A diet high in fat has shown to provide the most benefit to canine cancer patients, because cancer cells have difficulty using fats for energy, and also because cancer cachexia, or weight loss despite the amount of calories consumed, is caused by depleted body fat stores and can be very hard to reverse once it develops.  The type of fats consumed though, do matter significantly.  In the research conducted at Colorado State University, Dr. Ogilvie and his team found that Omega 3 Essential Fatty Acids (EFA’s), specifically in the form of fish oil, “may prevent the development of carcinogen-induced tumors, the growth of solid tumors, as well as the occurrence of cachexia and metastatic disease in experimental tumor models. Fatty acids of the n-3 series have [also] been shown to normalize elevated blood lactic acid and insulin levels in non-malignant conditions.”  In contrast, Omega-6 fatty acids, “appear to enhance tumor development and metastases.” (Nutrition and Cancer: Frontiers for Cure!)

Look for high quality fish oils or EPA/DHA supplements that are free of mercury or other toxins, are fresh, and have a good ratio of EPA to DHA (with a high level of DHA). 

Please Note: Stay away from Cod Liver Oil for this purpose, as Cod Liver Oil has a high level of Vitamin A, which can be toxic in high doses, and always store your fish oil supplements in a cool, dark place.  Do not add before cooking, as heat rapidly destroys the benefits of Omega 3 fatty acids.  Also, fish oil supplements can have a blood thinning effect, so be sure to tell your vet about any supplements you are giving your dog, especially if your pup will be having surgery.