A study published in Current Biology shows that foods with calories trigger more brain responses linked to eating than calorie-free foods do, regardless of whether they taste better. The Yale University Psychologist and study author Dana Small explains that there appear to be two unrelated brain circuits that kick into gear when people consume things. One is related to consciously liking flavors, while the other responds to glucose in the blood.
Previous animal studies have found that mice, rats, and fruit flies are able to sense nutrition independently of taste. Even when researchers genetically engineered lab animals not to be able to taste sweetness, they learn to prefer mixtures containing real sugar instead of artificial sweetener.
In this study, the researchers had people drink artificially flavored drinks. Some of the drinks were calorie-free, while others contained maltodextrin, a carbohydrate that people can’t taste, but still has calories. In a series of experiments, the researchers found that over three weeks’ time, people come to like the flavor paired with maltodextrin more, even though they couldn’t taste the added carbohydrate. Still, they only increased their liking from “mildly pleasant” at the start of the study to “moderately pleasant” at the end of the study.
The researchers saw a larger effect when they took functional magnetic resonance imaging (MRI) scans of people’s brains while they sipped the different flavors. Although they gave everyone 112.5 calories’ worth of maltodextrin in their caloric drinks, different people have different metabolisms, so the blood sugar levels differed after drinking the maltodextrin drink. The researchers saw that two regions in the brain, the hypothalamus and the nucleus accumbens, responded to the drinks in such a way that people with more blood glucose after drinking had more of a brain response. Meanwhile, the hypothalamus and nucleus accumbens responses weren’t related to how much people said they liked their drinks.
“Overall, the results suggested that there’s a real tight metabolic control of flavor reinforcement,” said Small. “So it’s metabolism controlling the mind, in a way.”
Small’s hypothesis is that people who overeat don’t necessarily like calorie-dense foods more. Instead, some people may be more at risk for overeating because their blood glucose levels—and brains—are reacting more strongly than other people’s.
