I don’t think supplementing with vitamins, especially if you are feeding a food that’s complete and balanced, is a good idea. If you must supplement for a specific condition, that would be different. An example is, in a dog with arthritis, fish oils may be beneficial to reduce inflammation. Here’s a link which shows that several studies have found that humans taking multivitamins has actually caused a shortened lifespan and some medical issues in others:
http://www.theatlantic.com/health/archive/2013/07/the-vitamin-myth-why-we-think-we-need-supplements/277947/
