Nutritionists recommend eating fish regularly, twice a week, because it is an excellent source of proteins, vitamins, and omega-3 fats. These omega-3 fats are nicknamed the ‘healing fats’ because they are pivotal in preventing heart disease, certain forms of cancer, and other illnesses. But for the last decade, people have been worrying about eating too much fish because of possible contamination by methylmercury.
Because of its mobility, mercury is named after the messenger of the Gods in Roman mythology and the symbol Hg is derived from the Latin “hydrargyrum,” meaning liquid silver. Metallic mercury is not regarded as a serious contaminant in water because, like most metals, it is almost completely insoluble in water. In fact, the practice of storing metallic mercury under water was widely recommended to prevent release of toxic mercury vapor. Even soluble inorganic salts of mercury, in trace concentrations, were not considered hazardous to fish. How, then, does mercury accumulate in fish? When naturally occurring or industrial mercury is deposited in lakes and waterways, bacteria convert it to methylmercury, which then contaminates the food chain and builds up in tissues of fish and of wildlife that eat the fish. Methylation, which chemically bonds a carbon atom to the mercury atom, creates an organic compound that can move more readily through biological systems, and is more easily absorbed from the gastrointestinal tract. When methylmercury concentrates in the blood, it can affect the brain and the central nervous system by binding to the S-H groups of proteins, resulting in numbness, unsteadiness, tiredness, ringing in the ears, and problems with vision, hearing and speech. Moreover, eating fish to reduce the risk of having heart disease might not be such a good idea after all: recent studies have indicated that there could be a link between a high dietary intake of mercury from fish and coronary heart disease.
So, is eating fish good or bad? Let’s not forget what the father of medicine, Paracelsus, said during the Renaissance: ‘Only the dose makes the poison.’ The risks of mercury contamination in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury in the fish and shellfish. Since methylmercury concentration increases as you move up the food chain, small pan fish such as perch, rock bass and crappie are generally low in methylmercury. Older predatory fish such as walleyes, swordfish, sharks contain higher levels of methylmercury.
Pregnant women, women who might become pregnant, and children are advised to avoid some types of fish and only eat fish and shellfish that are lower in mercury. Methlymercury accumulates in the blood stream over time and is removed by the body naturally, but it may take over a year for the levels to drop significantly. Thus, it may be present in a woman even before she becomes pregnant, and affect the baby’s nervous system. This is why women who are trying to become pregnant should also avoid eating certain types of fish.
The question now, is how to benefit from eating fish and shellfish without being poisoned by mercury? First, avoid large fish because they contain higher levels of methylmercury. Second, eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, Pollock and catfish. Another commonly eaten fish, albacore (‘white’) tuna, has more mercury than canned light tuna. So when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week. If you are not sure about the fish you’re eating, you can always check the FDA food safety website (www.cfsan.fda.gov/seafood1.html).
As long as you don’t overconsume, fish is an excellent part of the diet. Scientists may someday expand healthy choices by engineering omega-3 acids into chicken or beef. Omega-3-rich eggs are already on the market but they do not provide as much of these fats as fish do.