More than 2,000 years ago, Hippocrates wrote about people who craved and consumed substances that wouldn’t typically be considered foods. Later, this eating behavior was called pica, after the Latin name for the magpie bird that was thought to be a rather indiscriminate eater.

The pica behavior is not only a matter of medical history. Pica remains a rather common phenomenon that occurs most often in pregnant women or children. Studies in some parts of the world report that more than 60 percent of pregnant women practice some form of pica. Some health professionals refer to pica as an eating disorder, but many researchers wonder whether there is a biological reason for such a common behavioral practice.

Question: What nonfood substances are most commonly consumed?

Answer: The most common forms of pica include the compulsive chewing on clay, dirt, starch, baking soda, paper, sand, coal, wood, ashes and even paint. Although ice is a food, excessive chewing on ice also is considered a form of pica when chewing ice is more than the occasional crunch on a partially melted ice cube in a drink.

The consumption of dirt (typically clay) has been considered to be one of the most common forms of pica. The medical term for adding dirt to the diet is called geophagy, which literally means the consumption of earth. This type of pica is especially common in parts of Africa. A 1988 study reported that more than 70 percent of pregnant women at a Zaire hospital engaged in geophagy.

Pica that includes the consumption of raw starch is called amylophagy. Cornstarch is the typical starch consumed in the U.S. However, in other parts of the world, pica might involve uncooked rice or raw flours from wheat, cassava or rice.

The craving and consumption of large amounts of ice is called pagophagy. People may eat several glasses of ice at a sitting, amounting to several pounds of ice in a day.

Q: Why is pica common in pregnant women?

A: No one really knows the answer. For some, pica might stem from cultural beliefs and superstitions. In some cultures, pica is considered normal during pregnancy.

Pica has been linked to diets low in iron and zinc and may be related to an increased physiological or psychological need for these nutrients. During pregnancy the recommended intakes of iron, zinc and iodine all increase by about 50 percent. Low status for both iron and zinc is known to affect the senses of taste and smell, and low iodine status can have widespread effects by reducing thyroid hormone production. So it is not too much of a stretch to expect any of these deficiencies to have the potential to affect cravings for foods or nonfood substances.

Interestingly, ice chewing is one type of pica frequently related to iron deficiency, even without pregnancy. This strong drive to chew ice is frequently a good indicator of iron deficiency. Ice pica often is reported to occur in those who have had the bypass types of bariatric surgery for weight reduction. The surgical changes in the stomach and small intestine decrease the capacity to absorb iron and can lead to iron deficiency over time.

Scientists struggle to explain pica because it is such a persistent behavior. Is pica triggered by stress or a rational physiological drive? At this time there doesn’t appear to be one theory that fits all types of pica.