Mercury in Fish vs. Omega-3 Fatty Acids Health Benefits

By Tim Fitzpatrick

[Dec. 2005]


In spite of the health benefits provided by omega-3 fatty acids found in fish oils, people are decreasing fish in their diets due to high levels of mercury found in some fish. Elevated levels of Mercury, a toxin affecting cognitive development in unborn children, is being found in many species of fish raising health concerns from eating mercury laden fish. Unfortunately, fish oil also contains Omega-3 Fatty Acids, which are a known disease retardant. While it is important to reduce the amount of mercury consumed as a result of eating mercury laden fish, it is equally important to have fish in one's diet in order to get enough Omega-3 fatty acids.

Pregnant women in particular have been advised to lower their intake of fish containing high levels of mercury such as tuna and swordfish, replacing them with low mercury species, salmon and shrimp. These mercury warnings, meant primarily for women of child-bearing age, have now had an impact on total fish consumption in the US. Researchers warn in a November 2005 study in the American Journal of Preventive Medicine that the effects of a lowered Omega-3 Fatty Acid diet, due to mercury concerns in fish, could lead to a higher risk of heart disease and stroke.

Mercury occurs naturally in the environment

Mercury is a metal found in various forms. Volcanoes, undersea vents, forest fires, and weathering of rocks all release it. It has been identified on the sun as well as in meteorites and moon rocks. Since mercury is an element, it is not biodegradable. Once it enters the environment, it stays there. There are three forms of mercury that exist today. The silver mercury found in thermometers, barometers, batteries, and dental fillings is metallic mercury. It is also used in gold mining and the production of chlorine gas. It stays liquid at normal temperatures but evaporates when exposed to atmosphere. From there, it can return to the soil and water through precipitation. Mercury combined with non-carbon elements such as chlorine, sulfur, and oxygen is called inorganic mercurials. This type of mercury is used in some skin-lightening creams, topical antiseptics, disinfectants, and antibacterials. Lastly, mercury that combines with carbon is organic mercury. It is the organic form, methylmercury, which has led to the fish consumption warnings.

Humans have artificially raised the levels of mercury

Mining and industrial applications of mercury have increased significantly since the Industrial Revolution. Due to its ability to conduct electricity, measure pressure, act as a biocide, preservative, and disinfectant, mercury has become a commodity. During coal combustion, two forms of mercury are released: oxidized (ionic) mercury and elemental mercury. Ionic mercury dissolves in water readily while elemental mercury has low solubility. Industrial activities, pulp and paper mills, chemical manufacturing, and wastewater treatment plants can all release mercury into the environment as well. As the mercury is emitted, it evaporates, and the wind disperses it. Mercury originally produced in one area can deposit in another country or region.

Methylmercury, a toxic collectors item

Methylmercury is a neurotoxin that may adversely affect the development of the brain and nervous system in children. According to the US EPA (United States Environmental Protection Agency), several instances of methylmercury poisoning have been documented. Symptoms included vision impairment, lack of motor coordination, and numbness. At high doses, seizures, severe neurological impairment, and death have all occurred. On an industrial scale, the "Minamata Disaster" brought to light how deadly organic mercury is. In the 1950's a Japanese chemical company was producing compounds derived from mercury. Although the company was successful, its ramifications to the environment were devastating. The company took its mercury waste and dumped it into Minamata Bay. As a result of this, the compound accumulated in fish, the staple of most Japanese diets. Subsequently, 3,000 victims were diagnosed with "Minamata Disease" or methylmercury poisoning. Many people lost their lives, suffered physical deformities, and had to live with the emotional pain of this disease.

How does methylmercury get into the fish?

Mercury is found in all fish. Bacteria convert elemental mercury into methylmercury through a process called biomethylation. Fish absorb this methylmercury through their gills and as they feed on aquatic organisms. The toxin biomagnifies as it moves up the food chain, and a little methylmercury can go a long way in an ecosystem. As larger fish eats little fish and as larger fish live longer, the problem exacerbates. Since methylmercury binds tightly to muscle tissue, it is in a fixed position. Cooking does not remove any appreciable amounts of the toxin. The further up you travel in the food chain, the higher the concentrations will be. Therefore, large predators (tuna, shark) will have higher levels than small predators.

