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What Causes Weight Loss in Dogs With Cancer?

What Causes Weight Loss in Dogs With Cancer

Cancer is sadly a condition that is affecting more and more dogs, especially the older ones, and weight loss in dogs with cancer is not unusual. After all, a good appetite for food is often associated with good health, and a lack of appetite is often associated with disease, but exactly why does cancer cause weight loss in dogs? The weight loss in dogs with cancer is often different than the weight loss seen in a dog who is losing weight because of increased exercise. When the loss of appetite, and its associated weight loss, is secondary to malignancy it's known as "cancer cachexia." When cancer cachexia affects dogs, there is weight loss of both fat and muscle despite feeding the dog an adequate amount of food. There are three main reasons why dogs with cancer lose weight, and often it's a combination of factors.

dog cancer weight loss

Loss of Appetite from Effects Of Cancer

Lack of appetite and its associated weight loss is often one of the first signs of cancer in dogs. While some dogs are finicky eaters in nature, a gradual or more sudden loss of appetite in a dog who has always loved eating is a big red flag for many dog owners.

Depending on the type of cancer, the appetite loss may sometimes be due to the direct effect of the cancer itself.

For instance, a dog with a cancerous growth in the mouth or throat may develop pain during eating or swallowing and may therefore start eating less due to the association of eating with pain.

Certain cancers affecting the dog's digestive tract may trigger chronic bouts of nausea, vomiting and diarrhea which negatively affect the dog's body absorption of important nutrients. For instance, a cancer in the dog's abdomen may happen to grow and end up pushing on some nerve receptors that may trigger nausea.

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Did you know? Mast cell tumors may also affect a dog's appetite as their release of histamines cause increased stomach acid production and consequently nausea.


Loss of Appetite from Metabolic Changes 

Cancer can cause great changes to the dog's metabolism. Cancer cells feed on carbs as their main source of energy and create waste products that take more calories to metabolize.

On top of that, cancer cells use up a lot of the dog's calories because they are very active considering how fast they tend to divide and making all these new cells takes a whole lot of energy, explains veterinarian Dr. Damian Dressler.

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Sadly, these cancer cells rob the dog's body of energy sources, depriving dogs from the intake of calories derived from the foods we provide them. It's as if, along with the dog, we are also feeding those hungry cancer cells. Of course, all of this has quite an impact and therefore it's not surprising why cancer causes the dog's metabolism to increase ten-fold, with the end result of causing weight loss.

Loss of Appetite from Drugs

Several drugs used to treat cancer may also cause a decrease in appetite in dogs. While dogs do not typically develop nausea and vomiting after a round of chemo as it often happens in people, some dogs may occasionally develop nausea and vomiting.

Radiation therapy may in some rare cases may cause systemic effects of loss of appetite, nausea and vomiting. When the beam accidentally contacts healthy tissue (radiation scatter), such as the mucus membranes lining the mouth, sores may develop making eating painful.

If radiation is targeted towards the head or neck area, the dog's salivary glands may be affected making eating and swallowing challenging, and if the dog's nasal area is targeted, there may be a reduction in dog's sense of smell which can impact his appetite.

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The problem with side effects of medications is that dogs can learn quickly (one-trial learning) and start associating eating with the subsequent nausea or belly pain. This taste aversion may be difficult to overcome. Dog owners are therefore often upset when their dogs stop eating due to the effect of medications or treatments.

It's not like in people where you can persuade your dog to eat and tell him how much better he'll feel if he takes a bite, explains Susan Harper, an Animal Health Consultant with a Diploma in Animal Healing. If your dog has developed loss of appetite or nausea or vomiting either after taking medications to treat his cancer, consult with your vet.

"If you’ve had a recent stomach virus, you probably remember the last thing you ate before it struck…and you probably avoided it for several days or weeks afterward. Our pets are the same way, and this is something to keep in mind during chemotherapy. "~Allison L. Martin


  • Dog Cancer Blog, Things You Need To Know About Radiation for Dog Cancer, retrieved from the web on January 1st, 2017
  • Dog Cancer Blog, Signs of Dog Cancer and Decompensation, retrieved from the web on January 1st, 2017

Photo Credits:

  • Flickr Creative Commons, Jack-JackT Quincy Yes, I'm skinny...for now. CCBY2.0

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