Listening to the Belly

You may have suspected it all along, but science is now proving it: your stomach has a mind of its own.

The digestive system has a rich network of neurons, which is not too surprising considering the important job it must do. The "enteric nervous system," as its called, mostly controls local feedback: helping the gut (which includes the stomach and the intestines) release the right chemicals, mix and churn food around for digestion, and squeeze food along the system at the right rate.

But researchers have discovered that the nerve cells in the belly are more than just the workhorses of digestion. In fact, they are saying we have a "second brain"—a simple nervous system in the belly that functions unconsciously, partly independent of the big brain in our heads. This collection of neurons doesn't just control digestion once the food's already down the hatch. It plays a role in appetite, eating, and perhaps even helps us decide what food we like to eat.

Psychologist Anthony Sclafani, who studies appetite and feeding behavior at the City University of New York at Brooklyn College, theorizes that there is a "hidden taste system" in the gut. This system seems to be sampling and savoring foods we eat without us ever knowing about it. "It's hidden from our view somewhere in the gut or beyond, but it's also hidden from our conscious appreciation," Sclafani says.

His theory that the gut has its own subconscious taste system comes from experiments with rats. Rats will quickly learn to associate a flavor like grape with high-calorie glucose drinks. They also learn to associate another flavor like orange with an equally sweet but calorie-free saccharin drink. Given the choice, they'll go for the grape glucose drink, apparently for the energy it contains. But rats will also learn to prefer sugar over saccharin when they can't taste the flavors in their mouths—when the researchers infuse the syrupy stuff directly into their bellies. He thinks that, unbeknownst to the conscious mind, the gut is "tasting" and evaluating foods on its own, influencing whether or not the animal chooses the food again in the future.

The anatomy supports it: Some cells in the lining of the digestive tract are very similar to "taste bud" cells on the tongue, and experiments show that the gut responds on its own to the sweetness of glucose. Some cells in the gastrointestinal lining also make the same proteins that in the tongue are used to sense bitter flavors.

The belly also seems to make up its mind on its own about foods that cause nausea—even if the sickness happens hours later. Food poisoning or other illnesses can result in intense, long-lasting aversions to a particular flavor. If you've ever had a bad clam, for example, it'll be a long time before you dare to eat those shellfish again. The very smell may be enough to make you gag.

Aversions can be a real problem for people getting chemotherapy: their stomachs "decide" that whatever they ate right around the time of treatment was what made them so sick. Pretty soon, it can be hard for a chemotherapy patient to choke down anything at all. Even though they know full well it was the drugs, not the food, that made them so nauseated, their stomachs don't agree.

Even an animal rendered unconscious learns to hate a food that made it sick, and the gut brain may be calling the shots. Rats fed a new food and then given a nauseating drug while under anesthesia will avoid that taste in the future once they wake up. We're biologically wired to make these long-term associations, Sclafani says.

Not much is known yet about how the stomach's taste system works, or what decisions it makes. But this neurobiology may shed light on why our eating habits sometimes don't seem to make much sense: Maybe we've just been thinking with the wrong brain.