Understanding junk food "addiction" in lab rats
Exploring the hypothesis that deficits in reward processing may contribute to obesity by making it difficult for certain individuals to stop eating once their energy needs are met—either because they are prewired with faulty reward systems or because "excessive consumption of palatable food can drive reward dysfunction"—researchers from the Scripps Research Institute examined how prolonged access to a high-fat diet influenced brain reward systems in rats.
The study published online this week in the journal Nature Neuroscience included several experiments that examined how rats' eating habits and reward systems (dopamine receptors) were effected by unlimited access to high-fat foods (including bacon, sausage, cheesecake and other items). The researchers found that rats with access to an unlimited high-fat diet were far more likely to overeat, get the majority of their calories from the high-fat foods—despite equal access to rat chow—and gain significantly more weight than those with restricted access to the junk food, or access only to rat chow.
What's more, later experiments showed that rats who consistently ate the junk food showed signs of increasingly dull reward response—that is, as with other addictive behaviors, they showed lower levels of dopamine receptors, meaning that they required higher levels of the high-fat foods to feel reward. In fact, one experiment found that rats inured to the high-fat diet were so motivated to continue consuming the junk food that they would keep eating even if it meant exposure to electric shocks. The findings, the researchers write, "...show that reward hypofunction arises in rats that volitionally overeat a palatable cafeteria diet similar to that consumed by humans and that this effect becomes progressively worse as they gain more weight." In other words, the more junk food the rats consume, the less their reward systems respond when they do eat the high-fat foods. This in turn drives them to continue eating more in search of that fulfillment. It's not surprising, the authors conclude, that there are many parallels with drug addiction. The findings, they argue, indicate that overeating junk food can prompt compulsive behaviors akin to those seen in drug addiction.