Fung actually says that to muscle loss, you should not go longer than 2 days if I remember correctly, I’d say like 36-40 hours is optimal. The longer fasts are really meant for obese people and medical condition cure. If you are like „bodybuilding“ you have to get protein in the body, especially if you continue training, and also keep doing cardio, which you should. I would say alternate day fasting is a good method here.