The classic slogan for Lay’s potato chips is “Betcha’ can’t eat just one!,” and anyone who’s ever sat down with a new bag of chips and systematically worked their way down to the bottom of the bag in an almost hypnotic state knows the truth of that message. Portion size has been shown in many studies to be a contributor to overeating, as scientists find that people tend to eat the food that’s placed in front of them rather than stopping when their hunger is satisfied. Some might say this behavior is culturally programmed by millions of mothers telling children to “clean their plates” – a good strategy for broccoli, but a rather unhealthy one when faced with a heaping mound of french fries.
But there may also be a biological reason driving people to eat whatever amount of food is placed in front of them, to the detriment of their own personal health. Today in the Journal of Neuroscience, University of Chicago neurobiologists Hayley Foo and Peggy Mason publish experiments that indicate rats get into a zone while eating or drinking something they like that actually reduces their sensitivity to pain. While eating a chocolate chip or having sugar water or regular water infused into their mouths, rats are slower to move their feet away from a hot light-bulb than when they are not eating or drinking. The implication is that rats are so focused on finishing the food in front of them, they are less susceptible to distractions…such as, for instance, a hot foot.
“It’s a strong, strong effect, but it’s not about hunger or appetite,” Mason said. “If you have all this food in front of you that’s easily available to reach out and get, you’re not going to stop eating, for basically almost any reason.”
In the wild, where food is scarce, a resistance to distraction while eating is a good skill to have. If a wild rat is eating a hard-earned nut, it would rather ignore that mild pain in its foot rather than flee the scene and risk losing the nut to another hungry animal. But for humans in modern society, where the next meal is only as far as the nearest supermarket or McDonald’s, an unshakable focus on finishing the food in front of you and drowning out distractions (like a little voice inside your head reminding you how many calories you’re consuming in that Big Mac), is decidedly unhealthy.
“We’ve gotten a lot more overweight in last 100 to 150 years,” Mason said. “We’re not more hungry; the fact of the matter is that we eat more because food is readily available and we are biologically destined to eat what’s readily available.”
But that’s not to say that there aren’t positive ways to exploit the ability of eating chocolate or drinking water to blunt the body’s response to pain. Studies have shown that giving sugar water to infants can reduce the pain of an injection, and the same method, I’m told, is used by veterinarians with dogs. Mason offers the Jewish rite of circumcision as a potential example of this phenomenon at work, where baby boys are given a small amount of wine as the foreskin is removed. The good news about Foo and Mason’s study then is that the substance used for this pain relief doesn’t have to be alcoholic, or even caloric – water was just as effective in rats as the chocolate chips.
Only substances that the rats did not naturally enjoy, such as a bitter quinine solution or very salty water, failed to trigger the reduced sensitivity to pain. Furthermore, the rat’s determination of what is good or bad to eat was also strongly influenced by learning in the study. When a good taste, such as sucrose, was conditioned by experimenters to turn bad via its association with sickness, ingestion of sucrose no longer triggered a reduction in pain. That result excited Don Katz, an associate professor of psychology and neuroscience at Brandeis University, who said it suggests an important biological purpose for taste.
“Here’s what you get out of tastes being palatable,” Katz said. “They’re saying that the purpose of the taste system is to give animal a cue that helps it decide what stimuli it should or shouldn’t pay attention to. It’s really important for them to know when they should stop eating if they don’t have to stop eating.”
This study is also not the first from Mason’s laboratory to show a “natural” form of pain relief – previous research has found that rats are less sensitive to pain while sleeping and even while urinating, two other behaviors you wouldn’t want to have easily interrupted in the wild. The blunting of pain during all three of these behaviors can be traced back to the brainstem, the primitive region of the brain responsible for reflexive actions. In Foo and Mason’s study, inactivating part of the brainstem called the raphe magnus removed the ability of water to change the rat’s response to pain, suggesting that this effect is as automatic as sweating during exercise.
“You’re essentially at the mercy of your brainstem, and the raphe magnus is part of that,” Mason said. “It tells you, ‘you’re going to finish eating this, whether you like it or not,’ just like you sweat while running whether you like it or not.”
And when the brainstem is in control, it’s hard to stop eating, as evidenced by the rats and their chocolate chips in Foo and Mason’s study.
“Every time, if the rat had energy to get up and get that chocolate, they never picked it up and said ‘I don’t want the rest of this chocolate,’” Mason said. “We’re talking 900-some chocolate chips; it was not a subtle finding.”