I'm not sure how you can say that fuel/food cannot build muscle. Amino acids, the building blocks of protein, are the basis for lean tissue creation within the body. Amino acids cannot be created by the body and must come from a food source. Without that, you're not laying the foundation for any muscle growth via exercise.
That's why I was asking if anyone doing extended fasting was concerned about the loss of lean tissue via fasting. Most research says anything beyond 16 - 18 hours becomes catabolic in nature. Beyond a certain point, the losses may minimize, but it's still loss of important tissue.