For some people, the urge to chomp on ice is irresistible, to the point that they tell their doctors (who then tell the Washington Post) things like "I love ice. It’s better than sex." This compulsive ice-chewing is called pagophagia, and it’s long been thought to be triggered by severe iron deficiency, or anemia, though scientists haven’t been sure exactly why there’s a link. But according to a new study in the journal Medical Hypotheses, it seems that ice-chewing increases alertness in an anemic person, kind of like the mental jolt you get from a morning cup of coffee.
To test her theory, University of Pennsylvania clinical psychologist Melissa Hunt and her colleagues gave both anemic and healthy people a short test designed to measure response time, and also gave them either a cup of tepid water or a cup of ice. After drinking the room-temperature water, the anemic people performed worse on the test than did the healthy people. But when the iron-deficient subjects were given ice, they performed just as well as their non-anemic counterparts.
It’s not immediately clear why chomping on ice might provide a mental boost, but Hunt’s best guess is that it could be explained by something called the mammalian diving reflex, a physiological response to very cold temperatures in which some biological processes are slowed or shut down in order to conserve energy for the organs most important to survival, like the brain. Since people with anemia struggle with fatigue and mental sluggishness because their bodies are lacking in oxygen-rich blood, Hunt believes the ice-chewing might drive more of that blood to the brain. (For healthy people, on the other hand, these processes are working properly, meaning they wouldn’t benefit much from an extra boost of blood.)
So this study provides a potential physiological explanation for one annoying human habit, but the biological drives behind constant leg-jiggling and incessant pen-tapping are as of yet unclear.