King Cobras (as opposed to "true" cobras) occur in their own genus, Ophiophagus - which refers to "snake eater." For King Cobras, snakes are described as a preferred food source (though rodents and lizards will apparently also be taken).
I can't find much in the literature that confirms health deficiencies in Genus Lampropeltis when other snakes are unavailable in their diet (or King Cobras for that matter). If it's out there, I'd be happy to read it. And I'd expect fledgling (or otherwise) Kingsnake keepers would appreciate seeing it too. Thanks !