You've received a lot of excellent advice. I would only add that you might also want to consult a dietician, as well. My husband is diabetic and discussed the fasting guidelines with a dietician to help him understand what his best choices would be.
An important thing to remember is what fasting really is. Although we often think of the Lenten fast, for example, in terms of food, it is, first and foremost, a fast from sin and self-indulgence. Abstinence from food is only a symbol of our true fast from sin and selfishness. Because fasting does not mean “giving up” certain foods, as a penance or to punish ourselves.
Any type of abstinence is a spiritual effort. Each time we tell ourselves "No," we become stronger, and each time we give in, we become weaker spiritually. So during Lent we give special emphasis to practicing self-restraint - not just in what we eat, but in all our life.