Would a rat risk dying just to satisfy its desire for chocolate?
I recently found out. In my laboratory, we gave rats unlimited access to their standard fare as well as to a mini cafeteria full of appetizing, high-calorie foods: sausage, cheesecake, chocolate. The rats decreased their intake of the healthy but bland items and switched to eating the cafeteria food almost exclusively. They gained weight. They became obese.
We then warned the rats as they were eating—by flashing a light—that they would receive a nasty foot shock. Rats eating the bland chow would quickly stop and scramble away, but time and again the obese rats continued to devour the rich food, ignoring the warning they had been trained to fear. Their hedonic desire overruled their basic sense of self-preservation.
Our finding mirrored a previous trial by Barry Everitt of the University of Cambridge—only his rats were hooked on cocaine.
So are the fat rats addicted to food? An inability to suppress a behavior, despite the negative consequences, is common in addiction. Scientists are finding similar compulsiveness in certain people. Almost all obese individuals say they want to consume less, yet they continue to overeat even though they know that doing so can have shockingly negative health or social consequences. Studies show that overeating juices up the reward systems in our brain—so much so in some people that it overpowers the brain's ability to tell them to stop eating when they have had enough. As with alcoholics and drug addicts, the more they eat, the more they want. Whether or not overeating is technically an addiction, if it stimulates the same brain circuits as drug use, in the same way, then medications that dial down the reward system could help obese people to eat less.
Until the early 1990s, society viewed obesity solely as a behavioral disorder: overweight individuals lacked willpower and self-control. Since then, the view has changed dramatically, in the scientific community at least.
The first change in opinion arose from pioneering work by Douglas Coleman of the Jackson Laboratory in Bar Harbor, Me., and by Jeffrey Friedman of the Rockefeller University. Experiments with two strains of mice, both genetically prone to obesity and diabetes, determined what drove the mice to overeat. The researchers discovered that one strain had a genetic defect in fat cells that secrete a hormone called leptin. Mice, like humans, normally secrete leptin after a meal to suppress appetite and prevent overeating. The obese mice had a leptin deficiency—and an insatiable appetite. Researchers later found that obesity in the second strain of mice was caused by a genetic defect in their ability to respond to leptin and regulate its actions. The findings seemed to make it clear that hormones regulate appetite and therefore body weight. A hormonal imbalance could lead to overeating; indeed, obesity runs rampant in certain human families that have a genetic deficiency in leptin.
Two observations suggest that viewing obesity as a hormone disorder is too simplistic, however. First, only a small number of obese people in the U.S. and elsewhere have a genetic deficiency in appetite-related hormones. Second, we would expect blood tests of obese people to show either a lower level of hormones that suppress appetite or a higher level of hormones that increase appetite. Yet the reverse is true. Obese individuals generally have a paradoxically high level of appetite-suppressing hormones, including leptin and insulin.
This is where the concept of food addiction comes into play. Appetite-controlling hormones affect certain pathways of neurons—feeding circuits—in the hypothalamus. They also affect systems in the brain that control feelings of reward, which makes perfect sense. If you have not eaten for many hours, you will spend a great deal of time, effort and money to obtain food—and it will taste very good! As the old adage says, “Hunger is the best sauce.”