Very interesting question mate
I have found when my dogs are being worked very hard everyday, they need extra fat and carbohydrate to keep their muscle up and a fine layer of fat over their ribs. The first place I notice wastage is around the loins and hips. Once I start to really see the bone poking out then I know I have to increase their food. I am referring more in relation to the greyhound types. But some Saluki types have leaner muscles anyways.
Now my collie hybrid puts on fat like a.... collie, so I feed him less and he can work all day. He starts the days work with a decent