A curious thing happens to some obese people who have gastric bypass surgery: they lose more than weight. They also lose their cravings for high-fat and very sweet foods, without which they might never have needed surgery.
Somehow the surgery changes the way taste is encoded in the brain, says Patricia Di Lorenzo, professor of behavioral neuroscience in the Psychology Department. “We don’t know why.”
“We also don’t know why some people overeat and why they can’t stop,” she says.
But Di Lorenzo’s research into taste may someday help take a bite out of the obesity problem by decoding how the properties of food are processed in the brain.
“It’s become a race against the clock to understand the taste system and its influence. We know that taste is the main factor that drives intake; it’s not just what something tastes like, it’s that it tastes good. We want to understand what parts of the brain make taste, taste good,” Di Lorenzo says.
How taste works is fairly straightforward: Bite into a piece of candy and your taste buds translate the chemical properties of the candy into electrical signals, which the nervous system sends to your brain.
The result? Mmm … chocolate.
What happens next is less predictable. Do you eat one piece? A handful? A bagful? Typically, when you are full, the brain sends a signal that says, “Stop. You’ve had enough.” If the signal fails, your hand goes back into the bag of candy over and over — a familiar scenario to anyone who struggles with weight.
Di Lorenzo’s research focuses on the earliest stages of taste, how nerve cells transmit information from the taste buds to the brain stem. So far she has learned that each of the five basic tastes — sweet, salty, sour, bitter and umami (savory) — prompts the firing of electrical impulses in a particular spiked pattern. Neurons in the brain stem “read” that pattern — called temporal coding — to interpret taste.
Here’s what else she has learned: If the temporal coding caused by a particular taste is “played back” in the brain, the subject perceives that taste, even if sipping plain water.
Does that mean we might someday have all the flavor of chocolate with none of the fat and sugar — indeed, none of the chocolate in any form?
Sidestepping the idea of virtual chocolate, Di Lorenzo says, “What we’re really interested in is why people overeat or why they stop eating. Both are biological mandates, but there are controls (for example, insulin and hormones) in most people that help regulate them.”
Tracking that information is akin to watching a train arrive at a station; people disembark and go their separate ways. Only here, the station is the brain stem and the passengers are bits of taste information headed into various parts of the brain.
“The information that says ‘spit it out or swallow it’ is going here; the information that says what taste quality it has is going there. The information that says ‘I’m enjoying this’ is going somewhere else,” she says.
Di Lorenzo’s research on taste and its implications in health issues ranging from obesity to brain-machine interface (using sensory feedback to enable, for example, a prosthetic hand to pick up a glass without grasping it so hard that it breaks) has earned the support of the National Institutes of Health.
She received a five-year, $1.125 million NIH grant in 2005. A second grant of $233,427 as part of the federal stimulus package began July 2009. And in December 2010, a $1.25 million renewal of her original grant kicked in, ensuring funding for the next five years.
Using what they’ve learned about the temporal coding of taste, Di Lorenzo and her assistants are doing research to:
• Understand complex tastes. “The majority of foods don’t have one dominant taste, but are a mixture of tastes,” Di Lorenzo says. “It turns out, mixtures are much more effective at activating taste neurons than are single taste qualities,” she says. Sometimes the mixture response is bigger than the sum of the components.
• Investigate the association of taste and smell, the two major components of flavor. “We don’t have a clue about how flavor is encoded in the brain, how smells interact with taste and vice versa,” Di Lorenzo says.
• Decode the taste of bitter. Because most medicines (defined as poisons taken in low doses) are bitter, finding a way to make them more palatable might ensure a patient takes the entire prescribed dose. Bitter is a challenge, Di Lorenzo explains: “There are 34 different types of receptors for bitter taste in rats.” And sweet? “Just one.”