A recent image study of the human brain actually shows that a hormone that is known for its ability to trigger appetite can stimulate parts of the brain to make food appear more appealing.

Ghrelin, the name of the appetite hormone, has a pleasure effect on the human sense and creates a happiness associated with food similar to those who suffer from drug addictions. Ghrelin encourages people to continue eating after the nutritional reasoning is finished and the stomach is already full.

"For hundreds of years, people used to think that you eat only because you're hungry," says study author Dr. Alain Dagher, an associate professor with the Montreal Neurological Institute of McGill University in Canada. "But we found that the actual system involves a drive for food that is not at all related to hunger."

"The reason for this," he adds, "is that almost every animal, including us, until very recently was living in a world where there wasn't enough food, so that the big risk is starving to death. This creates a real pressure to eat. And obtaining food is risky. It requires effort and putting yourself at the mercy of predators. So you need something to get you out of your cave, and the only way that's going to happen is if the food is attractive enough to get you to overcome those costs and risks. And we've found a hormone that does this by acting on the pleasure and reward centers of the brain and making food you see seem more appealing and more desirable."

Dagher and his colleagues reported their insights in the May issue of Cell Metabolism. They analyzed MRIs of activity in the brains of 20 healthy men while they observed pictures of food and drink. This was done 3 hours after they had eaten breakfast so that they weren't necessarily full nor hungry; all men viewed an initial series of 45 images during which they answered questions about their mood and appetite.

Immediately after the first viewing, 12 of the men received two injections of ghrelin, while the other eight men did not. The men then viewed a second set of 45 images after their blood was taken to observe the levels of the hormone in their bodies. It was concluded that more of the men who received grehlin injectionsreported hunger after looking at the second set of images than those who did not.

The increased hunger response correlated with an increase in brain activity in a broad range of brain regions associated with reward when viewing images of food and non-food "pleasure-related" items. The men who did not receive the ghrelin expressed no change in hunger over the course of the two viewing sessions and were less likely to remember the food imagery they saw following the viewings.

The researchers suggested that the findings could ultimately lead to treatments for obesity based on a disruption of the ghrelin hormonal effect.

"The problem today is that we have this evolutionary imperative to eat, but we now live in an environment where you don't have to spend any energy to get food," he noted. "Which means that it makes sense to think of appetite as a kind of addiction. So, if we want to address the fact that obesity is now the number one killer in the world, we're going to have to tackle the problem in the same way that we tackle cigarette smoking."

But Dr. Barbara B. Kahn, chief of the division of endocrinology, diabetes, and metabolism at Beth Israel Deaconess Medical Center in Boston, also spread caution that relating ghrelin-fueled overeating with drug addiction may be a disservice to the public.

"This study provides us with new information about additional ways in which this particular hormone may work," she said, "And overeating and drug addiction may converge on some of the same neurons. But other pathways are also involved. And from a biochemical point of view, the two are not the same thing. Drug addictions are much stronger. So to suggest that they are the same makes people feel that they can't do anything about overeating. That it's out of their control. So, I don't really buy that the parallel," added Kahn, "There may be aspects of overeating that may be related to aspects of addiction. But overeating is not just another addiction."

In the future, it may be valuable to have your grehlin levels checked or altered, but for now we'll just have to live with good old self-discipline and awareness.