From n=1 experience, if you exercise while water fasting you hardly lose any muscle, but if you don't, you lose a lot of muscle and quite quickly. (Which makes sense - maintaining muscles costs both energy and protein - if you have no intake and they are not in use, why spend that?)
In your personal experience, if you exercise when you are waterfasting, you maintain muscle. That is pretty cool, can you share your exercise routine? The parent commenter noted that exercising while fasting might be painful and I would appreciate hearing what you do. Thanks!
Mostly running at a leisurely pace - most people use many of their muscles when they run - I was doing 5km three times a week. Additionally, I was doing 2x1hr a week of karate practice (a whole body workout), And the occasional random set of 20 pushups / 20 sit-ups / 20 squats, probably 2-3 times a day.
I should note that anything aerobic like running was easy, anything anaerobic (like karate sparring) was not, but it wasn’t painful either.
I did pay attention to electrolytes - especially magnesium and potasssiun - I got cramps when water fasting and not supplementing.
I know my body well, and I think that’s essential to be able to continue - e.g. to know it is magnesium that’s needed now, and not (as one might think) the body is eating itself.
