In theory, we should want to spit coffee out upon first sip. It’s bitter, hot and has a distinct odor. As humans have evolved, we’ve adapted to reject bitter tastes as a natural warning to keep the body from ingesting potential toxins.

But new research from the Northwestern Medicine and QIMR Berghofer Medical Research Institute in Australia found that the more coffee someone drinks, the more sensitive they are to the bitter taste and essentially acquire a taste for the drink and how it makes them feel. The caffeine in coffee provides positive reinforcement, so individuals acclimate to the taste of coffee, despite how bitter it may seem because they like how it makes them feel, which overshadows the taste.

The sensitivity is caused by a genetic variant and means that people with a heightened ability to taste the bitterness in a cup of joe learn through stimulations, in this case the effects of caffeine, to associate good things with coffee.

In the study, scientists used Mendelian randomization, which can be thought of as a “natural” randomized controlled trial, to test the relationship between bitter taste and coffee consumption in more than 400,000 people.

The results also looked at alcoholic beverages and found the results intriguing.

The research found people sensitive to the bitter flavors of quinine and of propylthiouracil, a synthetic taste related to the compounds in cruciferous vegetables, avoided coffee. For alcohol, a higher sensitivity to the bitterness of PROP resulted in lower alcohol consumption, particularly of red wine.

"You'd expect that people who are particularly sensitive to the bitter taste of caffeine would drink less coffee," said Marilyn Cornelis, assistant professor of preventive medicine at Northwestern University Feinberg School of Medicine. "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."

The findings demonstrate that differences in bitter taste perception are causally associated with specific bitter beverage consumption behavior.

"Taste has been studied for a long time, but we don't know the full mechanics of it," Cornelis said. "Taste is one of the senses. We want to understand it from a biological standpoint."

The study was published in Scientific Reports.