Looking outside of the current repertoire of treatments might be important, as dieting alone has often proved to be an unsuccessful long-term strategy for people who struggle with overeating. The new study showed that after eating a diet full of sausage and sweets for 40 days—even though regular lab rat chow was available—the obese rats had little interest in reverting to the more healthful diet when the tasty stuff was taken away. In fact, after depriving the high-fat habituated rats of their human junk foods, the rats would refuse to eat their standard chow for an average of 14 days. "I was really shocked at the magnitude of the effect," Kenny says. "They basically don't eat anything. If that translates over to us as a species, that's a major problem."
The sticky part about studying food addiction is that, unlike cocaine or alcohol, humans can't exactly drop it—cold turkey or not. "You can't really quit food," Avena says. And humans are hardwired, thanks to eons of evolutionary selection, to seek high-calorie foods to keep us going through lean times. But with subsistence hunting, gathering and farming now little more than a niche lifestyle choice in wealthy nations, a brain set up to reward super-rich calorie snacks is more of a hazard than a help.
"In one sense, we're all addicted to food," Kenny says. He points out, however, that many of the food items widely available today, say cheeseburgers and milk shakes, are like superfoods in terms of their calorie quantities. "This energy-dense stuff is very new to us as a species. It's probably corrupting brain circuitry," he says.
Unlike rats, however, most people know that many of these high-fat foods are not a wise choice, especially when consumed in large quantities. But many continue to eat in excess of basic energy requirements anyway, putting on unnecessary pounds and possibly reinforcing unhealthful behavior. So the researchers designed an experiment to try to draw a parallel with the rats, training them to expect an electric shock when they saw a certain light cue. Unlike their chow-fed counterparts, obese rats accustomed to the high-fat diet would keep right on gorging even when they knew a shock was coming.
Although the current work focused on high-fat foods, Kenny notes that the full neurochemical and behavioral changes might be due to "a combination of both sugar and fat." Avena and her colleagues have been working to parse out the various nutrients in potentially addictive food products and what impact they have on the brain. They found, for example, that animals binge-eating fats and animals binge-eating sugars experience different physiological effects. "They affect the brain in very different ways," Avena says.
The big one-two punch for defeating healthy eating might in fact be a combination of neural effects from both of these ingredients. And, indeed, the sweet spot for the lab rats in Kenny's study seemed to be the food item that contained high quantities of both fat and sugar: cheesecake. Sara Lee, to be precise, Kenny reports.