The following is an excerpt from Petfinder.com’s The Adopted Dog Bible
It’s common for adopted dogs to be underweight, and if a pup moves from shelter to shelter, frequent changes in food types and feeding schedules can often lead to diarrhea and other gastronomic distress.
Unlike humans, who think that “thin is in,” being underweight is not healthy for dogs and can compromise their long-term health and quality of life.
If your dog is underweight — have this verified by your vet — start feeding him either a calorically and nutrient dense food that is formulated for performance, or a veterinary diet that is formulated for weight gain in chronically ill pets. These foods are both energy/nutrient dense and highly digestible.
If your dog does not begin to gain weight within a week or two, it’s possible that he is suffering from an undiagnosed illness or parasites, or is nauseous from a medication he is taking. Again, talk with your vet about any concerns.
Be sure that as your dog reaches his ideal weight, you adjust his diet accordingly, so he doesn’t inadvertently become overweight. As with any weight management program, the key is to go slowly to allow your dog time to adjust to his new meal plan.
In the meantime, keep in mind that underweight dogs (especially those with less fur) can get colder than dogs of normal weight, especially during the winter months. Provide your dog with warm sleeping accommodations and don’t be afraid to dress him in a chic canine coat!