Let us first explain the difference between fasting and abstinence. Fasting obliges one to control one’s appetite for food by eating only one full meal a day. Abstinence, commands one to abstain from fresh-meat on certain days.
From the Code of Canon Law ©1983:
Days of Penance Can. 1249 The divine law binds all the Christian faithful to do penance each in his or her own way. In order for all to be united among themselves by some common observance of penance, however, penitential days are prescribed on which the Christian faithful devote themselves in a special way to prayer, perform works of piety and charity, and deny themselves by fulfilling their own obligations more faithfully and especially by observing fast and abstinence, according to the norm of the following canons.
Can. 1250 The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Can. 1251 Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Can. 1252 The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance. Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast. Many who would be bound to fast are excused by virtue of a dispensation. Dispensations from fasting may be obtained from any good reason from the bishop, or the pastor, or some other priest have ordinary, quasi-ordinary, or delegated jurisdiction. In ability to fast may be physical or moral. It excuses the sick, convalescents, pregnant and nursing women, all who are in delicate health, the poor who cannot obtain enough food at any time to eat a full meal, as well as those who are obliged to perform hard bodily labor, or severe mental work, such as teaching; also, physicians, judges, and those whom fasting would hinder in the performance of pious and charitable works. The Church imposes fasting and abstinence as a means of furthering good works, and therefore does not wish this law to stand in the way of anything that is better or more necessary. Nevertheless, unless a person is clearly exempt from the fast, it is advisable that he consult his pastor or confessor and lay the case before him and get either a declaration of exemption or a dispensation. The priest may but is ordinarily not obliged to substitute some other good work, such as alms, prayers, Stations of the Cross.