Why are salted caramel confections so sweet?

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It’s one of life’s little ironies: Sweet foods get sweeter when you add a little salt. Now, scientists may have provided connoisseurs of salted caramel and grapefruit with the reason this culinary trick is worth its salt.

Your ability to savor food comes from the receptor cells in your tongue’s taste buds. Sweet tastes are detected by a family of receptors called T1R, which pick up both natural sugars and artificial sweeteners. Scientists originally thought disabling the T1R family would stop any responses to sweet stimuli. But in 2003, researchers showed that mice whose T1R genes had been genetically “knocked out” still liked the sugar glucose. The finding suggested there must be another way that mice—and possibly humans—sense sweetness.

Seeking an explanation, physiologist Keiko Yasumatsu of Tokyo Dental Junior College and colleagues turned to a protein that works with glucose elsewhere in the body: sodium-glucose cotransporter 1 (SGLT1). In the kidneys and intestine, SGLT1 uses sodium to carry glucose into cells to provide them with energy. Curiously, the protein is also found in sweet-responsive taste cells.

The researchers rubbed the tongues of unconscious T1R mice with a solution of glucose and salt—which contains the sodium SGLT1 needs to work—and recorded the responses of nerves connected to their taste cells. The salt seemed to make all the difference: It caused the rodents’ nerves to fire more rapidly, compared with mutated mice given only glucose. Conscious mice also seemed to show a preference for the sugar-salt solution. But this only worked with glucose; sweeteners like saccharin didn’t trigger a response.

Moreover, a compound known to inhibit SGLT1 appears to prevent the response to glucose. That suggests SGLT1 may be behind the “hidden” means of glucose sensing, the researchers report in Acta Physiologica. Although this pathway helped the knockout mice sense glucose, in regular mice, it likely boosts the sweet flavor picked up by T1R receptors. Yasumatu thinks the findings may even apply to humans—and could account for the enduring popularity of foods like salted caramel.

The researchers also concluded there are three types of sweet-sensitive taste cells. The first two, they say, use either the T1R or SGLT1 pathway; together, they help the body distinguish natural sugars from artificial sweeteners. The final type employs both pathways and also responds to fatty acids and umami flavors; the researchers propose that these provide a way to detect calorie-rich foods.

“This is an interesting piece of work that suggests the sweet taste is more complicated than we previously realized,” said Kathryn Medler, a taste signaling expert from the University at Buffalo who was not involved in the study.

“The evidence now that an SGTL contributes to detection of sweeteners by the taste system is irrefutable,” agrees Emily Liman, a neurobiologist at the University of Southern California. “We can now set aside the question of if … [and] ask how.” For researchers in this field, then, it’s time to go back to the salt mines.

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