PHILADELPHIA (AP) _ Dirt eating, a common practice in the rural American South and around the world, may have important medical benefits but could also pose serious health risks, researchers said Friday.

Dirt eating in the American South is most common among blacks, and the practice might have been brought by slaves from Africa, where it is also common, said Donald Vermeer, an anthropologist at Louisiana State University and one of the pioneers in the study of geophagy, as scientists call the practice.

Dirt eating is also seen in Southern whites, Vermeer said, and in American Indians.

Vermeer said at the annual meeting of the American Association for the Advancement of Science that in urban settings many dirt eaters turn to the consumption of laundry starch or even baking sda. Some northerners obtain clay from relatives in the South, Vermeer said.

Darla Danford of the National Research Council in Washington said that references to dirt eating go back as far as 300 B.C., in the writings of Aristotle. Dirt eating occurs ''in both sexes, in all races, and in animals as well as humans,'' she said. It is found at all social and economic levels and in all cultures.

For reasons that Vermeer cannot explain, dirt eating remains a little known phenomenon despite being so widespread.

''Surprisingly few investigations of the practice have been undertaken,'' Vermeer said. He began studying it 25 years ago when he observed it in West Africa.

In parts of Nigeria, clay is mined in large quantities and distributed for sale in markets all over West Africa, Vermeer said.

Most dirt eaters prefer some variety of clay, rather than sand or topsoil.

One form of clay, kaolin, is apparently used by many dirt eaters in part to control nausea and diarrhea. Kaolin is a principal constituent of Kaopectate, the commercial anti-diarrheal medication.

Clays have been shown to be able to remove toxins from the body. Vermeer speculated that clays eaten in Nigeria are high in calcium, for example, and thus their frequent use by pregnant women could be a way of adding calcium to the diet in a region where milk, an important source of calcium, is not consumed.

Timothy Johns of the University of California in Berkeley reported that clay is eaten in combination with wild, poisonous potatoes by Indians in the central Andes of South America and in the southwestern United States.

The clay removes the toxic substances, called glycoalkaloids, from the digestive system before they can cause illness, Johns said.

Danford warned, however, that dirt eating can lead to severe growth retardation, anemia and even death.

Certain soils can combine with minerals in the diet in such a way that the minerals cannot be absorbed by the body, she said.

Iron deficiencies can then result in anemia, for example, and zinc deficiency can produce growth retardation.

She described a ''geophagic syndrome'' that can be caused by dirt eating. The syndrome is marked by growth retardation, delayed sexual maturity and liver and spleen enlargement, she said.

Clay can also block the intestines, a condition that can be fatal, Danford said.

Vermeer said that ''no evidence exists that normal consumption of clays in the American South is either beneficial or harmful.''

Dirt eating in the South occurs in small rural communities that have their own local sources of clay. Many of these sources are in roadside banks of clay that have been exposed by road building.

''Continual excavation undermines the roadbank,'' Vermeer said. ''In Alabama the damage to roadbanks in some places has been so extensive that the highway department has posted signs requesting local residents not to dig into the roadbanks.''

Vermeer said dirt eating is especially prevalent in pregnant women. In both the South and West Africa, ''an ill-defined craving for clay is taken to be the first evidence of pregnancy,'' Vermeer said.