The knowledgable Chrisb1 as always summed it up beautifully!
As I fast for weight loss, I do advocate exercise within reason. Personally, I've found that 20 minutes of pilates in the morning has done wonders for my activity level and sense of well being for the rest of the day. I think it helps people shake out of their malaise. However, for those fasting for health/spiritual, I totally concur with Chris as they should spend their energies on their mission.
If it's heavy exercise that I've inadvertently gotten involved with (like helping to haul our production gear into a shoot, going up and down steps, being in the heat) then yes, I do feel it much more profoundly. My heart beat is far more obvious, a bit light headed and feeling like I couldn't breath enough. So I think I know where my comfortable boundaries are now during water fasting.
I'm now down 11.6 pounds on this Water Fast in 4 days so I am beyond delighted! Having scales that work absolutely rocks!