229,119 Men and 173,141 women asked about their diet and lifestyle to see which lifestyle factors were associated with longer or shorter life expectancy.
These men and women were then followed over many years. With such a large study, researchers were able to find thousands of cases of disease and mortality. As a result, they were able to see which lifestyle factors predicted early death, or a lower risk of death in this case.
Of note, coffee drinkers are also more likely to be smokers. Since smoking significantly increases a person’s risk of death, smoking was factored out of the analysis.
This is a higher quality study due to the number of patients involved. However, as with all observational studies, correlation doesn’t necessarily indicate causation. We cannot say whether coffee makes people healthier, or whether the types of people who drink lots of coffee are inherently healthier people. For example, it’s possible coffee drinkers tend to have healthier habits, genetics, or lifestyles. Its also possible that less healthy individuals are less likely to drink coffee.