Researchers think coffee is good for you but still can't be sure—here's why

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Despite decades of studies and headline like the recent one claiming that "coffee can help you live past 90," researchers overall still aren't sure whether the caffeinated life-source for over half of America really is good for you.

They think it might be, in some cases, but that's as far as they can go.

A recent comprehensive overview published in the Annual Review of Nutrition and highlighted by Knowable Magazine is the latest to make that hesitant claim. The researchers from the University of Catania in Italy found "probable" evidence that drinking coffee can reduce the risk of cardiovascular disease, Type 2 diabetes and some neurological conditions, such as Parkinson's and Alzheimer's disease. It also might prevent common cancers, including breast, colon and prostate, they said, echoing a claim from the World Health Organization in 2015.

Coffee beans contain caffeine and phytochemicals, which have antioxidant and anti-inflammatory properties, Knowable points out, as well as "specific effects on enzymes that regulate liver function, insulin and glucose metabolism and DNA repair." That explains its probable benefits for most consumers, with some exceptions, such as pregnant women, since researchers found coffee may be associated with an increased risk of miscarriage.

But of all the studies they looked at in their 127 meta-analyses, each comprised of groups of studies addressing the same coffee-related topic, none was ideal.

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As Knowable points out, "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."

Most of the studies were observational, and it's difficult to prove causality from an observational study, because the researchers have little control over the other variables in the experiment.

Maybe coffee really does help you live past 90, as researchers from the University of California, Irvine, recently announced, noting that "people who drank moderate amounts of alcohol or coffee lived longer than those who abstained." Or maybe coffee drinkers live longer because they also happen to drink less sugary beverages or because, on the whole, they're better off socioeconomically. The UC-Irvine findings haven't been officially published yet but researchers will have to control for these factors before they can responsibly claim that coffee is a life-extending elixir.

Frustratingly, there are also nuances that complicate most broad claims. Some findings, for instance, suggest that drinking very hot beverages at 149 degrees or more might increase the risk of esophageal cancer.

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Coffee contains the chemical acrylamide, a byproduct of the brewing process, which could cause cancer. The few studies that assess the risk of exposure of acrylamide in humans have been inconsistent and ultimately inconclusive. But it's currently the focus of a lawsuit in California deciding whether companies should have to provide cancer warning labels on their coffee cups. Several have settled and agreed to add a label, including most recently 7-Eleven.

Another reason it's difficult to generalize about health benefits is that different people respond to coffee differently.

A recent comprehensive overview of 700 studies about caffeine in the Atlantic pegged 400 milligrams as a safe daily limit, about the same amount the Italians claim provides the maximum benefit. That's two mediums at Dunkin' or a Venti at Starbucks. Any more, the researchers found, was associated with adverse effects: "Everything from depression and dysphoria (general unhappiness) to anxiety to hypertension to higher proportions of sperm with DNA damage," the Atlantic notes.

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Still, the researchers could not definitively claim that too much caffeine is dangerous, either. "There's a great deal of inter-individual variability in how people respond to caffeine. That's one of the research gaps. We need to better identify differences and identify people who are more sensitive," said Esther Myers, one of the authors of the paper.

And one study claims that, if you carry a specific gene variant, coffee might increase your risk of heart attack. "There are spectacular metabolic differences in people and to expect that coffee will have the same health effects on everyone is absurd," UCLA professor Sander Greenland told the Washington Post in light of the finding. "They want to come out with the generalized recommendations? It's laughable."

So it's good to digest all this research with a healthy dose of skepticism because, after thousands of studies, it appears coffee might cure heart attacks or incite them, and it could prevent cancer or cause it.

Overall, according to the most reliable and recent information, coffee is probably good for (some of) you, when consumed in moderation — and as long as it's not too hot.

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