Vitamins A and D: Dogs and cats cannot make vitamin D in their skin, so it needs to be in their diet. And the vitamin D needs to be D3, which comes from animal sources, not D2, which comes from plant-based sources. “People and dogs can use D2 to some extent, but cats really need D3,” Heinze says.
Taurine. Dogs can make taurine if provided the right building blocks through dietary protein. Cats cannot make their own taurine at all, so it is regarded as an essential amino acid in this species and must be present in adequate amounts in the diet. Both species can suffer taurine deficiencies.