Since all fish have mercury in them, what is a safe level?

The FDA has stated that a level of 1 ppm (part per million) is the threshold level for safe human consumption. Shark, swordfish, and certain species of large tuna sometimes have levels in this area. In March of 2004, the FDA and EPA jointly issued this advice to pregnant women and women who could become pregnant:

  • Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels or mercury.
  • Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury: shrimp, canned light tuna, salmon, Pollack, 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.
  • Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week.

Omega-3 Fatty Acids, Fish, and Disease

Nutritionally, fish are an important source of omega-3 fatty acids. Also referred to as fish oils, they help protect against heart disease and stroke. Since humans are unable to synthesize these fatty acids, they must be obtained through diet. Yes, they are fats, but contrary to popular belief, not all fats are damaging to health. In fact, the omega-three fatty acids are protective. In addition to protecting against heart disease and stroke, essential fatty acids are necessary for the formation of healthy cell membranes, proper development and functioning of the brain and nervous system, and for the production of hormone-like substances called eicosanoids. According to the study, consumers who completely avoid or decrease their intake of fish may be exposing themselves to higher risk of heart disease and stroke. In the 1970's scientists conducted studies on the Inuit people of Greenland and found far less incidences of heart disease, stroke, arthritis, and other diseases. The reason, they hypothesized, was their high fat diet of whale, seal, and salmon, all containing omega-3's.

Some Quick and Painless Chemistry

Key omega-3 acids include eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA). These are the two that are found primarily in cold water, oily fish, such as salmon, tuna, and mackerel. There is a third type of Omega-3 that is found in plants and that is called alpha-linolenic acid (ALA). This type is found in dark green, leafy vegetables and flaxseed oil. Fatty acids are organic acids with long strands of carbon atoms bonded together. The body takes ALA and converts it into DHA, which is then converted to EPA. All of these belong to a group called polyunsaturated fats. A double bond between two carbons and a few less hydrogen molecules is all it takes to make them polyunsaturated. These are primarily your oils (olive, vegetable, fish). Their cousins, the saturated fats, all have single bonds, no double bonds, and wherever there is space, there is a hydrogen molecule. In other words, the fat is "saturated" with hydrogen, making it a solid (butter and lard) instead of a liquid.

Women of Child Bearing Age Should Have the Salmon

The study looked at what would happen if people, namely women who are pregnant or about to become pregnant modified their seafood intake. The effects investigated were prenatal cognitive development. The "ideal" action, scenario one, had them eliminate consuming fish with "high" or "medium" methyl mercury concentrations and replacing them with lower concentration fish. According to the US EPA, seafood considered to be low in methylmercury includes shrimp, canned light tuna, salmon, pollock, and catfish. This scenario is considered optimistic because it still allows these women a source to omega-3 fatty acids while minimizing the risk to methylmercury. The resulting IQ score improvements were substantial. In scenario two, all women of childbearing age reduced their total fish consumption by 17%. This action reduces the risk to methylmercury but at the same time, also reduces fatty acid intake. IQ improvements remained positive but the gains were 75% less than scenario one.

A Decrease in Fish Consumption

In scenario three, all members of the population reduce their consumption of fish by 17%. The IQ results were again 75% less than in scenario one. However, the elderly were at elevated risks for heart disease and stroke due to the reduced omega-3 fatty acid intake. In particular, men, aged 75 to 84, showed heart disease mortality risk increases by approximately 2 in 10,000.

An Increase in Fish consumption

Scenario four assumes that fish consumption increases among all adults with the exception of women of child-bearing age. Heart disease and stroke both decreased due to the higher diet of fish with the effect being the greatest on elderly men. For men aged 75-84, annual heart disease mortality decreased by 5 in 10,000.



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