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.
Researchers have also shown that the risk rises and falls with changes in coffee consumption. In studies that tracked thousands of men and women over two decades, scientists found that when coffee drinkers increased their coffee intake by an extra cup or two a day, their risk of diabetes fell 11 percent. But when people decreased their coffee intake by roughly the same amount, their likelihood of developing diabetes rose by 17 percent. Scientists did not see the same effect when they looked at changes in tea consumption.
Experts say that coffee is more than just a delivery system for caffeine. It has hundreds of other compounds that can have surprising effects on our metabolism. In the short term, when you drink coffee – especially if you don’t drink it regularly – the caffeine it contains triggers the fight-or-flight response. This stimulates higher adrenaline levels, increased blood pressure and blood sugar levels, and a reduction in your insulin sensitivity.
“You get a stress response,” said Rob van Dam, an expert on coffee’s health effects and a professor at the Milken Institute School of Public Health at George Washington University.
These physiological changes are why scientists believed decades ago that drinking coffee was generally harmful.
“At the time, what was mostly available were these trials where you give people just coffee or just caffeine and you look at them over a couple hours and you see these clear detrimental effects,” said van Dam, who is also an adjunct professor of nutrition at the Harvard T.H. Chan School of Public Health.
But as any longtime coffee drinker can attest, when you consume coffee long enough, you build up a tolerance to its stimulating effects. The jittery, unpleasant and seemingly detrimental physiological response becomes less pronounced.
“Within a week these responses are largely gone,” said van Dam. “You don’t have a big stress response anymore, and you don’t see the effects on blood pressure or glucose levels.”
At the same time, coffee’s other properties begin to work their magic. Coffee is a rich source of polyphenols – compounds in fruits, vegetables, whole grains and other plants that are known to confer health benefits.
A cup of coffee contains about double the concentration of polyphenols contained in a cup of green or black tea. “There are hundreds of phytochemicals in coffee,” said van Dam.
A cup of coffee also contains fiber – up to 1.8 grams, or roughly half the amount you’d find in one serving of broccoli.
Coffee in many ways is akin to a liquid vegetable, said Hubert Kolb, a visiting scientist at the West-German Center of Diabetes and Health in Düsseldorf who studies coffee’s health effects.
“One helping of vegetables is a small cup of coffee if you compare the amounts of polyphenols they contain,” he added.
One of the most potent and abundant polyphenols in coffee is chlorogenic acid, which has been shown in some studies to improve insulin sensitivity and blood sugar control. Chlorogenic acid and other polyphenols in coffee help to reduce inflammation and increase the production of proteins involved in repairing and protecting cells and their DNA.
Studies indicate that these effects occur in organs throughout the body, but in particular in the liver and in the beta cells of the pancreas, which produce insulin and play a critical role in the development of Type 2 diabetes. According to Kolb and his colleagues, habitually drinking coffee probably lowers the risk of diabetes because it helps to prevent the deterioration of liver and beta cell function.
According to van Dam at George Washington University, while there are clear health benefits to drinking coffee, they are not enough to suggest that every adult should start drinking coffee or increase the number of cups they consume.
Coffee certainly has its downsides. Depending on how much you consume, it can disrupt sleep, worsen anxiety and cause headaches, nausea and other side effects. Drinking more than two cups of coffee a day can also increase the risk of complications in pregnant women.
For these reasons, health authorities generally recommend that healthy adults consume no more than about 400 mgs of caffeine daily, which is equivalent to four or five cups of brewed coffee. Coincidentally, studies show that two to five cups daily is the range in which people are most likely to see health benefits such as a reduced risk of diabetes, heart disease and some cancers, said van Dam.
But even that may be too much for some people. Doctors may recommend cutting back on coffee if you have a sleep disorder, cardiac issues or glaucoma.
If you don’t drink coffee and don’t particularly enjoy it, then don’t feel pressure to start. But for those who do drink it daily, it’s nice to know that your morning coffee – in addition to tasting delicious and providing a pick-me-up – may be doing more for your health.
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