Puppy mill dogs are often suffer severe nutritional deficiencies during their lives as breeder dogs. Many unscrupulous puppy millers feed cheap food of very poor quality, just enough to keep the dogs alive and breeding. You can help to mend their battered bodies by researching and carefully selecting high quality, nutritious food. The best possible food would be a home cooked, well balanced diet approved by a veterinary nutritionist. Balanceit.com is a great website for well balanced recipes. If time constraints or lack of cooking ability (we understand that) are issues, myollie.com will cook and deliver pet meals right to your door.
When feeding commercial food, our number one suggestion is to stay out of the grocery stores and big box stores. You aren't likely to find good quality dog food there. The best dog and cat foods are the ones you have never heard of. That is because they spend their money on making high quality food rather than advertising that they have high quality food when they really do not.
The website dogfoodadvisor.com has a great rating system for dog foods. We recommend no less than 3.5 start (preferably 4 or 5) when selecting a brand. Also check out their pet food recall section and sign up for alerts.
Consult your holistic veterinarian or veterinary nutritionist regarding the best nutritional profile for your dog's age, breed, health conditions, etc. For example, many high quality foods may be too high in protein for a dog with kidney or liver issues, causing further damage, and a dog prone to pancreatitis would not do well with high fat content. The best bet is to select high quality food with a well rounded balance of protein, carbohydrates and fat.