Just like humans, almost all pets do benefit from a vitamin supplement. And just as there has been considerable controversy in the medical fraternity over whether humans need extra vitamins, so too do many vets dispute the need for nutritional supplements for pets.The argument usually goes something like this: The majority of commercial petfoods contain added vitamins. So even though your pet's diet may be inadequate, the vitamins added to pet food compensates for this.Wrong! Many human brea...