Your body burns alcohol first, then carbs and finally fat. When you fast, you only have the last one - but you likely have enough of it for a few days (likely close to a month) before your body needs to break down muscle and convert it to carbs for fuel (gluconeogensis).
It may not feel very good in the first few days, you will need a lot more electrolytes, and it will take a few days before your body is doing it perfectly; fasting puts you into ketosis, with all that entails.
When I fast, I make sure to exercise more than usual, and especially cover all muscle groups (which I don't religiously do when exercising for fun). It is my experience that my body will keep muscles i use, but not keep those I don't. YMMV.
Also, listen to your body but do not extrapolate; the first two days of full fast may seem impossible, and if you extrapolate them to a full week may seem like certain death or madness; but after 48 hours or so, there's a phase change. I stop when I start feeling hungry - usually after a week or so, but once or twice after a month.