Pope John Paul II 'regularly whipped himself'
The book also described how, as a bishop in Poland, the future pontiff would often sleep on a bare floor as an act of self-denial and asceticism.
Msgr Oder, who like the former Pope is Polish, also revealed in the new book that John Paul had decided that in the event of an "incurable sickness" he would voluntarily step down rather than stay in office until his death.
He declared in 1989 that in "the case of sickness that is deemed to be incurable, which prevents me from sufficiently fulfilling the functions of my apostolic ministry," he would renounce his positions as the head of the Roman Catholic Church and the Bishop of Rome.
Senior Vatican cardinals would then have been entrusted with the task of putting the resignation into effect, after which a new Pope would have been elected.
Had he stepped down, he would have been the first Pope to do so willingly since 1294.