The Secret of Sanctity of St. John of the Cross, by Fr. Lucas of St. Joseph, O.C.D., Bruce, Milwaukee, 1962. (Fr. Lucas was martyred by the Communists in Spain in 1936.)
EXPLAINING THE LAWS OF FAST AND ABSTINENCE, FOR DAYS OF FAST AND ABSTINENCE On days of fast, only one full meal is allowed, at which meat may be taken. Two other meatless meals, which together are less than the full meal, are also permitted. Only liquids may be taken between meals. The law of fast must be observed by all between the ages of 21 and 59 inclusive under pain of mortal sin.
If fasting poses a serious risk to health or impedes the ability to do necessary work, it does not oblige.