I am a firm believer that animals know instinctively what is good for them and what isn't. Stick a herd of cattle out on the range and they steer clear of any loco weed. Most canines are meat eaters. But every so often you will see them eating grass or some other vegetation. Vets, like doctors don't know EVERYTHING! They do know that chemically, there are foods like chocolate that are certainly not good for a dog. But this has been learned the hard way. If a dog loves bread, I say let him have it in moderation.
I know we are a bit off topic, but......
Foods That Can Be Poisonous to Pets : The Humane Society of the United States