Three things can cause a dog to be underweight: parasites, disease or illness and being underfed. Finding out what is causing your dog to be underweight is important because until you address it, it will be very difficult to help your pet gain the appropriate weight.
Assess your dog's weight. You should be able to feel your dog's ribs but not see them clearly. When looking down on your dog, you should see a slight tuck at the waist. From the side, the belly should be tucked up and a flank fold present.
Have your dog checked by a veterinarian. Among the most common causes for a dog to be too thin are parasites and disease. You need to eliminate these causes before making any dietary changes.
Weigh and record your dog's weight. This will give you an idea of how much your dog has gained later on.
Determine your dog's Resting Energy Requirements (RER). This is done by multiplying the dog's weight in kilograms by 30, then adding 70. For example, if your dog weighs 22.7 Kilogram, or 22.7 kilograms (kilograms = pounds divided by 2.2). 22.7 x 30 = 681 + 70 = 751. This will give you a rough estimate of how many calories your dog would need if he was resting all day long.
Multiply the RER by 1.6 if your dog is neutered and has a normal activity level. Multiply the RER by 1.8 if your dog is not neutered. If your dog is more active, you will want to multiply this by more, anywhere from 2 (for light work) up to 8 (for heavy work) times.
Feed your dog a high-quality dog food with lots of protein. Check the ingredients on the dog food--the first ingredient should be "chicken," "turkey," "lamb" or even "beef." Avoid foods where by-products or non-protein sources are listed first.
Feed your dog at least two times a day. In some cases, you may choose to leave dry food in front of him at all times and providing canned food twice a day.
At least once a week, weigh and record your dog's weight. See if there is any improvement. If not, and your dog is healthy, try increasing the amount you are feeding slightly.
Adding some high protein "people" foods to your dog's diet is OK providing it does not exceed 10 per cent of the total. This can include cooked meats without the bones and dairy products such as cottage cheese, low-fat yoghurt or hard-boiled eggs
Use a measuring cup to ensure your dog is getting the same amount each day. Gradually increase the amount you feed your dog.