Mercury is a highly toxic heavy metal; it is the second most toxic element in the world. There is no safe level of mercury for humans.
Mercury comes in the following three forms:
- elemental
- methylmercury
- inorganic complexes.
The elemental form is found in dental amalgams (fillings), thermometers, and gas meters. The methylmercury form is found in fish.
Mercury in Fish
Elemental mercury naturally occurs in lakes and streams, but emissions from industrial and mining process and the burning of fossil fuels concentrate elemental mercury in the environment. Once in the environment, microorganisms within the lakes and streams convert the elemental mercury to the methylmercury form.
Methylmercury travels from the ocean to the dinner plate, within seafood, through the following process:
- Methylmercury is taken up by bacteria and plankton
- Small fish consume the mercury by eating the plankton and bacteria
- Larger predatory fish eat the small fish. The larger fish accumulate large amounts of methylmercury in their tissues
- Humans and animals catch the larger fish. This exposes them to large amounts of methylmercury.
Methylmercury binds tightly to the protein in fish tissue and is concentrated in large fish, due to being high up in the food chain. Fish such as shark, swordfish, tilefish, and king mackerel are high risk fish. These fish are included in the document “What You Need to Know about Mercury in Fish and Shellfish” produced by the U.S. Department of Health and Human Services and the U.S. Environmental Protection Agency.
Effect of Mercury Toxicity
Mercury from fish (methylmercury) has an absorption rate of 95%, whereas, elemental mercury is poorly absorbed through the digestive tract but is efficiently inhaled (80%). Both methylmercury and elemental mercury, once absorbed, can cross the blood-brain and placental barriers.
Over time mercury accumulates and is slowly converted to inorganic mercury. As inorganic mercury, it binds to sulphur containing molecules such as haemoglobin in the blood and the powerful antioxidant glutathione, depleting their levels. The increased danger associated with methylmercury is that it can also have a direct effect on cells without being converted into the inorganic mercury form.
Methylmercury accumulates mainly in the kidneys, liver, and brain. Elemental mercury may accumulate in the brain, lungs, fatty tissues, kidney, liver, and digestive tract.
Both forms of mercury affect the following systems:
- Nervous system – mercury can affect brain neuron degeneration
- Cardiovascular system
- Immune system
- Reproductive system.
Fetal development is the most dangerous time for mercury toxicity.
Mercury Testing
Hair testing will provide reliable information on the level of mercury from seafood sources; however, it is not as reliable for elemental mercury. A urine test is required for elemental mercury. This involves providing a urine sample then ingesting a chelating (binding) agent, which removes the mercury from the tissues, and then following up with another urine test. This measures the level of mercury before and after chelation. The urine test can also provide assurances of an accurate hair test for methylmercury.
Mercury is highly toxic and extremely damaging to cellular and tissue function. It is important everyone is aware of the fish most at risk of mercury toxicity, especially pregnant women.
References:
Crinnion, W. J. (2000). Environmental medicine, part three: long-term effects of chronic low-dose mercury exposure. Altern Med Rev, 5(3), 209-223.
Fleisher, M. A. (2001). Mercury Detoxification. Townsend Letter for Doctors & Patients, May 2001.
The National Academy of Sciences (Ed.). (2000). Toxicological Effects of Methylmercury. USA: The National Academy of Sciences. From nap.edu
World Health Organization. (2000). Mercury from euro.who.int
Melissa Peterson - Melissa Peterson is a naturopath specializing in nutrition education. Melissa has been writing and editing technical and patient-friendly ...
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