hair growing waist-lenght in half a year!?

If you've read "Catch Me if You Can" by Frank W. Abagnale and Stan Redding, you will find a most amazing situation that's hardly seen in the recent film of the same name. When Abagnale was doing time in a French provincial prison, he spent 6 months wasting away in an approximately 5' by 5' by 5' dungeon, with little food, no light or toilet or practically anything else for that matter, according to what is written in the book. This was in or around 1968 and the prison was amazingly primitive, even by France's hard standards. They write that Abagnale's hair grew to his waist (I imagine from being short) and his beard grew to his belly, in only those 6 months ("I am hirsute by heritage - I.E. Abagnale has strong hair). This seems hard to believe. Is it possible? Could it be that it grew particularly quickly due to his body eating its own nutrients due to poor nutrition? Or did the book exaggerate that scene (Abagnale himself claims his co-author overdid some things in the book for dramati purposes, and there is a disclaimer inside)? Is this for real?

Yes, in fact I first saw the movie and liked it so much I read the book. In the film, Leonardo DiCaprio exits the jail with grown-out long hair and a beard, but nothing like what the book describes.