Commercial foods are subjected to high heat which causes the vitamins to lose their potency. There are many good natural supplements on the market made for pets that will ensure your dog is getting the proper nutrition. If you homecook or are feeding raw, adding the right supplements in proper amounts is very important too. I supplement my dogs with glucosamine/chondroitin, omega 3, Dr. Mercola's Spirugreen, COQ10, B complex, Vitamins D & E and calcium in the form of ground eggshells. Check with a holistic vet to find which ones to use and dosages for your particular dog. I would recommend reading "Raw and Natural Nutrition for Dogs" by Dr. Lew Olson. There's a wealth of information that is very helpful whether you feed commercial food or raw or homecooked.
