There is commandment laid on us to fast. For fasting helps to destroy sin, and to raise the mind to thoughts of the spiritual world. Each man is then bound, by the natural law of the matter, to fast just as much as is necessary to help him in these matters. Which is to say that fasting in general is a matter of natural law. To determine, however, when we shall fast and how, according to what suits and is of use to the Catholic body, is a matter of positive law. To state the positive law is the business of the bishops, and what is thus stated by them is called ecclesiastical fasting, in contradistinction with the natural fasting previously mentioned.
--St. Thomas Aquinas