Sorry, we can’t REALLY tell you once and for all if coffee is healthy, because that’s not how science works.
Coffee has been linked to a lower risk of Parkinson’s, which is a neurological disease that can cause tremors, balance problems, and mobility issues. In one 2000 study in the Journal of the American Medical Association, researchers looked at 30 years of data from over 8,000 Japanese-American men in Hawaii. They found that the more daily coffee they drank (up to 28 ounces, or three to four cups), the lower their risk of Parkinson’s compared with those who drank little or none. (The same seemed to be true of caffeine-containing beverages in general, so the theory is that the caffeine may be responsible.) There is so much research on this that the amount of evidence for the link is "substantial," according to a 2017 review, but still "not conclusive," so do with that what you will.
Improve performance in terms of speed AND endurance
Long-studied as a mild (legal) performance enhancer, coffee — or more specifically, the caffeine in coffee — has been shown in multiple studies to actually increase speed and endurance a bit in sports like running, cycling, rowing, and others. You're not gonna win the Olympics after a trip to Starbucks. But the consensus seems to be it’s safe for athletes to consume in moderate amounts, e.g., the amount found in one to two cups of coffee about an hour before exercise. (More isn’t necessarily better, and might be detrimental.)
Yes, but it’s still a bit iffy
Coffee drinkers might have a lower risk of Alzheimer’s disease than non–coffee drinkers. But some of the research is in animals, which is generally considered not the best indicator of what will happen in humans. And a 2010 analysis published in the Journal of Alzheimer’s Disease suggested that people who drink three to five cups daily in middle-age have a lower risk of Alzheimer’s in old age. But the jury still out — there isn’t a definitive randomized clinical trial to look at the issue. (And because such studies are pricey and few people want to spend a lifetime skipping/drinking coffee in the name of science, don’t expect such a study any time soon.)
Yup, pour me another cup
Coffee contains antioxidants such as polyphenols. (Chlorogenic acid is the main one in coffee.) The amount is on par with tea, cocoa, and red wine. While some people say antioxidants are the reason for potential health benefits of coffee, it’s not clear if those particular compounds will snatch you back from the yawning abyss of death. So yeah, drink it if you like it, antioxidants be damned.
Reduce the risk you’ll get them
The evidence suggests that coffee can help prevent gallstones, rock-hard bile deposits that can be hideously painful and even lead to gallbladder removal surgery. For example, one 1999 study of more than 46,000 men found that those who drank coffee had a lower risk of going on to develop gallstones or need surgery during the 10-year study than those who didn’t drink it. (They didn’t find a risk reduction with decaf.) Coffee is thought to stimulate contraction of the gallbladder and reduce cholesterol crystallization in bile, among other things, so there’s a mechanism here that might explain the link.
A lower risk of cirrhosis and liver cancer
In a 2016 meta-analysis (a study that combines a bunch of other research on a subject) researchers found that one to five daily cups of caffeinated coffee, and to a lesser extent decaf, was linked to a 14–27% lower risk of liver cancer. And multiple studies have linked coffee intake with a lower risk of cirrhosis, a scarring of the liver.
Increase the effectiveness of certain pain relievers
This seems to be all about the caffeine. A review of 20 studies suggests that the amount of caffeine found in a cup of coffee — about 100 milligrams — can increase the effectiveness of pain relievers like acetaminophen (found in Tylenol and other products) and ibuprofen (found in Advil and others). An additional 5–10% of people found pain relief with caffeine + pain reliever vs. pain reliever alone. Caveat: Most of the research looked at caffeine, not coffee itself. But there’s research on this going back for decades, so there’s good evidence here.
Research suggests that coffee drinkers — decaf or caffeinated — are LESS likely to get Type 2 diabetes. (That’s the most common form of the disease, the one that can sometimes be delayed with dietary, exercise, and lifestyle changes.) A 2014 review in the journal Diabetes Care that included 28 studies and more than 1 million people suggested that coffee drinking was linked to a small reduction in Type 2 diabetes risk.
A number of studies have found an association between coffee and better cognitive function. For example, a 2002 study of 890 older women found that coffee consumption was linked with better performance on cognitive tests. (But the link wasn’t found in men or decaf drinkers.) Caffeine actually increases blood pressure, although that effect tends to wear off as your body becomes accustomed to it. And a 2009 review found that caffeine can make symptoms worse in people with anxiety and panic disorder, according to the American Psychological Association.
Live longer, non–coffee drinkers
There’s some really large studies that suggest this is true, including one published in 2017 of more than 500,000 people that found that coffee drinkers (two to four cups a day) had an 18% lower risk of dying during the study than nondrinkers. But again this one might be complicated by the fact that people who have health conditions may cut back on coffee — which would mean that coffee isn’t really the magic longevity elixir of your dreams. But if coffee is giving you the strength to make it through your daily grind, we give you permission to pretend it is.
All of the above
Let’s be honest — caffeinated coffee will mess up your sleep if you drink it before bed. And by “mess up” we mean harder to get to sleep, more episodes of wakefulness, and less total sleep time. However, other research suggests that if you consume 200–250 milligrams of coffee (that’s about the amount in one to two cups), take a nap, and wake 30 minutes later, you’ll be more alert than you would with a snooze alone. But these studies are small so feel free to take a caffeine-free nap and just enjoy your coffee free of any urges to “game” the system. And if you leave enough hours between your last caffeinated drink and your bedtime — at least six hours — you should be OK.
Both are true. Fun!
Caffeine is a vasoconstrictor — it constricts your blood vessels. If you drink caffeinated coffee daily, and suddenly stop, you can get rebound headaches, which happen from caffeine withdrawal. Conversely, blood vessels tend to expand just before a migraine, according to the National Headache Foundation. So caffeine can help mitigate the pain, and indeed, caffeine is an ingredient in some over-the-counter headache remedies.
Switch to decaf or cut back to a cup a day
OK, we’ll be honest — there’s a lot of conflicting information on this. It’s always best to talk to your doctor for the final word. But some research suggests that caffeine can affect fetal growth so it’s not a bad idea to limit caffeine. The American College of Obstetricians and Gynecologists says that less than 200 milligrams a day of caffeine, about the amount in one 12-ounce cup of coffee, is generally recognized as just fine. (But again, your doctor.)