What diet should we feed a cat or dog with cancer?

There is very little evidence to support the use of a special cancer diet in people or dogs. There is even less evidence in the cat. If someone was to require proof of this, then exploring the advice given to human cancer patients is a good start – moderate recommendations to eat a healthy balanced diet including plenty of fresh fruit and vegetables are not unusual. Thus, unless a specific pet has certain needs to be addressed (for example, renal insufficiency or cachexia), a dietary change is likely to be of limited value or possibly dangerous. So, in principle, this could be a very short article; however, there is a very important point to make – so please, read on!

What is the lie of the land?

The internet is full of reports of ill-evidenced cancer diets and miracle treatments based on nutraceuticals. Owners of dogs or cats with cancer will understandably be drawn to these. Studies from around the world have shown that 30 to 40 percent of owners of cancer-bearing pets feed a home-cooked diet (often involving raw food), 40 percent use food supplements and over 50 percent distrust conventional diets (Bianco et al., 2020).

We need to stay part of the discussion on feeding and supplement […] to ensure that the animal is not receiving a dangerous diet and to make sure no space is created for unqualified individuals to give advice

In view of the lack of evidence supporting these measures, it is easy to dismiss them and concentrate on treatments that can make a difference; however, the same studies highlighting the high prevalence of dietary change, raw feeding and use of supplements also consistently tell us that pet owners regard veterinarians as the most important source of nutritional advice. Like it or not, we need to stay part of the discussion on feeding and supplement use for two reasons: to ensure that the animal is not receiving a dangerous diet and to make sure no space is created for unqualified individuals to give advice!

Consideration of a pet’s diet can have a tremendously positive effect for the owner. Many of our owners are not scientifically minded and find it difficult to understand the biology of the disease afflicting their best friend and the treatments we propose. This creates a feeling of powerlessness and distress. Use of simple dietary-based measures can provide an important psychological crutch, empowering people to play a role in their pet’s treatment. The benefit of this to the owner can be hard to overstate. Furthermore, if the veterinary surgeon doesn’t engage in a discussion on nutrition and supplements, someone else will – such a person may not be considered qualified to do so! 

Use of simple dietary-based measures can provide an important psychological crutch, empowering people to play a role in their pet’s treatment

We will now explore two of the common dietary alterations that owners often want to discuss with me: low-carbohydrate and raw diets.

Low-carbohydrate diets

Often, pet owners will ask me some form of this common question: “I am trying to feed him as low carbohydrate a diet as possible. What do you recommend for this?” So, let’s explore the growing interest in low-carbohydrate diets for cancer patients.

A little bit of history

In the early 1920s, German biochemist Otto Warburg made the following observations of cancer tissue:

• Cancer tissue has no difference in oxygen consumption compared with normal tissue
• BUT cancer tissue also uses 10 times the amount of glucose and produces 100 times more lactate!

These observations were very interesting. To understand them, let’s (briefly!) revise a little biochemistry from vet school. 

Breaking down the biochemistry

Normally, a glucose molecule is first broken down into pyruvate through glycolysis. This process releases a little energy but doesn’t require oxygen. Further breakdown of pyruvate occurs in the Krebs (or citrate) cycle. This also only releases a little energy but provides important hydrogen ions, which then form the substrate for oxidative phosphorylation in the mitochondria, releasing lots and lots of energy (Figure 1). All processes after glycolysis require the presence of oxygen.

Where oxygen is absent, cells are able to “tick over” by going as far as glycolysis alone. The accumulating pyruvate would halt this equilibrium reaction if it were not for its conversion to lactate, which then diffuses out of the cells and allows the equilibrium reaction to continue (Figure 2). This is an incredibly inefficient process, but it allows essential energy release to continue until oxygen (hopefully) becomes available again.

