healthy weight.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Independent, 17 November 2015
The Daily Telegraph, 17 November 2015
Mail Online, 17 November 2015
Metro, 17 November 2015
Links to the science
Circulation. Published online November 16 2015