When you look at your dog's body, if you can see its ribs clearly, then your dog is underweight, according to the Iams website. Visible ribs on a dog are a sign that your dog has little body fat and may suffer from some type of illness. To treat your dog, adjust its diet and have it examined by a veterinarian for signs of a medical condition.
Several types of medical conditions can cause weight loss in dogs or may contribute to a dog's inability to gain weight. Take your dog to a veterinarian to see if it suffers from an intestinal parasite, liver disease, pancreatic disease, intestinal blockage or blood disease -- all of which can cause the dog pain and reduce its desire and ability to eat, according to PetPlace.com. Other diseases that affect the dog's sense of smell or irritate its throat can also contribute to a lack of appetite. Have the doctor check the dog's teeth, as poor dental health makes it uncomfortable for your dog to eat and chew food.
Ask your veterinarian what the normal acceptable weight for your particular breed of dog would be. Use this number when determining how much food to give your dog each day. Read the package directions on the canned or dry food you plan to give the dog. Feed the dog the daily recommended serving, based on the ideal, rather than actual, weight of the underweight dog. Divide this amount of food into four smaller portions to feed it throughout the day at six hour intervals, to allow the dog's stomach to adjust to the food, recommends the PetMD website.
To promote weight gain in your dog, feed it a food labelled as nutritionally complete, made with meat-based proteins. Give the dog a food designed for puppies, labelled for growth, or a special, prescription weight-gain diet available from your veterinarian. Include an omega-3 or omega-6 fatty acid supplement mixed into the dog's food, for a dog recovering from starvation. Tempt a dog with a poor appetite to eat the food by warming it for a few seconds in the microwave to increase its aroma. Add some chicken or beef broth to the food to moisten and provide extra flavor.
Underweight dogs can become chilled easily, so you need to keep them warm. Dress the dog in a warm sweater in the winter months and put a warm coat on it when taking it outdoors. Provide the dog with a comfortable heated dog bed at night to provide warmth, and a cushion for the dog's body, which has little fat on it. Never feed large meals to an emaciated dog; it can lead to vomiting and actually discourage eating. If a dog does not eat for several days, the lack of nourishment can cause severe damage to its organs and lead to death.