That is One Spicy Cola

Taking a sip of cola usually delivers a powerful sweet flavor to the taste buds, but that’s not exactly what your body registers.

The carbon dioxide in fizzy drinks, including sodas, triggers the same pain sensors in the nasal cavity as mustard and horseradish, though at a lower intensity, according to new research at USC.

“Carbonation evokes two distinct sensations,” said Emily Liman, senior author of a study published online in the Journal of Neuroscience, in a press release. “It makes things sour, and it also makes them burn. We have all felt that noxious tingling sensation when soda goes down your throat too fast.”

The burning sensation comes from a system of nerves that respond to sensations of pain, skin pressure, and temperature in the nose and mouth.

“What we did not know was which cells and which molecules within those cells are responsible for the painful sensation we experience when we drink a carbonated soda,” said Liman, associate professor of neurobiology at USC College.

The researchers made the discovery by flowing carbonated saline onto a dish of nerve cells from the sensory circuits in the nose and mouth. They found that the gas activated only a particular type of cell.

“The cells that responded to CO2 were the same cells that detect mustard,” Liman said.

These cells, which express a gene known as TRPA1, serve as general pain sensors. Cells from mice missing the TRPA1 gene showed “a greatly reduced response” to carbon dioxide, Liman said. Adding the TRPA1 genetic code to CO2-insensitive cells made them responsive to the gas.

Despite being linked to pain circuits, people still seem to enjoy drinking the beverages. Liman cited studies going back as far as 1885 that found carbonation dramatically reduced the growth of bacteria.

The pain-sensing TRPA1 provides only one aspect of carbonation’s sensory experience. In 2009, a group led by Charles Zuker of the University of California, San Diego and Nicholas Ryba of the National Institutes of Health showed that carbonation triggers cells in the tongue that convey sourness.

Liman's collaborators were lead author Yuanyuan Wang and second author Rui Chang, both graduate students in neurobiology at USC. The National Institutes of Health funded the research.

Located in Los Angeles, the University of Southern California is one of the world’s leading private research universities. With a strong tradition of integrating liberal and professional education, USC fosters a vibrant culture of public service and encourages students to cross academic as well as geographic boundaries in their pursuit of knowledge.