The best thing to do is to determine, first of all, whether the dog really is underweight. You should be able to feel the dog's ribs with only a thin covering of skin. In a short-coated dog you should not be able to see the ribs and the bones should not be prominent. Due to the fact that they are active many dogs are lean this is not a sign that they need more food.

Increasing the amount of food you feed your dog will reduce the effectiveness of digestion so the dog may not put on weight. Increased feeding may even cause health problems.