How appetite affects the brain


Scientists often distinguish between “wanting” (“when I get this thing, life is better”) and “needing” (“unless I get this thing, life is worse”). In other words, “want” applies when you’re still trying out something; “need” applies when it owns you.

There is an equally interesting, if subtler, distinction between “wanting” and “liking.” Many times, we want more of something without liking it more. This category often covers things that we use instrumentally to achieve a long-term goal: For example, “I don’t like these gewgaws, but I want them all very badly because there’s a great potential market of people who want them even worse.”

A recent paper by Alison Jing Xu of the University of Minnesota and colleagues, published in the Proceedings of the National Academy of Sciences, shows how brain wiring can throw a wrench into this logic of wanting versus liking—so that we end up wanting something that we not only don’t like but also can’t use to better our lives in any way.

We start with a no-brainer: When our bodies are low on calories, we feel hungry. We animals then hunt more tenaciously, climb higher to search in trees for fruit, look longer for bugs to eat and—if we are pets—push our empty food bowls more irksomely against our owners’ ankles. In other words, hunger motivates food-acquisition behavior.

As it turns out, hunger also subliminally shifts our cognition. Prof. Xu’s team had volunteers look at a screen where combinations of letters appeared for a fraction of a second, and subjects had to rapidly say whether the letters formed correctly spelled words. Subjects who were hungry were more accurate at recognizing food-related words (”I have no idea if that was ‘broom’ or ‘broon,’ but that sure was ‘cookie’.”) So our basic animal wiring (“me hungry, me get food”) can unconsciously infiltrate how we perform the very sophisticated human task of reading comprehension.

But Prof. Xu’s team found something even more intriguing. When subjects doing the word/nonword task were hungry, they not only improved on food-related words but get better by an equal measure at assessing the accuracy of words related to acquisition in general (e.g., want, obtain, gain). Hunger doesn’t just prime us to think about food acquisition; it primes us to think about acquisition in general.

The researchers also found other evidence of this motivational shift. They gave 77 subjects a list of five food and five nonfood items and asked them to rate how much they wanted each one. Hungry subjects wanted both the food and the nonfood items more intensely than sated ones did.