If you've ever looked at a half-eaten plate of congealing poutine or grease-soaked pizza and thought, "Ugh. Why did I ever think that looked good to eat?" science has an answer for you.
Food looks more appetizing when we're hungry because of a hormone called ghrelin. Produced in the gut when the stomach is empty, it travels through the bloodstream and makes our brains' pleasure centres light up when we see food in the same way a smoker's brain responds to the sight of someone puffing on a cigarette, researchers at McGill University have discovered.
Twenty lean and healthy research subjects looked at photos of food such as spaghetti, cake and hamburgers. Those who got a shot of ghrelin paid more attention to the food pictures and remembered the images more strongly, according to research published in the May issue of Cell Metabolism.Conventional wisdom has held that there are two different kinds of hunger: the metabolic, empty-belly kind and the pleasure-seeking, mmm-that-cake-looks-yummy variety. This new research shows that those two sources of hunger are connected. The hormone ghrelin is the link between belly and brain.
"We have a profound desire to eat - evolution made us that way," says neurologist Alain Dagher of the Montreal Neurological Institute at McGill, one of the study's authors.
Early humans had to work hard to get food, he explains, especially fatty food that required hunting. So our bodies evolved with a number of strong mechanisms - ghrelin (pronounced grell-in)being one of them - by which the stomach motivates the brain to seek food and eat. It worked great in a hunter-gatherer society, but it's problematic now that obesity presents a greater challenge than slaying woolly mammoths.
Dr. Dagher says he doesn't know whether ghrelin production leads to obesity, which probably stems from multiple causes. But he believes the hormone hurts our ability to lose weight.
"As soon as you try to diet, all these gut hormones signal, 'Emergency! Eat!' " he said.
Pharmaceutical companies are working on ghrelin-blocking pills as a possible diet aid, but Dr. Dagher says it remains to be seen whether you can block the hunger hormone without messing up the brain's whole reward and pleasure system.
"Very likely it would cause weight loss, but we don't know what else it would do," he says. "You run the risk of affecting the basic system that controls emotion in ways that are unpredictable."
The findings hold more direct promise for people who need to gain weight. Cancer patients and others suffering appetite loss as a result of chronic disease may benefit from a dose of ghrelin.
The new research also has implications for social policy on obesity. When research subjects got a shot of ghrelin and saw food, MRI scans show that their brain lit up in the same way an addict's brain responds to images of cocaine.
"It makes sense to think of hunger as a kind of addiction," Dr. Dagher says. Unlike most illicit drugs, food is good for us and we need it to live. But the research helps explain why, in some cases, losing weight can be as hard as kicking a drug habit.
"It makes sense to look at obesity as a brain disease, and it makes sense to apply some of the same approaches," Dr. Dagher says. This new research could bolster efforts to treat fast food more like cigarettes - for example, banning its sale in school cafeterias - he adds.
The brain-scan research on ghrelin is part of a growing body of knowledge about the power of gut hormones. Ghrelin was first identified in 1999, and its link to appetite was found in 2001. Scientists are still discovering new gut hormones, Dr. Dagher says, giving credence to the notion of gut feelings.
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