The besieged need bread
In May of 1590, the city of Paris came under siege. France was in the throes of the wars of religion, and the city was held by the supporters of the ultra-Catholic "Holy League." Henri III, the last of the Valois kings, had been assassinated in 1589, and his heir was Henri of Navarre, a Protestant, whose accession the leaders of the League were determined to resist.1 Henri of Navarre moved to enforce his claim to the throne by force of arms, and on May 7th, his troops began to advance on Paris.2 The siege of Paris (along with much that led up to and followed it) was chronicled in the diary of one Pierre de L'Estoile, clerk-in-chief of the French Parlement. It is to his Memoires-Journaux that we owe the account of a strange experiment (reminiscent of the giant in the story "Jack and the Beanstalk") in the making of bread from human bones.
tldr; they did it
http://www.troynovant.com/Franson-JM/Essays/Bones-to-Bread.html