The brain can count calories more than you think is possible

Studies offer clues about what makes us start and stop eating

- GINA KOLATA

Do we really have free will when it comes to eating? It's a vexing question at the heart of why people find it so difficult to stick to a diet. To get answers, one neuroscientist, Harvey J Grill of the University of Pennsylvania, turned to rats and asked what would happen if he removed all of their brains except their brainstems. Would they know when they had consumed enough calories?

To find out, Grill dripped liquid food into their mouths. "When they reached a stopping point, they allowed the food to drain out of their mouths," he said.

Those studies, initiated decades ago, were a starting point for a body of research that has continually surprised scientists and driven home that how full animals feel has nothing to do with consciousness. The work has gained more relevance as scientists puzzle out how exactly the new drugs that cause weight loss, commonly called GLP-1s, affect the brain's eating-control systems.

Neuroscience offers clues about what makes us start eating, and when we stop.

While most of the studies were in rodents, it defies belief to think that humans are somehow different, said Dr Jeffrey Friedman, an obesity researcher at Rockefeller University in New York.

As they have probed how eating is controlled, researchers learned that the brain is steadily getting signals that hint at how calorically dense a food is. There's a certain amount of calories that the body needs, and these signals make sure the body gets them. The process begins before a lab animal takes a single bite. Just the sight of food spurs neurons to anticipate whether a lot of calories will be packed into that food.

The neurons respond more strongly to a food like peanut butter, loaded with calories, than to a low-calorie one.