Pure, fresh cod liver oil is the one supplement that Dr. Billinghurst recommends above all others. He states "if you supplement with nothing else, supplement with pure, fresh cod liver oil."
Why? Cod liver oil is anti-inflammatory, meaning it can help or prevent arthritis in dogs. It is a natural source of Vitamins A and D, Omega-3 fatty acids EPA, DHA. It promotes healthy cell function, bone growth, eye health, and promotes skin, coat and joint health.
Here is a comprehensive article appearing in the Whole Dog Journal that every dog lover should read.
Always supplement your dog's fresh diet with cod liver oil. Do not supplement your dog's commercial diet with cod liver oil. The article above gives complete details.