Our drive to eat can be based on physical hunger or desire. The two aren’t as separate as once thought.

Imagine you haven’t eaten in days. Then, someone places two plates in front of you. One holds a chocolate truffle, the other, a large turkey sandwich. No matter how much you adore chocolate, you will likely opt for the more filling sandwich. But given the choice in a different situation—after a big dinner, for example—the chocolate might look vastly more appealing. Are these choices due to pure reasoning or an innate desire mediated by chemicals in your brain?

New research suggests the latter. Your level of hunger affects how much pleasure you’ll get out of eating chocolate or a turkey sandwich. In a recent experiment, mice placed a higher reward value on a calorie-rich drink than on an artificially sweetened drink after they’d been deprived of food. It’s the first experiment to show that hunger levels affect the reward value of food.

The choice to eat something is influenced by a mélange of messages: how good something tastes and smells, the time since your last meal, your mood, what emotions or memories you associate with a food.

“Our new findings provide an experimental approach for studying how feeding pathways, including those that sense hunger and pleasure, are wired together into one grand circuit,” says HHMI investigator Jeffrey Friedman, who led the study.

Friedman, at the Rockefeller University, studies the complex chemical signaling in the brain that controls hunger, appetite, and eating behaviors. In 1994, he published results describing his discovery of leptin, a hormone that controls appetite. When a mouse loses weight or hasn’t eaten recently, the leptin levels in its blood fall, making it hungry and likely causing it to eat. After a large meal or weight gain, leptin levels rise, and the mouse loses its appetite and eats less.

Illustration: Martin Nicolausson

	PAGE 1 OF 2	Continue