Let’s start with some numbers. According to the International Coffee Organization, the USA bought enough coffee beans in 2014 to brew something like 100 billion cups of coffee. That’s roughly one cup of coffee per day for every single US resident. Including babies. Coffee is everywhere, all the time.

So here’s a perfectly reasonable question: is coffee good for you? Humans have been drinking coffee for at least 600 years — plenty of time to come up with an answer.

The answer is a resounding “maybe”. Consider:

• A 1985 article in the New York Times suggested that 5 or more cups a day increases risk of heart disease. Specifically, it described a study which concluded that drinking that much coffee will triple the risk of heart disease, relative to people who drink none. (Coffee bad.)
• A 2012 article in the New England Journal of Medicine reported a correlation between increased coffee consumption and a decrease in “all-cause mortality” rates, essentially a measure of the likelihood of death. (Coffee good!)
• A 2013 study by the Mayo Clinic concluded that 4 cups per day increases the likelihood of all-cause mortality. (Coffee bad.) They recommend that young people limit their consumption of coffee to less than 4 cups per day.
• A 2014 study in The Annals of Internal Medicine concluded that “Regular coffee consumption was not associated with an increased mortality rate in either men or women. The possibility of a modest benefit of coffee consumption on all-cause and CVD mortality needs to be further investigated.” (Coffee good?)

I could go on. And on. And on. There’s an overwhelming supply of studies that will support either side of this argument. We haven’t even started talking about coffee’s effect on specific medical conditions; there are studies out there addressing coffee’s effect on the incidence and/or severity of Parkinson’s, liver disease, diabetes, Alzheimer’s, anemia, depression, dementia, athletic performance …

Superficially, this is bonkers. Coffee is everywhere. If you are not currently drinking coffee, you probably could be five minutes from now if you put your mind to it. I’m writing this post while sitting in a coffee shop. How can we still have so much uncertainty about something so ubiquitous?

The answer is typically science-y, of course. Coffee’s effect on the human body is complex because the human body is complex. These confusing and contradictory experimental results can motivate scientists to seek deeper, more complete answers to difficult questions. In the case of coffee, some very recent work suggests that your genes determine whether “coffee good” or “coffee bad”. Brave readers with lots of free time might want to stick around for the epilogue.

Anyway, I assert that coffee is hard to understand, despite its ubiquity. Do you know what else is ubiquitous and hard to understand?

Did you just yell “NEUTRINOS!” at the top of your lungs? Yeah, that’s the answer I was going for.

Subatomic particles, generally, are so, so far outside our everyday experience as human beings. Chances are, you’ve never had any reason to care about muons in your daily life. The subject just doesn’t come up, right? What about cosmic ray flux? Or neutrino oscillation rates? Not as often as you drink coffee, amirite?

So here you are, minding your own business, when a physicist starts blogging at you about neutrinos. They’re all around you, he says. Trillions of them pass through your body every second, he says. What are you supposed to do with that? To be blunt, how are you supposed to believe something so far outside your daily experience, when you don’t even know whether “coffee bad”?

Here, I don’t mean “believe” in the sense of truth vs. lies. I mean, how can you know that your body is permeated by neutrinos in the same way you know that gravity pulls you down to Earth, or that snow is made of frozen water, or that Daniel is handsome? You have direct, personal experience, through your senses, that these things are true. There are no intermediate steps. You don’t have to consult a scientific instrument to know that things fall down — you can feel the pull of gravity and you can see its effects on everything around you. Likewise, you don’t need to read a book in a library to know that coffee tastes amazing at 8 am.

But are your senses the only reliable source of truth? Are you skeptical about neutrinos because you can’t see them? You haven’t seen live dinosaurs either, and your day-to-day experience suggests that the Earth is flat, not round. Maybe you should limit your appreciation of truth to what you can sense for yourself.

More than two thousand years ago, the ancient Greeks kicked that idea right in its butt. This is a long discussion and I can’t do it justice in an already-long blog post. Essentially, your perception can change depending on circumstances. For example, maybe a fig tastes sweet to you. But if you eat honey before you eat figs, maybe those figs won’t seem so sweet anymore. What can you say that you know (like, really really know) about the taste of figs?

Your perceptions can be unreliable. Just think about the last time you got hangry. You skipped breakfast, maybe, and then right around 11 am the world started to suck, right? The line at the coffee shop started to seem unreasonably long, or the barista’s haircut seemed unreasonably annoying, or the guy behind you in line was talking unreasonably loud on his phone. In that moment, are you really perceiving an objective reality? Do you have well-deserved, righteous indignation about the barista’s haircut? Maybe you should get a muffin with that coffee.

Our senses, by themselves, are not the sole arbiters of truth. They are vital, beautiful, and useful, but they are not the whole story. Humans reach for truth in ways besides immediate sensory experience. One of those ways is called science. We have built tools and systems of thought in order to help us reliably, repeatably demonstrate complex and obscure phenomena.

Leon Lederman is a Nobel laureate, a former director of Fermilab, and the co-author of a truly enjoyable book with an admittedly silly name: The God Particle: If the Universe is the Answer, What Is the Question? (1993, Bantam Press). Here’s a particularly relevant excerpt. (Note for young people: TVs used to be bulky vacuum tubes with electron beams inside.)

The lady in the audience was stubborn. “Have you ever seen an atom?” she insisted.  … My attempts to answer this thorny question always begin with trying to generalize the word “see”. Do you “see” this page if you are wearing glasses? … If you are reading the text on a computer screen? Finally, in desperation, I ask, “Have you ever seen the pope?”

“Well, of course,” is the usual response. “I saw him on television.” Oh, really? What she saw was an electron beam striking phosphorous painted on the inside of a glass screen. My evidence for the atom, or the quark, is just as good.

Sometimes, you need to reach for truth through a pair of glasses. Or a television. Or a particle accelerator.

I’ll be talking about these ideas in more depth on December 6th at an event called Ask A Scientist. It should be fun! Hope to see you there.

Epilogue for Sticklers

For those of you still reading, I should admit to being a little glib for rhetorical reasons. As I said before, the human body is an incredibly complex system. To pose a binary question about whether coffee is categorically good or bad is to be ridiculously reductive.

Sometimes, the questions worth asking have complex answers. The questions we ask should allow for answers complex enough to be correct. As they say on Twitter, you should want better for yourself.
Arguments about all-cause mortality are statistical in nature and difficult to apply to a specific individual with her own specific physiology, metabolism, gut flora, lifestyle, etc. And in fact, there are a couple studies I’ve seen recently that bear this out.

• Does coffee increase your risk of heart disease? Well, you’ve got a gene called CYP1A2 that tells your liver how to make enzymes that help to metabolize caffeine. If you’ve got the CYP1A21A allele, your liver will make enzymes that help you to metabolize caffeine quickly; in that case, “coffee good”. But if you’ve got the CYP1A21F allele instead, you metabolize caffeine slowly and coffee might increase your risk of a heart attack. (Coffee bad.) This is hard to summarize in a paragraph-friendly way. Check out the article for better information.
• Likewise, there seem to be genetic factors that influence the effect of coffee on the risk of Parkinson’s disease.

Probably, then, the question “is coffee healthy” is a bad question to ask since the answer depends so much on individual factors. Perhaps a better question would be, “will I personally benefit from drinking coffee?” And perhaps you can’t answer that question without doing some of your own research, listening to your body, … I’ve even heard of people ordering genetic tests for themselves so that they can have some certainty about this.

Tip your baristas, ladies & gentlemen.