Gut counts calories even when we do not.

With holiday feasting in full swing, the digestive tract is working overtime, not just processing all the turkey, green beans, and apple pie, but also keeping tabs on the nutrients ingested.

For the first time, scientists can glimpse the extensive array of sensors that do this tracking, says Terry L. Powley of Purdue University in West Lafayette, Ind. And contrary to what scientists thought, the body closely monitors every morsel, from the tongue's first taste of it to its passage through the intestines.

Researchers have long known that the vagus nerve connects the digestive system to the brain, Powley explains. But because of the limitations of their research techniques, they viewed the connections as sparse and simple.

He and Purdue graduate student Feng Bin Wang now know better. Just a few nerves, called afferent fibers, extend from the central nervous system to the stomach, Powley agrees. But from there they send out a dense array of branches that reach down to the intestines and up toward the throat. He and Wang reported their findings this week in Miami at the annual meeting of the Society for Neuroscience.

Even before dessert, the body has begun to assess the caloric and nutrient intake from the meal's earlier courses, adds Timothy H. Moran of Johns Hopkins University School of Medicine in Baltimore. The tongue recognizes familiar flavors and clues the brain in on fat or carbohydrate consumption. Also, some foods may sneak past the stomach and activate the nerve endings at the top of the small intestine, he explains.

These endings react to the volume, osmotic pressure, acidity, and quality of the food in the gastrointestinal tract. The sensors assess the meal and signal the brain to slow consumption, end the meal, or wait awhile before starting the next one, they suggest.

"The sensory characteristics of the gut have been tremendously underestimated," comments Gerard P. Smith at New York Hospital-Cornell Medical Center in New York City. "It's like a vast sensory sheet." Smith says that the gut knows, and gets the brain to respond differently to, particular fats such as oleic or linoleic acids.

Just as the retina sends the brain visual information, this "sheet" relays food data. Therefore, controlling diet is as much a neurobiological issue as a nutritional one, these researchers assert. They hope this new view will lead to a better understanding of, and perhaps treatments for, eating disorders.

Surprisingly, rather than relay all this information directly to the brain, the fibers can process the data they collect, Powley says. The brain then receives an integrated message. This processing also results in local adjustments to the digestive tract that ensure maximum nutrient absorption, he adds.

Various chemicals, including hormones and protein fragments, or peptides, take part in gut-brain communication and in the regulation of food intake, says Sarah F. Leibowitz from Rockefeller University in New York. She and her colleagues described how insulin puts the brakes on the production of a peptide called galanin, which stimulates fat intake (SN: 11/13/93, p.310). Galanin also helps control the release of insulin and stimulates brain messengers that reinforce eating. Stress or overeating, especially consuming high-fat foods, can diminish insulin's effects, she adds.

As proteins and fats enter the gut, the intestine releases a different peptide, cholecystokinin, Smith says. This stimulates the gut's nerve endings, which tell the brain to reduce food intake, he reported at the meeting.