ARGGGHH!
Ok, as this one has touched a deep nerve with me, I'm going to ignore the very sound advice on my signature and jump in. I almost wish I could get permission to repost this post on my website.
Whenever I see a new client I always ask, of course, what they are feeding. And if, say, a seven year old dog is doing well- good weight, good coat, no allergies, good stools, good appetite, and is on a decent food like Purina, Beneful, Pedigree, I would NEVER suggest that they 'upgrade' to something better with 'better ingredients.' And I wish the busybodies would stop doing that as well. Now, if you are starting out with a puppy - sure, we can discuss the relative merits of foods. If there is any sort of problem, of course, 'upgrading' food is the ultimate in natural, holistic health improvement.
But this idea that people whose dogs are doing just fine have to keep searching for the Utopian absolutely best possible food is alarmist and harmful. This poster's experiences are a perfect example of the problems that such 'advice' can provoke.