From the Pet Connection (one of our favorite sites) comes a review of some of the common vitamins your dog needs and the signs of either a vitamin deficiency or too much of any vitamin.
Becoming acquainted with the signs of vitamin deficiency and excess can help us identify contributors to chronic ailments in our companions. We can then “consider the source” of vitamins and then add foods rich in daily requirements that might otherwise be missing from the diet.
Be sure and read the whole article. After the problems with Blue Buffalo, this is an important topic.