Perhaps the most straightforward reason for weight loss in dogs is that they’re not consuming enough calories. If you’ve recently changed up your dog’s food, ensure they are receiving enough caloric intake for their body weight.
For example, 1 cup of their old food may not have the same number of calories as 1 cup of their new food. This could explain a rapid loss of body weight. Make sure you follow the feeding instructions on your dog’s food or ask your veterinarian for a feeding guide.
Similarly, if their physical activity level has increased, your dog may require more caloric intake. If your dog doesn’t like their food and refuses to eat it, this will also contribute to weight loss. As dogs age their nutritional requirements also change, so feeding a diet that was appropriate at one stage may lead to weight loss at a later stage (ex. A senior dog should not be eating an adult maintenance diet).
Decreased Appetite or Anorexia
When your dog is feeling unwell for whatever reason, it can lead to a decreased appetite. With a short term illness, a day or two of decreased appetite will not lead to significant weight loss. Chronic illnesses, however, tend to cause a more gradual loss of appetite. By the time a dog is completely anorexic (not eating at all), significant weight loss has often occurred.
Examples of chronic illness that decrease appetite include chronic kidney disease, liver disease, and even dental disease. Cancer is also a common cause of decreased appetite and weight loss. Tumors release chemical messengers called cytokines that kill appetite and contribute to muscle wasting.
Unfortunately, because weight loss in these cases tends to be gradual, owners often fail to notice until the disease process is fairly advanced. With most chronic illnesses, there will usually be other symptoms, so keep an eye out for changes in your dog’s behavior.
Unlike dogs with kidney or liver disease, dogs with diabetes lose weight despite maintaining a good appetite. In fact, one of the main symptoms of diabetes is an increased appetite. Without insulin, dogs with diabetes cannot move glucose from their diet into their cells, and the result is essentially starvation of the cells in their body, despite ingesting plenty of calories.
The earliest symptoms of diabetes are increased appetite, thirst, and urination. If left untreated, significant weight loss, and eventual loss of appetite will occur. Diabetes can become a life-threatening emergency if it’s not diagnosed early.
Contrary to popular belief, parasites are a fairly uncommon cause of weight loss in pets. An extremely heavy parasite burden could contribute to weight loss by causing diarrhea and decreased appetite. Parasites are more common in puppies and present as failure to gain weight rather than true weight loss.
Gastrointestinal Disorders including IBD
Conditions that affect the gastrointestinal tract can inhibit your dog’s ability to absorb essential nutrients and calories. Inflammatory Bowel Disease (IBD) is the most common example of this in dogs.
Dogs with IBD typically present with chronic vomiting and/or diarrhea in addition to weight loss. Another condition called lymphangiectasia causes proteins to leak out of the small intestines which can lead to considerable weight loss.