Realistically if you eat adequate protein and workout throughout the week or month, would 1 day a week or a couple days of fasting result in any muscle tissue loss, during weight loss? I understand glycogen stores deplete and all but what about the tissue itself?
Our ancestors thousands of years ago routinely had to go 1-3 days or more on severe caloric restriction. If muscle attrition was significant, we would not be here. You will lose more from sedentary lifestyle.