+1 for Sandymere,if you are resorting to extra vits etc then you aint feeding your dog right...full stop. As far as electrolytes are concerned then there are times when they are invaluable,but that is not something that is needed as an ongoing part of any dogs diet,they are for rehydrating a dog,not a supplement.Plus,I'm sure no-one runs their dogs anywhere as hard as you do.
Martin