“It’s impossible that it’s 2017 and we still struggle to decide if coffee is healthy or unhealthy,” says Giuseppe Grosso, a nutritional epidemiologist at the University of Catania in Italy. Good for hypertension one week. Bad for hypertension the next. To address this vexing situation, Grosso and his colleagues collected all existing studies on the health effects of slurping down coffee, systematically reviewed the evidence, then offered up their bottom line in the Annual Review of Nutrition.
Specifically, they looked at 127 different meta-analyses, which lump together and statistically analyze studies on similar topics. A few of the studies were randomized controlled trials on coffee or caffeine administration, but most were observational studies of real-world coffee and caffeine consumption habits. (None of the review’s authors were paid by any food or beverage company.)
For each meta-analysis, the team calculated the strength of the study’s designs and conclusions and then ranked its evidence for relationships between coffee and health on a scale from “convincing” all the way down to “limited.” No studies showed a “convincing” level of evidence — not surprisingly, since observational studies lack the rigor of gold-standard, placebo-controlled trials. But several found “probable” evidence that coffee-drinking is associated with a decreased risk of many common cancers including breast, colorectal, colon, endometrial and prostate, with a 2 to 20 percent reduction in risk depending on the cancer type. The review also found risk reductions of 5 percent for cardiovascular disease, and around 30 percent for both type 2 diabetes and Parkinson’s disease. A coffee habit was also associated with a lowered rate of death from any cause during the course of a study. Great news for coffee drinkers!
But there is one group who should exercise caution with coffee: pregnant women. Some studies showed a link (with a “probable” score on the scientists’ scale) between caffeine or coffee intake and increased risk of miscarriage. Grosso notes that fetuses lack the enzyme needed to metabolize caffeine and they accumulate it when the mother drinks coffee. He points out that these findings are still uncertain, but out of an abundance of caution he recommends that women give up, or severely limit, their coffee intake while pregnant.
The team also solved some earlier discrepancies involving coffee and the risk for high blood pressure and death from all cancers (when lumped together). The confusion, they found, stemmed from failure to adequately control for smoking — a habit that’s strongly linked to coffee consumption. When non-smokers alone were considered, the data indicated that moderate coffee drinkers gained some protection from these diseases.
Most studies didn’t measure exact coffee volumes or caffeine levels. But in those that did, maximum benefits occurred around four to five cups (about one liter) per day — the equivalent of two Starbucks “grande”-size drinks. That’s roughly 380 to 475 milligrams of caffeine per day for typically brewed coffee drinks. (You’ll find at least 95 mg in an 8-oz cup.)
So how do the positive effects work? Coffee-shop denizens probably benefit from two main mechanisms. First, coffee beans contain phytochemicals (some of which are also found in fruits, vegetables, chocolate and tea) that have antioxidant and anti-inflammatory properties. All of the diseases linked to protective effects from coffee start with low-level inflammation in the body, and anti-inflammatory dietary chemicals circulating in the body could calm it down.
Second, caffeine and other phytochemicals have specific effects on enzymes that regulate liver function, insulin and glucose metabolism, and DNA repair. All could act favorably to fend off Parkinson’s, type 2 diabetes and cancer.
Sadly, Grosso notes that none of the analyzed data pertains to his home country’s coffee habits: “For 99.9 percent of Italians, coffee is espresso and anything else is ‘dirty water,’” he says. Italians’ typical espresso intake is only about one fluid ounce (30 milliliters) a day — a paltry 50 to 75 mg of caffeine. In other parts of the world, he's seen people guzzling much larger volumes of coffee and tea, he says. “It was absolutely important to know if this was having an effect on health.”