Modern Catholic Dictionary:
FASTING. A from of penance that imposes limits on the kind or quantity of food or drink. From the first century Christians have observed fasting days of precept, notably during the season of Lent in commemoration of Christ’s passion and death. In the early Church there was less formal precept and therefore greater variety of custom, but in general fasting was much more severe than in the modern Church. In the East and West the faithful abstained on fasting days from wine as well as from flesh-meat, both being permitted only in cases of weak health. The ancient custom in the Latin Church of celebrating Mass in the evening during Lent was partly due to the fact that in many places the first meal was not taken before sunset.
The modern Church regulations on fasting, until 1966, prescribed taking only one full meal a day, along with some food for breakfast and a collation. Days of fast and abstinence for the universal Church were Ash Wednesday, the Fridays and Saturdays of Lent, Ember days, and the vigils of certain feasts. Days of fast only were the rest of the days of Lent, except Sundays. Special indults affected different nations and were provided for by canon law.
With the constitution Paenitemini of Paul VI in 1966, the meaning of the law of fasting remained, but the extent of the obligation was changed. Thus “the law of fasting allows only one full meal a day, but does not prohibit taking some food in the morning and evening, while observing approved local custom as far as quantity and quality of food are concerned.” To the law of fast are bound those of the faithful who have completed their eighteenth year and up until the beginning of their sixtieth year. Prescribed days of fast and abstinence for the whole Church are Ash Wednesday and Good Friday. Nevertheless, as with abstinence, so with fasting or other forms of penance, “It is up to the bishops, gathered in their episcopal conferences, to establish the norms . . . which they consider the most opportune and efficacious” (Paenitemini, III). In the Eastern rites it is the right of the patriarch, together with the synod or supreme authority of every rite, to determine the days of fast and abstinence in accordance with the decree of the Second Vatican Council for Eastern Churches.
My doctor would have my head if I fasted.