Researchers at Yale University sought to find out what exactly triggers eating and overeating in the brain. They found that compared to calorie-free foods, foods with calories trigger bigger responses in the brain, regardless of whether the food tastes better.

Dana Small, a Yale University psychologist, led a team of researchers that found that two unrelated brain circuits were triggered when people eat: one that's related to liking the food and another that responds to changes in glucose levels in the blood, Popular Science reported.

"The thing the brain really cares about are the calories," Small said.

The study comes after years of research that tested whether mice, rats, and fruit flies were able to sense nutrition independently of taste.

The researchers then tested 14 human participants who were given artificially flavored drinks. While some of the drinks were calorie-free, the other ones had the tasteless carbohydrate maltodextrin, which still had calories. They found that over the course of three weeks, people claimed to like the drink with maltodextrin more, even though they only went from describing the drink as "mildly pleasant" to "moderately pleasant".

In addition, using magnetic resonance imaging (MRI) scans, they were able to track brain responses when the participants drank the drinks with maltodextrin, which each had 112.5 calories. Considering that each person's metabolism and blood sugar levels were different after drinking, they found that people with the highest levels of blood glucose also showed more response in their brains. The response was located in the hypothalamus and the nucleus accumbens regions of the brain.

These responses in the brain were unrelated to how each person liked the flavor of the drinks, suggesting that people who overeat don't necessarily like calorie-dense foods, Popular Science reported. Instead, the body's metabolism is spiking glucose levels up, causing the brain to react, and the person to eat.

Small believes this comes from an evolutionary background.

"All of these brain circuits evolved millions of years ago in animals that lacked consciousness but still needed to incorporate fuel," she told Popular Science. "From our research, it appears that those unconscious circuits that are caring about that energy are alive and well in our brains."