Is coffee good or bad for you? That may depend on your genes

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Latte, pour-over or double double: no matter how you grind or brew it, many Canadians start their day with a cup of coffee.

Coffee — more specifically, caffeine — can be a great way to feel more alert and awake. But what else is that cup of coffee doing to our health?

According to experts, it depends.

“In popular press, one day coffee is good for you, another day it’s bad for you, another day it doesn’t do anything,” said Sara Mahdavi, a clinical scientist at the University of Toronto.

But whether a certain level of caffeine has health benefits, is neutral or harmful depends on how our bodies respond to coffee, which happens through a genetic pathway — something many studies don’t take into account, she said.

“For the most part, people are not looking at the genetics of a population who consumes this coffee,” said Mahdavi, who has done research into caffeine and genetics.

LISTEN | What impact does coffee have on our health?

The Dose24:47How does drinking coffee affect my health?

For many of us, coffee is an essential part of our day. So what impact is it having on us, beyond just waking us up in the morning? To try to answer that question, we speak to Thomas Merritt, a geneticist and professor at Laurentian University in Sudbury. For transcripts of this series, please visit:

Thomas Merritt, a geneticist and professor in the school of natural sciences at Laurentian University, agrees that much of it comes down to our genes.

“There’s some people who can pound back pot after pot with no adverse effects. And there’s some people, they have a sip of coffee and they get really amped up and over-jittery,” Merritt told The Dose‘s hostDr. Brian Goldman.

“So there’s a lot of variation in how individuals respond to that cup of coffee.”

So how much coffee should we drink?

Mahdavi recommends limiting your caffeine intake to 200 milligrams per day, which she said her research shows is safe for everyone.

How much coffee is that? According to Health Canada’s caffeine guidelines, an eight-ounce cup of coffee can contain between 118 to 179 milligrams of caffeine, depending on how it’s brewed.

A double shot of espresso — which is more concentrated than regular brewed coffee — has about 150 milligrams of caffeine.

Health Canada’s guidelines recommend up to 400 milligrams of caffeine per day for adults who aren’t pregnant or breastfeeding.

In general, most people can tolerate one to four cups of coffee a day before they start to get jittery, said Merritt.

But the important thing, he says, is to pay attention to how coffee makes you feel.

“Do you feel uncomfortable after a cup of coffee? Well, you should probably scale it back or think about something like a decaf coffee. And if you feel like you’re waking up and you’re still comfortable with the coffee after a couple of cups, that’s great,” he said.

Hands hold a mug of coffee.
People generally drink as much coffee as their bodies can tolerate and no more than that, says Marilyn Cornelis, an associate professor of medicine at Northwestern University, who has researched the genetics of coffee consumption. (Shutterstock)

How to say coffee wake us up?

The caffeine molecules in coffee look a lot like a naturally-occurring neurotransmitter in our bodies called adenosine.

“Adenosine is involved with keeping us drowsy, keeping us asleep,” said Merritt.

The caffeine binds to our adenosine receptors, but because it looks slightly different, it turns off the sleep pathway and turns on a wake-up pathway instead.

“It breaks that normal sleep cascade and breaks that cycle, and it actually fires off a different cascade that leads to a series of things, one of which is waking us up,” said Merritt.

And it’s not just alertness. He said drinking coffee can also provide a hit of dopamine, a chemical neurotransmitter that sends messages of pleasure to the brain.

“So to drink a cup of coffee does wake you up, but it has this euphoric effect to it, in addition to just that buzz of a cup of coffee.”

Why our genes are important

Though the mechanics of the jolt of caffeine will be the same for everyone, experts say the degree to which an individual’s reactions can vary wildly — because of how quickly or slowly our bodies metabolize caffeine.

If you’re a fast caffeine metabolizer, you can tolerate a lot more coffee, whereas if you’re slower to metabolize that caffeine, you’re likely more sensitive to coffee, says Marilyn Cornelis, who researches the genetics of coffee consumption.

And those genes often affect how much coffee you naturally drinksaid Cornelis, an associate professor in the department of preventive medicine at Northwestern University’s Feinberg School of Medicine.

“As an example, if I have a genetic variant related to a higher caffeine metabolism, I will generally consume more dietary caffeine or coffee,” she said.

“We actually see that the genetics alter our behaviors.”

Earlier this year, Mahdavi published a study suggesting that the genetic variants that affect our caffeine metabolism have a big impact on whether coffee is boosting our health or harming it.

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For the study, Mahdavi and her co-authors looked at data from more than 1,100 people between the ages of 18 and 45 over a period of 16 years.

The participants all had stage one hypertension, or high blood pressure.

“We were able to demonstrate that of course those individuals who are slow metabolizers, the more coffee they consume — particularly more than 300 milligrams equivalent of caffeine per day — there are different health deterrents,” said Mahdavi.

Her study measured three markers of kidney health, including hypertension, and found that the group that metabolized coffee more slowly showed a decline in kidney function.

When she looked at the results from fast caffeine metabolizers, they told a different story.

Regardless of the amount of coffee they drink, their kidney function stays the same instead of decreasing, which suggests the speed of caffeine metabolism makes a big difference.

“It was really quite miraculous when you looked at their graph over time,” said Mahdavi. “There was literally no difference between those who consumed no coffee, some coffee and a lot of coffee, with relation to those three same markers of their kidney [health].”

Mahdavi and Cornelis say research shows that about half of the general population metabolizes caffeine more slowly, while the other half does it more quickly.

So how do you know which kind of caffeine metabolizer you are?

There’s a simple genetic test that will tell you, said Mahdavi, although it usually costs hundreds of dollars and isn’t covered by insurance. But you may not need to worry about it.

“Based on my research, we generally consume within our tolerability of coffee,” said Cornelis.

That means if you’re a slow metabolism, you can likely feel how coffee affects you and cut back on your own.

A man sits in a booth in a restaurant and pours coffee into cups on a tray from an ethnic-looking pot.  A vessel containing what looks like smoldering ash sits on the table beside him.
Habtamu Lamu, owner of Awash Ethiopian Restaurant in Edmonton, pours a cup of coffee. A recent study from the University of Toronto suggested that people who metabolize caffeine more quickly often get health benefits from drinking coffee. (Nola Keeler/CBC)

Is coffee doing us any good?

For those who metabolize coffee more quickly, there are definite health benefits, said Mahdavi.

“We saw that the more coffee they consumed, actually their rate of heart attack went down,” said Mahdavi of the participants in her recent study who were fast caffeine metabolizers.

More recent research shows that coffee is generally good for us, including reducing the risk of both Type 2 diabetes and Parkinson’s diseasesaid Cornelis.

“Twenty years back … coffee and caffeine in general really had a bad rap. But with better research they’ve actually shown that at least coffee, it’s actually showing to have a much more beneficial impact on health generally,” she said.