I'd say it is always a question of balance...is it a water or other fast you are doing?
if on water i'd say be careful!
you should do little exercice and not push too strong, if another fast i'd say you can go for more...
it will help you loose more in harmony, speaking about the silhouette amd also some exercice, if not brought to the extrreme, helps to keep a good level of energy!
it is important to move during a fast.
hope this helps a bit