Has evolution hardwired us to snack?



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No matter what your size and what your goal, dieting is never easy. And even when you can stick to the prescribed reduction in calories at meal times, it's those perilous moments in between when you hear the cookie jar calling that might very well be your downfall. You like snacking but you really don't like beating yourself up afterward. It turns out it might not be your lack of willpower that's to blame. Scientists at Howard Hughes Medical Institute (HHMI) say your brain might be hardwired to nibble even when you know you shouldn't.


Hungry for answers

The researchers at HHMI's Janelia Research Campus, who published their work in the journal Nature, explain that the irresistible call of the kitchen might actually have an evolutionary explanation. They say that "hunger-sensitive cells" in the brain known as AGRP neurons are responsible for that nagging little voice that always wants a treat.

For animals in the wild — and that includes our earliest descendants — the search for food and water means making life-and-death choices. How much risk those animals are willing to expose themselves to can make all the difference. "We suspect that what these neurons are doing is imposing a cost on not dealing with your physiological needs," says Scott Sternson, a group leader at Janelia.

The AGRP neurons do not make the animal hungry. Instead, they provoke a response to "sensory cues" from the presence of food. In environments where food might be scarce, the cues drive the animal to take what is available even in the face of potential danger. "We suspect that these neurons are a very old motivational system to force an animal to satisfy its physiological needs. Part of the motivation for seeking food is to shut these neurons off," says Sternson. Though, for us at least, the dangers of acquiring food may be gone, evolutionarily speaking, it seems, the need to gather what's available remains.


Eating their words

When the scientists began to research the function and effect of hunger, there was some confusion over how the brain handled deprivation of food and water — the team found a separate yet similar set of neural connectors (called SFO neurons) that affect thirst — and how a shortage affects emotion. "There was an early prediction that there would be neurons that make you feel bad when you were hungry or thirsty," says Sternson. "This made sense from an intuitive point of view, but all of the neurons that had been looked at seemed to have the opposite effect." He says that, until now, what was already known about those neurons did not completely match up to the fact we all take for granted: Hunger is unpleasant. They also knew, of course, that hunger makes food taste good.

So couldn't the neurons be controlling food and water intake by positivity — that is, rewarding the eating of food with positive emotions? The answer is no. By conducting some simple experiments with mice, researcher Nicholas Betley and graduate student Zhen Fang Huang Cao discovered they could pinpoint exactly how the neurons were working. The mice were given two tasty but entirely nutrition-free gels. By switching off the AGRP signals in a mouse's brain while it was eating one of the gels, the mouse developed a preference for that specific flavor in subsequent tests. By switching the neurons on for a specific flavor, a mouse would develop an aversion to it. Each came to associate the flavor with either hunger or lack of it.


Chewing it over

The neurons don't make food taste good or even make you hungry. The tests on mice show that an animal's behavior is driven by a need to shut off the activity of the neurons for hunger or thirst. The scientists were even able to control the behavior of the mice in other ways. By switching the neurons on or off, the mice came to seek out certain areas of their environment and avoid others.

What was surprising, Sternson says, is that mice did not actually have to eat to quiet the neurons. Instead, the cells ceased activity as soon as an animal saw food — or even a signal that predicted food. And their activity remained low while the animal was eating. This wouldn't be true if it was the role of the neuron to control eating. Instead, it is the eating that controls the neuron, neutralizing the unpleasant emotions it causes. "There's a similar motivational quality to hunger and thirst," Sternson says. "You want them to end."

As his team's investigations progress, Sternson hopes to learn more about how to interfere with the functions of AGRP neurons. Though a miracle cure for snacking is not promised, this research could potentially lead eventually to help for those looking to suppress those irresistible cravings when they're trying to lose weight. Which has to be good news. Now, where's my cookie?