In TIME, science writer Maia Szalavitz dissects a recent rat study that was reported as if it showed that junk food is "as addictive" as crack. Some rats were assigned to the equivalent of an all-expenses-paid cruise: nearly continuous access to a spectacular array of fatty delicacies including bacon and chocolate frosting. (Rats have good taste. No doubt they'd say the same about humans. We're co-evolved species and it shows.) Another group was allowed to visit the buffet only once a day. An unlucky control group subsisted on regular rat chow and water. As you might expect, the Royal Caribbean rats gorged themselves to the point of obesity.
It seemed as if, the more weight they gained, the more they had to eat to get the same level of satisfaction—like junkies who always need a bigger hit to maintain the same high. Szalavitz writes:
But what shocked the researchers was that extended-access rats also showed deficits in their "reward threshold." That is, unrestricted exposure to large quantities of high-sugar, high-fat foods changed the functioning of the rats' brain circuitry, making it harder and harder for them to register pleasure; in other words, they developed a type of tolerance often seen in addiction; an effect that got progressively worse as the rats gained more weight. "It was quite profound," says study author Paul Kenny, an associate professor of neuroscience at the Scripps Research Institute. The reward-response effects seen in the fatty-food-eating mice were "very similar to what we see with animals that use cocaine and heroin," he says.
Kenny's study did not include rats exposed to drugs, making direct comparison tricky, but other studies have found that chronic cocaine or heroin exposure leads to reductions in reward thresholds of 40% to 50%.
However, Szalavitz argues that this is not cause for a national junk food addiction panic. Just because rats will gorge themselves on food, or drugs, in the laboratory does not prove that these substances have magical addictive properties. Rats, like humans, are highly social animals. They crave cognitive and social stimulation. When you lock them in tiny cages with one bright spot in their lives, be it food or drugs, they will overindulge. Previous research with opiates has shown that when rats are given plenty of space and social interaction, they lose interest in morphine, even when it's freely available.
This study does hold object lessons for the human obesity crisis. The issue is not so much that junk food is dangerously addictive, but rather that people who are isolated, sedentary, and stressed are far more susceptible to its charms than those who have a more balanced lifestyle.
Photo credit: By flickr user hamron, distributed under Creative Ccommons.