WASHINGTON (AP)  -- Children sacrificed by the Inca appear to have been "fattened up" in a yearlong ritual, new research suggests.

Researchers studied hair from the heads and in small bags accompanying four mummies of children sacrificed in Inca rituals. Their findings are reported in Tuesday's issue of Proceedings of the National Academy of Sciences.

The children's hair had been cut first a year and then six months before they were killed. By studying the chemicals preserved in the hair researchers can calculate the diet of the children.

The Inca lived in the area that is now Peru and were conquered by the invading Spanish in the early 1500s.

The children's diet was initially focused on vegetables such as the potato, but in the last year of their lives it was enriched with corn -- an elite food -- and protein probably from llama meat, according to the researchers led by Andrew Wilson of the University of Bradford in England.

"Given the surprising change in their diets and the symbolic cutting of their hair, it appears that various events were staged in which the status of the children was raised," Wilson said in a statement. "In effect, their countdown to sacrifice had begun some considerable time prior to death."

Changes in the hair samples indicate that in their last 3-to-4 months the children began their pilgrimage to the mountains, probably from Cuzco, the Inca capital.

The scientists are not certain how the children died, but think they were given maize beer and coca leaves.

"It looks to us as though the children were led up to the summit shrine in the culmination of a yearlong rite, drugged and then left to succuмb to exposure," said co-author Timothy Taylor, also of the University of Bradford.