lenten fasting questions

[Poche]

The instructions say that if the fasting or abstinance negatively effects your health then you are obligated to it. It doesn't say that you  have to have a note from your doctor or that it has to be certified in any way. If you do not feel up to fasting then why not substitute something else. Go to daily mass, make the stations of the cross, pray the rosary. visit the sick, be nice to someone you particularly dislike, the list can go on. 

[GangGreen]

On a side note, since I figure this the place to ask. If liquids don't break a fast, to what extent do they not?

For example, I assume a glass of milk doesn't break a fast. Does a whey protein shake break a fast? Whey protein is technically just extracted from milk, but it can also have a bunch of other stuff in it. I'd assume that once you start adding stuff to drinks is when it breaks the fast, so adding a powdered substance would kind of be borderline to me.