Here's a pretty good link on vegetarian diets for dogs. It reviews one of the articles that I quoted above (
E. Kienzle and R. Engelhard. A field study on the nutrition of vegetarian dogs and cats in Europe.) vegetarian pet food for dogs and cats?
Overall, the website looks a little like a Dog Food Advisor for the U. K. I'll try to see if I can locate the qualifications and educational background of the website manager...