@thor Your body will always burn fat and carbs first. But it will also convert proteins to ketones and use those for energy after it burns whatever carbs and far is availible to it or if protein is in excess and is to be stored for later.

If you dont lift weights and have no muscle to repair the protein wont go to making new muscle, it will be stored as fat.

In the end any talk about which type of energy source is used first is pointless, the amount of energy your body needs is determined by your activity, and your body will burn what it needs to get that energy and will always store the rest as fat if there is no active protein being needed.

The reason body builders can eat massive calories and not get fat is not their focus on protein, but rather the fact that weight lifting over long periods burns tons of energy and therefore they can eat a lot and still have a deficit.