Researchers started with data from surveys of adults in the United States that asked how much coffee they consumed, as well as other foods and drinks, and then they looked at their rates of death and disease over the following two decades.
The study was large, including more than 200,000 women and 50,000 men.
At first, researchers did not see an obvious relationship between coffee consumption and death rates. Study participants who drank between less than a cup of coffee and three cups a day had 5% to 9% lower risk of dying than those who drank no coffee. Those who drank more than three cups a day did not see any benefit. The finding was murky, like previous studies, some of which suggested a benefit and some did not.
But when the researchers looked at coffee consumption only among people who said they never smoked, the relationship became clearer: Those who drank between less than a cup of coffee and three cups a day had 6% to 8% lower risk of dying than noncoffee drinkers. Those who drank three to five cups and more than five cups had 15% and 12% lower death rates.
"The lower risk of mortality is consistent with our hypothesis that coffee consumption could be good for you (because) we have published papers showing that coffee consumption is associated with lower risk of type 2 diabetes and (heart) disease," said Ming Ding, a doctoral student in the Harvard School of Public Health department of nutrition. Ding is the lead author of the study, which was published on Monday in the journal Circulation.
It might have been hard to see the link between coffee consumption and lower death rates because coffee and smoking often go hand-in-hand, and any benefits associated with the first could have been canceled out by the second. Although the study participants were asked about smoking, there might have been a tendency, especially among heavy smokers, to underestimate the average number of cigarettes they smoked per day, Ding said.
What's behind the lower death rate?
It's possible that people who drink a lot of java have healthier diets overall and drink less soda, which has been linked to higher rates of death and heart disease, or that they have healthier diets overall.
But that's probably not what links coffee to lower death rates -- researchers took into account the health benefits of drinking less soda and eating well. They also took into account the fact that coffee drinkers were more likely to have vices such as drinking alcohol and eating red meat.
At least some of the health benefits associated with coffee consumption are probably a direct result of the ingredients in coffee, Ding said. It contains chemicals such as lignans and chlorogenic acid that could reduce inflammation and help control blood sugar, both of which could help reduce the risk of heart disease.
In keeping with this possibility, Ding and her colleagues found that coffee drinkers were about 10% less likely to die of heart disease. They were also between 9% and 37% less likely to die of neurological diseases such as Parkinson's and dementia.
The researchers also found that study participants who drank at least a cup of coffee a day had between 20% and 36% lower rates of suicide, although those who drank less than a cup had 36% higher rates.
Several other studies have hinted at an association between coffee consumption and lower suicide rates, but it was a bit unexpected to see, Ding said. It is not clear whether chemicals in coffee have a direct effect on mental health or whether people who drink a lot of coffee have higher rates of employment or certain lifestyles that are associated with lower suicide rates, she added.
Although previous studies have suggested that drinking coffee could protect against cancers such as prostate and liver, the current study did not find lower rates of cancer deaths among java drinkers.
However, there may not have been a large enough number of deaths because of specific cancers, such as liver cancer, to be able to see a difference between coffee drinkers and nondrinkers, Ding said.