EWTN on fasting
Jimmy Akin on the question of beverages and fasting
From the above link:
The above is helpful to me, because as someone who deals with those pesky migraines on occasion, I used to just accept the fact that I would just about always have a migraine on the Thursday after Ash Wednesday and on Holy Saturday. It was just part of the fasting package.
When we do that, it is immediately clear that in interpreting the Church's laws regarding fasting the terms "meal" and "food" are understood as being food rather than beverages. If you go look in old moral theologies, they invariably talk about the fact that you can drink things--including things other than water--on days of fasting.
Some moralists have considered alcoholic beverages contrary to the spirit of the day, but they don't consider beverages other than water to be contrary to the spirit. Examples they commonly cite of beverages that are okay to have on fasting days are milk and fruit juices and coffee with cream, all of which contain calories.
Beverages just are not included under the law of fasting.
This means that, if I wanted, I could drink can after can of low-carb protein shakes on Ash Wednesday and Good Friday and consume 3500 calories of them and still be within the letter of the law.
I would not, however, be within the spirit of the law, which is to encourage moderate hunger as a form of spiritual discipline. If I use calorie-laden beverages (or hunger suppressants) to get around that then I am violating the spirit though not the letter of the law.
But when I found out that it was okay to drink a little fruit juice or milk between meals if necessary, I also discovered that the post-fasting migraine wasn't a foregone conclusion. True, as Jimmy writes, you aren't supposed to be drinking calorie-rich beverages in order not to feel hungry--but if you are drinking them to ward off mild hypoglycemia that isn't in itself bad enough to exempt you from fasting (which some of my skinny sisters suffer from), or to keep from the kind of blood-sugar crash that is implicated in some types of migraine, or to be able to serve at the altar at an evening E.F. Ash Wednesday Mass without fainting halfway through, or to be able to drive safely home from work or to that Mass, then you can drink the beverage with a clear conscience, because such beverages are not forbidden on fasting days.
Now, if you do suffer from a bona fide health condition which makes fasting unduly burdensome or even dangerous, or are pregnant or nursing, chances are that you aren't subject to the strict fast anyway. When in doubt, ask your pastor (preferably before the fast day begins). Of course, not being subject to the strict fast does not preclude offering up some voluntary sacrifice to unite yourself with the spirit of the fast day.