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But the question is, does this data really show that the ubiquity of cheap junk food will turn us all into junkies? Not really. A closer look at certain key elements of this study and of prior research helps clarify who — mouse or man — is most likely to get hooked and who isn't, and why addiction involves a lot more than mere exposure to a substance.
While Kenny's research can be read to suggest that simple long-term exposure to enticing foods leads to obesity and reduces the ability to obtain pleasure, there's actually at least one other major factor at play. Consider the living conditions of the rats in the study: solitary cages. Like humans, rats are highly social animals that suffer when deprived of contact with others. But in the experiment, the rats were not only isolated from other rats, but were also given no toys or exercise wheels; their diet options were either monotonous rat chow or cheesecake and bacon.
The human equivalent would be to study whether prisoners in solitary confinement choose to subsist on bread and water or eat junk food to excess. In such a situation, human obesity and overindulgence in the only source of environmental pleasure would be no surprise.
In one classic rat study by Bruce Alexander, emeritus professor of psychology at Simon Fraser University, researchers measured the impact of the social and physical environment on the risk of morphine addiction. They found that rats kept in small, isolated cages readily chose to self-administer high, frequent doses of morphine. But rats that lived in "Rat Park" — an earthly rat paradise with plenty of friends and potential mates, nesting materials, toys and room to run and play — voluntarily took significantly less morphine, preferring activity with friends and family to getting high. Under some conditions, Rat Park rats took 20 times less morphine than caged rats. And some rats that had been forcibly made physically dependent on morphine chose to suffer withdrawal symptoms while in Rat Park rather than seeking the drug.
"We showed that an enriched environment made drug use less likely. I think from human research we can say clearly that enriched environments reduce all kinds of addictions, not just to drugs or alcohol," says Alexander, author of The Globalization of Addiction and designer of Rat Park.
The new study did not measure stress in rats, but Kenny concedes that their living environment could have affected their response to junk food. "In some ways, you could draw parallels to humans," he says. "Some people live in enriched environments. They're well educated, have good backgrounds and have other sources of reinforcement. People who come from an impoverished background don't have the same access to sources of [meaning and pleasure]. They tend to migrate toward [things like junk food] that are now readily available and very cheap."
Past studies have found that socioeconomically disadvantaged people and others in high-stress situations with little social support are at much greater risk for both addiction and obesity. For example, people who were displaced for more than two weeks after hurricanes Katrina and Rita were 56% more likely to have a substance abuse disorder a year later, compared with those who survived the hurricanes but were not displaced. Overall, about 1 in 8 people who lost their homes — and who were also much more likely than the general population to be poor and unemployed — suffered from substance abuse disorders, compared with about 9% of the general U.S. population.
"All of these factors — environmental exposure, environmental stress — can have an impact on the vulnerability to obesity or drug addiction," says Shertleff. Whether or not cheesecake is as addictive as crack, "what this research is really showing us is that we can learn something about compulsive eating from the addiction world," he says.