Pregnant women experience it to varying degrees. Others report that it’s simply the result of iron deficiency. Called pica, it’s a disorder defined by inexplicable cravings for unusual foods like dried pasta or eggshells—even inedible objects like chalk, paste, or clay.
An extreme form of pica is pagophagia, a compulsive desire to chew ice. Although the vast gamut of pica-related case studies remains a mystery to scientists, a new theory could explain why some iron-deficient people report a burning desire to chomp on frozen things.
Melissa Hunt, a clinical psychologist at the University of Pennsylvania, gave both iron-deficient and healthy participants either a cup of ice or lukewarm water before they took a 22-minute attention test (typically administered to diagnose attention deficit hyperactivity disorder). She found that iron-deficient participants performed just as well as healthy participants if they’d had the cup of ice; if they had sipped lukewarm water, their performance was greatly inhibited. Meanwhile, there was no difference in performance for the healthy participants.
Hunt and her colleagues published their results in the journal Medical Hypotheses and concluded that ice helps alert people who might otherwise favor a cup of coffee if they weren’t iron deficient.
Here’s Meeri Kim, writing for the Washington Post:
Hunt points to a phenomenon called the mammalian diving reflex as a possible reason the ice-chewing caused better test performance. When submerged in water, most air-breathing vertebrates slow down their heart rate and constrict blood vessels in their arms and legs. This decreases the oxygen supply to the body’s periphery, saving it for vital organs.
“If you think about whales and dolphins diving, the water gets colder and their peripheral blood vessels constrict and shunt all the blood to the internal organs and the brain,” she said. “It is sort of vestigial, but humans do show the dive reflex.”
Crucially, the reflex is triggered by the face having contact with cold water, but not warm water. So perhaps the chill of chewing on ice cubes may lead to an increase of oxygenated blood to the brain, providing the cognitive boost that anemic patients need. For those with enough iron, Hunt speculates, there would be no additional benefit to more blood flow.
Doctors have long known that they can track iron-deficient and anemic patients’ progress by monitoring the disorder’s intensity. But Hunt’s dive reflex theory helps explain why the behavior is classified as a disorder and not just a learned habit. It may be that other pica-related eating disorders have roots in their own curious, vestigial instincts.
Photo Credit: Robert Fornal / Flickr (CC BY-NC-SA 2.0)