Have you had a cup (or two or three) of coffee this morning?
Most likely you have. According to a survey taken earlier this year, 64 percent of Americans over the age of 18 drink at least one cup of coffee daily.
Yet, given coffee’s bitter taste, our fondness for the beverage is, well, somewhat odd. Bitterness is, after all, an evolutionary tool that helps us avoid the consumption of harmful foods and other substances.
Yet it gets even odder than that. For, according to a study published late last week in the journal Scientific Reports, people with a genetic variant that makes them especially sensitive to caffeine — one of the bitter compounds in coffee — tend to drink more coffee than those without that variant.
“You’d expect that people who are particularly sensitive to the bitter taste of caffeine would drink less coffee,” said Marilyn Cornelis, the study’s senior author and an assistant professor of preventive medicine at Northwestern University, in a released statement. “The opposite results of our study suggest coffee consumers acquire a taste or an ability to detect caffeine due to the learned positive reinforcement (i.e. stimulation) elicited by caffeine.”
In other words, coffee drinkers learn to associate caffeine’s unpleasant taste with its pleasant “buzz” of alertness, and thus will drink — and even enjoy — the substance anyway.
Those people, do, however, tend to drink less caffeinated tea than individuals without the variant. But that finding may be explained by the fact that they are too busy consuming coffee, says Cornelis.
To do their study, the researchers used an epidemiological technique known as “Mendelian randomization.”
“We knew from previous research that inherited factors play a role in the amount of coffee and tea a person drinks a day, and that the ability to digest caffeine plays an important role in the people’s consumption of caffeinated beverage,” writes Daniel Liang-Dar Hwang, one of the authors of the study and a postdoctoral researcher at the University of Queensland, in an article for The Conversation.
“But we didn’t know whether genes for bitter taste perception were involved in determining consumption of bitter-tasting beverages,” he adds. “Previous studies with small sample sizes reported no or inconsistent relationships.”
The researchers began with data from a large Australian study involving more than 1,700 twins and their siblings. It showed that three particular gene variants are associated with a trio of distinct bitter tastes: caffeine, quinine (also found in coffee, as well as in tonic water), and an artificially made substance called 6-n-propylthiouracil, or PROP (a molecule that has the same bitterness as Brussels sprouts, according to Hwang).
The researchers then analyzed data from more than 430,000 adults participating in UK Biobank, a large database set up in the U.K. to help that country’s health officials collect information about preventing, diagnosing and treating a wide range of diseases. They compared the participants’ genetic data (specifically, whether they had any of the gene variants associated with sensitivities to caffeine, quinine or PROP) with their self-reports of how much coffee, tea and alcohol they drank each day.
“Compared to an average person, we showed that people who carried the bitter taste [gene] receptor for caffeine were more likely to be heavy coffee drinkers, meaning they drank more than four cups of coffee a day,” writes Hwang. “Every extra copy of the bitter taste receptor gene [led] to a 20% higher chance of being a heavy coffee drinker. These ‘super-tasters’ of caffeine also drank less tea.”
“In contrast, people who carried the bitter taste receptors for quinine or PROP drank less coffee and more tea,” he adds. “Compared to an average person, every extra copy of the quinine or PROP receptor gene was linked with a 9% or 4% chance of being a heavy tea drinker (meaning they drank more than 5 cups of tea a day.”
The study also found that people who had the gene variant associated with a greater sensitivity to PROP tended to drink less alcohol, particularly red wine.
Not only a matter of genes
The study comes with caveats, of course. Most notably, it relied on the participants’ own reports about how often they drank different beverages. Such self-reports can be unreliable. Also, most of the study’s participants were of European ancestry. The findings may not be applicable to other demographics.
Still, the findings are in line with other research that has identified genetic reasons for certain taste preferences — for example, why some people find cilantro delicious, while others complain it tastes “like soap.”
But, as Hwang points out, we can’t blame all our dietary preferences on our genes.
“Even if as a child or right now you dislike the bitterness of coffee or tea, you may have noticed that your taste and dietary behaviour change over time as you grow,” he explains. “So, even if you carried the ‘wrong’ genes in terms of tasting bitter flavours, you could still learn to enjoy deliciously bitter-tasting foods and beverages.”
FMI: You can read the study in full on the Scientific Reports website.