Keep in mind, there is a significantly larger exposure to toxic chemicals in animal products compared to plant foods. By eating lower on the food chain and reducing our intake of animal products, one automatically reduces exposure to toxic chemicals. Plants have the least fat-soluble pollutants, animals that eat plants have more, and animals that eat animals have the hightest levels of these toxic compounds. Fish that eat smaller fish will store the toxic compounds from every fish it ever ate, including all the fish eaten by the fish it just made a meal of. It is important to avoid lobster, shellfish, catfish, and predator fish such as tuna, bluefish, striped bass, shark, and swordfish, where toxins such as PCB, DDT, dioxin, and mercury are likely to build up due to the compounding effects of eating lots of smaller fish. One gets larger doses of more toxic compounds from these contaminated animal products than would be possible to take in from produce.
This passage is from the chapter entitled "Understanding the Causes of Cancer and Other Illnesses" from Joel Fuhrman M.D.'s new book Disease-Proof Your Child.