Some coffee drinkers could face kidney dysfunction if they don't reduce caffeine, new U of T research finds

New University of Toronto research found some coffee drinkers may need to start putting up boundaries with caffeine or face an increased risk of kidney dysfunction.

Sara Mahdavi, lead author of the study published in January, said about half of the population has a genetic variant that means their body is slow at metabolizing caffeine. Coffee enthusiasts who drink more than three cups a day – and are also slow metabolizers – are at risk of kidney dysfunction, the study found.

“Those individuals when we look at how quickly they can clean their blood of caffeine, they are generally anywhere from three to four times slower at cleaning out caffeine from the blood than what we call the fast metabolizers,” she said.

The study found slow metabolizers who drink more than three cups of coffee per day are nearly three times more likely to develop kidney dysfunction, which can impede on the organ’s ability to filter waste from the blood. Unmanaged, it can lead to long-term damage, such as kidney failure.

“Drinking a large amount of coffee which contains caffeine can lead to extra stress for the kidneys to work harder,” Mahdavi said.

Much of her work dove into measuring that extra stress. Caffeine can become a toxin in the human body when it’s in a high concentration, Mahdavi explained. The key difference is slow metabolizers break down those toxins slower, which increases the likelihood of adverse effects.

Over 16 years, Mahdavi and her co-authors Ahmed El-Sohemy and Paolo Palatini observed caffeinated coffee consumption for more than a thousand people in Italy between the ages of 18 and 45.

For fast metabolizers, they found it didn’t matter how much caffeinated coffee they consumed, there was no link to kidney dysfunction.

This research, published in the journal JAMA Network Open, followed El-Sohemy’s 2006 study, which found that coffee increases the risk of a heart attack, but only for those who have the slow metabolizer gene.

AM I AT RISK?

The recent findings only showed a link between caffeine intake and kidney dysfunction for people who drank more than three cups of caffeinated coffee a day.

In the study, participants drank espresso, which typically has about 100 mg of caffeine in a single shot. Mahdavi notes that a medium coffee at Tim Hortons or Starbucks can have anywhere from 200 to 300 mg of caffeine in it. Meaning, coffee drinkers can reach that “heavy” drinker threshold much quicker in North America.

Current Health Canada guidelines indicate that a moderate daily caffeine intake of up to 400 mg per day is not associated with adverse effects.

“For at least half of the population, that recommendation is actually harmful,” Mahdavi said, pointing to her findings, which shows slow metabolizers would be at risk of kidney dysfunction if they drink 400 mg of caffeine a day.

For those wondering if they have the slow or fast metabolizer gene, Mahdavi said a simple saliva sample genetic test would reveal the answer.

Mahdavi said she appreciates the immense joy that coffee brings to many people and believes there are workarounds for slow metabolizers, like one regular cup of caffeinated coffee in the morning paired with two cups of decaf later in the day.

“We don't want to give coffee as a whole a bad name,” she said.