yah, tell that to all the monks that fast. they look to be in pretty good health and shape to me! they look much better then most of our western population!
personally,i'm 5 ft 1 and about 100lbs. i've done 2 fasts in the last 2 yrs. last month i did one for 8 days. i did lose some weight but not much. muscle can be built back up with protein and exercise and like with any cleanse or fast you should replenish and rebuild your body after.
look at it this way: a fast or cleanse is like taking your clothes off at the end of the day and bathing.
tomorrow you will get dressed again and sweat, shed skin cells and get dirty right? but with a shower you will have a clean start!