Decades ago, many scientists believed that drinking coffee was bad for your health. But coffee has experienced a remarkable turnaround. Study after study has found that enjoying a daily cup or two of Joe — either caffeinated or decaffeinated — may lengthen your life span and lower risk for chronic disease.

One of the most striking findings is that coffee drinkers are less prone to developing Type 2 diabetes. Many large studies have found that people who drink three to four cups of coffee daily have about a 25 percent lower risk of the disease compared with people who drink little or no coffee. Your likelihood of developing diabetes decreases about 6 percent for each cup of coffee you consume daily — but only up to about six cups.

Many of the studies on coffee and health come with an important caveat. They are usually large observational studies, which show correlations — not cause and effect. This means that it’s possible that something else could explain the findings. Perhaps coffee drinkers also are more likely to exercise more, drink less alcohol, eat healthier diets or engage in other habits that boost their health.

But there are other reasons to believe that the findings are not a mirage. Coffee’s protective effect against diabetes persists even when scientists take these other lifestyle behaviors into account. The effect has been found in dozens of studies involving more than a million participants across Europe, North America and Asia. It’s been found in women and men, in young and old people, in smokers and nonsmokers, and in people with and without obesity.