Seeing Hungry

Scientists know our guts secrete ghrelin, that ghrelin levels peak before meals and plummet afterwards, and an injection of extra ghrelin boosts peoples’ appetite. But they don’t know exactly how the hormone affects the human brain. Alain Dagher and colleagues from the Montreal Neurological Institute are working to find out.

By scanning the brains of people dosed with intravenous ghrelin, the researchers found out that ghrelin can activate pleasure centers and visual processing areas in the brain, “in effect making food appear more attractive,” says Dagher.

People who received ghrelin and looked at food pictures showed enhanced activity in areas of the brain involved in reward processing. image: Alain Dagher

The study suggests that if ghrelin is coursing through your veins as you walk past the neighborhood burrito vendor, “you would be more likely to be attracted to that, and you would be more likely to buy food and eat it,” he explains.

For their experiment, the scientists gave volunteers a shot of ghrelin and monitored brain activity using a functional MRI scanner. At first, reward-processing brain regions were unperturbed. Then, the volunteers looked at food pictures, and their anterior insula, amygdala, and other brain regions involved in reward processing, motivation, and appetite lit up. That means mere images of food can give us pleasure, as we anticipate the reward of actually eating.

What’s more, people who’d received ghrelin could better remember details of the food pictures they saw. Dagher explains, “Ghrelin increased brain activity in the visual areas of the brain, in other words, making the food pictures more easy to remember and more easy to perceive.”

Dagher says, “The most exciting part of the study is that we demonstrated that ghrelin acts on this reward processing network in the brain, I think this is quite novel.”

Two Kinds of Chow

Although everybody needs food to survive, most people also indulge in some “A cupcake? Don’t mind if I do!” eating too, even if we know better. To Dagher, the study suggests that these two kinds of eating—for survival and for pleasure—may be “controlled by the same parts of the brain,” he says.

Our guts release ghrelin when we’re low on calories and it’s time to eat, but Dagher says ghrelin is also involved when “you’re not particularly hungry but you see a piece of chocolate cake. It’s very appealing, and that will also trigger hunger.”

Does that mean people who overeat have too much ghrelin in their systems? Dagher says the jury’s still out, although injecting people with ghrelin does make them hungry. Even so, he says that “whether ghrelin is normal or abnormal in overweight people, it’s likely that ghrelin is a good target for treating obesity.”

Dagher says his study supports the idea that hunger is “an addiction to food,” since other addictions like smoking also activate the same reward centers in the brain. When smokers see videotapes of people smoking, and when people with high ghrelin levels see food pictures, he says, “more or less the same brain regions” respond. “This confirms the idea that areas of the brain that are important for eating are also the targets of addictive drugs,” he says.

When people get a shot of extra ghrelin they become extra ravenous. So if researchers found a drug that could block the body’s natural ghrelin, perhaps the drug could combat obesity. But Dagher says it might not that simple, and ghrelin-blockers potentially could cause problems like depression.

“We show that ghrelin acts on the brain reward centers, and these areas of the brain don’t just control hunger. They control our response to all kinds of emotional stimuli, so if you block ghrelin, you run the risk of causing side effects,” he says.

Researchers have only known about ghrelin since 1999, and Dagher says, “We still have much to learn about it.”

In the study, Dagher and colleagues conducted fMRI brain scans of twenty healthy volunteers as they looked at food pictures and as a control, at scenery. At the beginning, food pictures had little effect the brain’s reward circuitry because all the volunteers had low levels of ghrelin, since they’d just eaten. Then, halfway through the study, twelve people received ghrelin injections. The ghrelin group promptly proceeded to get the munchies. That meant the ghrelin was working.

Back in the brain scanners, the ghrelin group still showed no special response when staring at seascapes or other non-edible images. But when the ghrelin recipients saw pizza, the scientists found fireworks. The combination of ghrelin and food pictures activated the brain’s reward and pleasure centers.

Can merely seeing food or thinking about it trigger your body to produce more ghrelin, making us hungry when we don’t need actually need food? The answer is still unknown, but if you’re trying to shed a few pounds for bathing suit season, it’s probably best to flip right on past the food channel.

This study was published in the journal Cell Metabolism in May 2008 and was funded by Unilever PLC, the Canadian Institutes for Health Research, and the Fonds de la Recherche en Santé du Québec.