Although fish is an extremely healthy, low calorie protein source that contains nutritious omega-3 fatty acids, it is possible to eat too much of it. The first thing to consider is that since healthy eating means a balanced, portion-controlled diet, eating too much fish or any other food is not recommended. Beyond that, high mercury levels are the main reason for not consuming large amounts of fish.
Many health experts warn that eating too much fish can create a toxin build up of mercury in the body. Mercury is a metallic element that is toxic in high doses. It gets into the water supply from traces of medical waste or power plants. Fish swimming in that water then become contaminated with mercury.
Fish can be a good part of a balanced diet. Avoiding fish is thought to place individuals at risk for brain, nerve, and heart problems. Most health experts recommend avoiding too much fish, due to the mercury risk, but to consume it weekly in limited amounts. The recommended amount of most fish and shellfish per adult per week is a maximum of 12 ounces (340 g). Larger fish such as tuna, shark, golden bass, swordfish, and king mackerel should be limited to 6 ounces (170 g) weekly at the most.
Pregnant women, nursing mothers, and young children should avoid larger fish varieties. Instead, they should eat other types of fish and seafood in small quantities. Toxic levels of mercury are said to be found mostly in the larger species of fish. Babies, young children, and women of child-bearing age are especially susceptible to the possible harmful effects of mercury. If a person from a lower risk group is going to eat tuna, it's generally considered best to choose chunk light, water pack types rather than albacore or oil packed versions.
Mercury poisoning, possibly from sources such as too much fish, may cause birth defects or brain damage. Symptoms of mercury poisoning in adults or children include pink or red cheeks and fingertips, as well as loss of hair, teeth, and nails. Skin rashes and organ damage are also thought to be caused by high mercury levels such as those from too much fish. Vision, speech, and hearing disorders have also been linked to mercury poisoning.
Another problem that may be caused by eating too much fish involves the ingestion of antibiotics in the fish. Antibiotics are medicines given to fish to control bacterial diseases. While wild fish may still absorb antibiotics from toxic waste in the water, traces of the drugs are most likely to be found in farmed fish. Further complicating the problem is the lack of antibiotic dosing regulations in the fish farming industry in some parts of the world. Fish farming involves raising food fishes in a limited area; antibiotics are used to keep bacteria from spreading throughout the stock.