If you haven't already heard this phrase, you are probably wondering exactly what it means. Cancer does not just affect the part of the body where the tumor exists. It has far-reaching effects on the animal's entire system. For cancer to thrive, it has to have a source of energy or fuel. To do this, the cancer disrupts the metabolic pathways in the body and robs the animal of the ability to extract adequate nutrition from the food he/she eats. (Of or relating to metabolism. Metabolism is defined as: the organic processes (in a cell or organism) that are necessary for life)
What all this amounts to is that the animal's body only gets the leftover energy, while the cancer itself thrives. Although your dog may be eating normally, he/she is in effect, starving. This state of accelerated starvation is called cancer cachexia. (General physical wasting and malnutrition usually associated with chronic disease)
Tumor cells rely heavily upon carbohydrates for their energy and rob the body of amino acids. On the other hand, tumor cells cannot utilize lipids (fats) for energy while the rest of the body can. Therefore it is possible that diets with increased fat content may slow tumor growth, allowing the patient to fight against the tumor. Protein content must be maintained a levels sufficient for tissue repair, but carbohydrates should be held to a minimum.
It is very important for a dog with cancer to be eating a specific diet design to support the patient. The result of not eating a correct diet is decreased quality of life, shortened survival time, and decreased response to cancer treatment.
Although many experts agree with the adage "Feed the dog, starve the tumor", there are many different opinions on the specifics of a cancer diet. Once again, you will have to use your best judgement based on what foods your dog will eat.