Coffee continues to come up smelling good in the research on its health effects. Two new, large studies published in the Annals of Internal Medicine found that coffee drinking was associated with longer life and lower risks of various common diseases including type 2 diabetes.
The findings confirm previous, smaller studies that also found signs that there might be benefits to moderate coffee consumption. One of the studies looked in particular at whether the perceived benefits, which had looked largely at populations of European decent, also extended to other racial and ethnic groups. They did.
The subjects, 185,000 of them, were followed for about 16 years. Those who drank at least two cups of coffee a day were 18 percent less likely to die during that period. The researchers controlled for various factors that might also affect longevity, such as smoking, eating habits and obesity levels.
And the more coffee that subjects drank, the less likely they were to die of a variety of the most common ailments that kill Americans including diabetes, heart disease, cancer, stroke and chronic lower respiratory disease. It didn’t matter whether people drank decaf or regular. Not affected were rates of death from Alzheimer’s disease, pneumonia or flu.
The second study was the largest ever conducted on coffee’s possible health effects, examining the records of more than 521,000 people in 10 European countries, also over about 16 years. In addition to similar results, it found, looking at a subset of about 15,000 participants, that the coffee drinkers also had better biomarkers associated with type 2 diabetes.
No one can say at this point that the coffee caused the lower rates of disease, however, because the study wasn’t a randomized trial in which people were assigned to drink various amounts of coffee, or abstain from it, for 16 years. That would be nearly impossible; few people with a coffee habit would give it up for a study, and few who detest the beverage would agree to drink several cups for years on end. The study merely found a correlation between drinking coffee and better health.
The researchers warned that other factors could have affected the results. For example, they did not control for income; wealthier people might get better medical attention or might engage in other activities that maintain better health. Or it’s possible that people who drink more coffee tend to socialize more; social interaction is also associated with better health.
All the same, there’s reason to think that coffee consumption itself might be behind the changes, at least where diabetes is concerned. A 2014 Harvard study published in the journal Diabetologia found that among coffee drinkers, those who started drinking more had a lower risk of developing type 2 diabetes, and those who drank less were at elevated risk. Changes in tea consumption didn’t have the same effect
“Our findings confirm those of previous studies that showed that higher coffee consumption was associated with lower type 2 diabetes risk,” Shilpa Bhupathiraju, the lead author, said in a statement. “Most importantly, they provide new evidence that changes in coffee consumption habit can affect type 2 diabetes risk in a relatively short period of time.”
Still unexplained, though, is exactly how coffee might bring these benefits.
Researchers have repeatedly noted that coffee contains polyphenols, which act as antioxidants, playing a role in preventing cellular damage from free radicals.
But other foods contain polyphenols as well—blueberries are one famous example. Is the possible difference that black coffee is also free of sugar and calories? Or that people take in a lot more coffee each day than they do blueberries or goji berries?
Or, could it be more complex, involving the specific types of polyphenols?
Scientists are just beginning to parse the answers.
A 2016 Japanese study in the journal Nutrition and Diabetes examined two specific polyphenols found in coffee: caffeic acid and chlorogenic acid. Researchers measured the blood-plasma concentrations of the two polyphenols and found that high levels of chlorogenic acid were strongly correlated with lower fasting blood glucose and other biomarkers associated with type 2 diabetes. Caffeic acid and overall coffee polyphenol levels were more weakly related to the better biomarkers.
In addition, coffee contains magnesium, which can contribute to increased insulin sensitivity.
But just to make things more complicated, even though multiple studies have indicated a beneficial long-term link between coffee and type 2 diabetes, some studies have found that drinking caffeinated coffee can, in the short run, have the opposite effect, increasing blood-glucose levels in people who already have diabetes. The effect isn’t well understood; results seemed to vary depending on whether people exercised or were obese.
The search for definitive answers goes on.