What Warburg discovered was the fact that cancer cells were choosing to do just the glycolysis part, even when oxygen was plentiful (Figure 3). Why is this? Warburg’s observations were very hard to explain at first, but we now understand that the breakdown of glucose to pyruvate produces lots of precursors for cellular macromolecules (Figure 4). In rapidly dividing tissue, these are in much shorter supply than energy. Secondly, there is now evidence that lactate can act as a signalling molecule, serving to promote malignant properties: for example, angiogenesis.

Why is this relevant?

So, biochemistry lesson over! But why is this relevant? Because misunderstanding of the clinical relevance of the “Warburg effect” has led to almost 100 years of people trying to cure or prevent cancer by feeding a diet depleted of carbohydrates. This can include some extreme and potentially dangerous diets (for example, those that can promote ketosis). The central feature of misunderstanding here is that providing the animal is not diabetic and has a functional liver, normoglycaemia is maintained regardless of what is fed. If there is no glucose in the diet, then the liver will create some through gluconeogenesis, catabolising the body’s tissues to do so.

Providing the animal is not diabetic and has a functional liver, normoglycaemia is maintained regardless of what is fed

A study in dogs with lymphoma compared a group fed a high-carbohydrate diet to a group fed a high-fat (low-carb) diet while undergoing the same chemotherapy treatment (Ogilvie et al., 1993). It concluded that there is no difference in remission duration, survival or cancer burdens between groups. There are many, many such studies in human medicine too.

So, the bottom line is that cancer’s exposure to glucose is largely unchanged, regardless of what the patient is fed. It is certainly true that the natural diet for cats in the wild is a low-carbohydrate diet (a rat carcass, for example, is only 2 percent carbohydrates), and we find the higher-quality feline diets usually have a low carbohydrate content. But we shouldn’t seek to push this carbohydrate level lower in our feline friends, or else we risk a lack of dietary balance and exacerbating weight loss. 

What about raw feeding?

Raw feeding is a practice that, I might suppose, few of us will promote. In the case of cats, there is some basis for saying that raw feeding is more natural for a pet that would otherwise gain food through hunting. But the term “natural” is not always synonymous with “better”. And in dogs, there is very little support to say that raw food is natural – just look at street dogs in developing countries as proof of this. Dogs are scavengers and have adapted to eat whatever is available (rarely prime cuts of raw meat!). There are further claims that raw feeding is more palatable and digestible and can improve immune system function and prevent or treat many diseases, including cancer. These claims are unproven.

The term “natural” is not always synonymous with “better”

Most veterinary nutritionists will tell us that raw feeding is risky for a number of reasons, and here are a few:

• The majority of home-made raw diets have major imbalances in nutritional content (Dillitzer et al., 2011). These imbalances include both toxic and insufficient levels of vitamin D, as well as imbalances in calcium and phosphate and often undetectable levels of vitamins A and E
• Commercially made raw diets may claim to be balanced in terms of the ingredients, but few manufacturers conduct AAFCO digestibility or feeding trials on the finished product. The amount of quality control procedures is variable but is, in general, much lower than those of cooked AAFCO-conforming brands
• The freezing or pressure-washing procedures often applied to raw foods do not kill all the necessary pathogens. Salmonella spp. have been found in 20 to 48 percent of commercially prepared raw foods and over 80 percent of home-made raw foods. There is an extremely long list of other pathogens isolated from raw food, including Toxoplasma spp., Mycobacteria spp., Clostridia spp. and Echinococcus multilocularis

Final thoughts

In summary, when advising owners on nutritional choices for cancer patients that are maintaining a stable bodyweight, have no significant comorbidities to address and are otherwise eating well, there are crucial things to consider. These are:

• The best diet to feed a pet with cancer is one they like! Feeding palatable foods will increase intake, not only minimising the risk of cachexia but improving quality of life
• The diet should be nutritionally complete and balanced, which typically means feeding a commercial diet, but if owners prefer to home cook, a veterinary nutritionist can advise them on formulating a correctly balanced diet
• Please avoid raw food for reasons of infectious disease, lack of dietary balance and (often) lack of quality control