Like liver, kidneys are also classified as offal and should constitute a maximum 10% of the diet. They are a great source of good quality protein, essential fatty acids and many vitamins including all the fat soluble vitamins A, D, E and K, plus all the B group vitamins, in particular B6 aids the creation of antibodies in the immune system and B12 which protects the nervous system and aids digestion. Kidneys are good for dogs who may be constipated.  They are also a rich source of iron and have good levels of zinc