On a PetMD.com blog, Dr. Jennifer Coates discusses how nutrition impacts the immune system. No argument here, of course what you, your cat, your dog, or your pet elephant eats has multiple effects on the immune system and every other body system as well:
“Supplemental arginine (an amino acid) has been linked to increased T-cell immune function. T-cells direct and regulate the body’s immune responses and/or directly attack infections and cancers. Because arginine levels do not have to be disclosed on a pet food’s label, it is hard to determine exactly how much of the amino acid a particular diet contains. However, certain ingredients like flax seed, soybeans, chicken, salmon, and eggs all contain high levels of this important nutrient.”
Actually, all animal proteins contain abundant arginine. I still would not recommend feeding a cat or a dog any soy-containing products. At least 95% of soy in the U.S. is genetically modified, making it a very undesirable ingredient in pet food. Many dogs also have a hard time digesting soy. So stick with a species-appropriate, meat-based diet for your cat or dog…and don’t believe everything you read on the internet!