There's a well-established link between impulsive behavior and addiction in scientific literature, but which came first: impulsiveness, or the addiction?
While the question poses a difficult research challenge in humans, scientists from the Boston University School of Medicine believe that in rats, at least, impulsive behavior precedes food addiction.
"We now know with this study that impulsivity is present beforehand, and can present risk factors for overeating," said lead researcher Pietro Cottone, Ph.D. of BUSM's Laboratory of Addictive Disorders. "We observed that the [rats] who were more impulsive were also the ones who showed the food addiction behavior."
Cottone hopes that his findings will help researchers develop a way to identify people at risk for addiction before it starts -- especially food addiction, which could be one of several complex causes of obesity.
In the past few years, research has emerged to support the notion that some types of overeating are like an addiction, which could explain why some people eat more than they should, despite wanting to cut back. Foods high in salt, sugar and fat, activate reward signals in the brain and override feelings of fullness -- similar to the way drugs and alcohol can affect the brain's pleasure centers and leave a person wanting more.
Some signs of food addiction, according to Yale University's Rudd Center for Food Policy & Obesity, include eating until you are sick, going out of your way to obtain foods if they aren't available and letting the constant consumption of food interfere with normal, everyday activities.
Food addiction could affect as much as 5 percent of the general population, according to a 2013 study published in in PLos ONE. Researchers surveyed 652 adults recruited from the general population and found that 5.4 percent of them qualified as food addicted. Those who were food addicted were significantly more likely to to have a higher BMI, have more body fat and more abdominal fat. Obesity affects over a third of adults in the U.S., and the medical costs for people who are obese are estimated to be $1,429 higher than those of normal weight, according to the Centers for Disease Control and Prevention.
Cottone first determined which rats were impulsive based on a lever test. Rats pressed a lever and were rewarded with a sweet solution -- but only if they waited patiently for 15 seconds. Rats that pressed the lever repeatedly during the waiting period ended up delaying their reward (every extra lever press would add another 15 second). The thoughtful rats who were able to inhibit their behavior were rewarded more, while the rats who couldn't wait to press the lever were classified as impulsive.
Then Cottone exposed all the rats to "junk food," or food that was specially formulated to be tasty and sugary, for only one hour per day. This gave the rats a chance to develop a taste for the junk food, and some of the rats binged on the stuff.
Finally, Cottone placed the rats in what he called an "aversive context." Because rats are nocturnal and feel safest in dark spaces, Cottone placed the junk food in an open, well-lit space. Those rats who ventured out from their safe, dark homes to seek out the junk food despite the brightly lit conditions were defined as compulsive. The compulsive rats that had also binged on the junk food were considered food addicted, and they were also more likely to be the rats Cottone had initially identified as impulsive.
The impulsive rats also showed increased levels of a protein called Delta-FosB in the area of the brain associated with reward evaluation and impulsiveness, which hints at a possible biological factor in their behavior.
"Typically, rats don't want to put themselves at risk to get the food," explained Cottone. "The ones who were less impulsive at the beginning were also less likely to become food addicts."
James MacKillop, Ph.D., a professor at the University of Georgia specializing in alcohol and drug addiction research, said Cottone's study makes a significant contribution in research on impulsiveness and addiction.
"What this study provides is a clear link between impulsivity as a general trait and the probability of developing compulsive eating habits," said MacKillop to HuffPost. "This could either suggest a parallel mechanism in humans, or if nothing else, this is proof of concept that there does seem to be an interaction between innate characteristics and food exposure in an animal model."
MacKillop published a study in the journal Appetite last January in which he found that people with impulsive personalities were more likely to report higher levels of food addiction, and is currently conducting research on how to analyze people's brain activity of people when they make decisions about food. His one caveat about the findings: while impulsivity has historically been discussed as a singular personality trait, research is uncovering an "increasing appreciation" about how impulsivity could encompass a group of many behavioral characteristics.
"We're increasingly referring, not just to impulsivity, but different types of impulsivities," MacKillop concluded. Unpacking those behaviors could be key to public health officials identifying a segment of the population that is especially at risk for addictions of all kinds, he said.