WSAVA nutrition toolkit: tools for vets and pet owners to carry out nutritional evaluation, choose the best diet for our patients, etc.
Pet nutrition blog oriented to pet owners, by Dr. Lisa Weeth (DACVN)
Website from AAFCO, Association of American Feed Control Officials, regarding commercial manufacturing of pet food
Website from FEDIAF, European Pet Food Industry Federation, regarding commercial manufacturing of pet food. Nutrient requirement tables can be found here.