Children selected for Inca ritual sacrifice were "fattened up" with high-protein diets in the months leading up to their deaths, a new study has found.

Researcher Andrew Wilson and his team conducted DNA and chemical tests of hair samples taken from four child mummies found in the Andes mountains in the 1990s. (See a photo gallery of the frozen Inca mummies.)

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By studying the ratios of chemicals present in the hair, the team helped show how victims were prepared for death as far as a year in advance, sent on grueling highland journeys, and drugged before the sacrificial ceremonies.

"The findings offer insights into the preparatory stages leading to Inca ritual killing, as represented by the unique capacocha rite," the report reads, referring to the Inca tradition of mountaintop child sacrifice.

Hair logs a chemical record of what an individual consumes, and the information can stay intact in archaeological remains, the study authors point out.

"It is chilling that the children themselves, through their own tissue, give us graphic details and evidence that they were not killed on a whim but were part of a complex process for which they were selected some considerable time before," said Wilson, a lecturer in archaeological sciences at the University of Bradford in Britain.

The study appears online this week in the Proceedings of the National Academy of Sciences and was funded by the U.K. charity the Wellcome Trust.

"Fattened Up" and Drugged

One extremely well preserved mummy - a 15-year-old girl known as "La Doncella" or the "Llullaillaco Maiden" - appears to have been selected for sacrifice a year in advance, Wilson said.

Tests show the Maiden - whose hair was 9.8 inches (25 centimeters) long, representing more than two years' worth of growth - was raised mostly on a protein-poor "peasant diet" rich in potatoes.

"But 12 months before her death, her diet becomes protein rich," Wilson said, adding that she was likely fed "elite" food such as maize and llama meat.

"We can equate a change in diet of this magnitude with a change in her status, one that occurred as part of her final demise."