someone like me with health issues like diabetes, certain medications etc. should not fast at all. the strict medical diet I am on is fast enough, by like poster above says, prayer makes the difference. In my morning offering I follow Matt Talbott’s advice and offer up all the sacrifices and sufferings of the day to the Lord.
As a general rule of spiritual direction, one does not undertake fast, abstinence or other penitential practices beyond the standard practice of the Church, without permission from one’s confessor or spiritual director. Standard practice being what is ordained by law and custom–Friday abstinence, the manner of fasting prescribed for Lent etc., or any of the fasting disciplines of one’s religious order or secular institute, or connected with an approved devotion, such as consecration to Mary etc.
Of much more relevance than making up disciplines is what classic monastic direction calls “the food thought.” What is eaten, the amount, the quantity, the quality, why you are eating, when you are eating, with whom you are eating, when to stop eating. The disciple pays conscious attention to each of these, does not eat mindlessly, eats in community when that is appropriate, appreciates and gives thanks for whatever is received, prepares and serves food with love as if Christ himself is being served, does not eat anything detrimental to health, and so forth. In the same way pay attention to hunger, as it brings its own spiritual message as well as physical demands.
When we fast under self-imposed guidelines we run the risk of being tempted by pride, or of falling into scrupulosity if we have that tendency, that is why spiritual direction from a personal director or from a classis spiritual writer in the mind of the Church is so essential. It is easy to lose sight of the reason for fasting or any spiritual discipline: fostering humility and obedience.