Thanks to years of toxic chemical pollution and runoff into our waterways, most freshwater fish and many varieties of ocean fish have become contaminated with persistent organic pollutants (POPs), a class of chemicals that never degrade in the environment, such as the pesticide DDT and the now-banned flame retardants PCBs. Those toxic pollutants may cause neurological damage in the region of the brain that controls mood and appetite, damage that in animal studies has led rats to develop a subconscious drive to self-medicate with high-fat foods. The most problematic POPs have now been banned in the U.S., but they're still used in other countries. The easiest way to avoid them, if you love seafood, is to consume fish low on the food chain; these chemicals build up in fat and exist in higher amounts in large predatory fish like swordfish and shark. Stick with tasty sardines, which are omega-3 powerhouses owing to the fact that they survive on sea vegetables and algae.
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