Micronutrients: Mercury

Micronutrients: Mercury
GETTYIMAGES.COM/bhofack2

Not all metals are essential micronutrients. Mercury, notably, does not play a role in promoting human health. Rather, it is considered the most toxic heavy metal in the environment and, according to the World Health Organization, one of the top 10 chemicals of public health concern.

Mercury exists in three forms: organic (e.g. methylmercury in fish), inorganic (e.g. batteries and disinfectants) and elemental (e.g. dental amalgam and thermometers). All types of mercury accumulate in the body over time.

Found naturally in the environment, soil and as a byproduct of pollution in the air, mercury is transformed by bacteria into the harmful organic compound methylmercury. In the United States, more than 3,000 lakes have been closed to fishing due to mercury contamination.

Seafood recommendations and safety
Fish and shellfish contain protein, omega-3 fatty acids and other essential nutrients. The 2015-2020 Dietary Guidelines for Americans recommend adults eat a variety of protein foods, including two servings of seafood per week. Unfortunately, consuming seafood is the most common way people in the U.S. are exposed to mercury.

While small amounts of mercury from seafood do not pose serious health concerns for most people, pregnant women and young children are more susceptible to potential neurological development issues. Pregnant and lactating women should eat eight to 12 ounces of low-mercury seafood per week, and children should be served smaller portions one to two times per week based on their age and calorie needs.

Nearly all fish and shellfish absorb and accumulate methylmercury. Larger, longliving fish such as swordfish and bigeye tuna accumulate the highest levels and pose the greatest risk.

Mercury levels range from 0.003 parts per million in scallops to 1.123 parts per million in tilefish from the Gulf of Mexico. Methylmercury in fish is 95 to 100 percent absorbed in the intestines. Centers for Disease Control and Prevention data shows most people have blood mercury levels below 10 micrograms/liter; levels under 20 micrograms/liter are normal and not associated with negative health effects.

Level Mercury per serving Examples
Best Choices ≤0.15mcg/g cod, flounder, haddock, scallop, tilapia, shrimp, canned light tuna, salmon, catfish
Good Choices 0.15mcg/g up to 0.46 mcg/g yellowfin and white albacore tuna, Chilean sea bass, grouper, halibut, Mahi Mahi, monkfish, snapper
Choices to Avoid >0.46 mcg/g shark, swordfish, king mackerel, marlin, orange roughy, bigeye tuna

The U.S. Environmental Protection Agency, or EPA, recommends eating low-mercury fish and limiting higher-mercury fish. Limit locally caught fish to one serving per week, if there is no advisory.

The EPA and U.S. Food and Drug Administration categorize fish in three levels for women of childbearing age (16 to 49), especially those who are pregnant or breast-feeding, and children older than 6 months. Adults should eat two or three servings per week of “Best Choices” or one serving per week of “Good Choices.”

Mercury poisoning
Mercury poisoning takes months or years to develop with excess exposure. Skin exposure, inhalation or ingestion of mercury can have harmful effects on the nervous, digestive and immune systems, lungs, kidneys, brain, heart and eyes.

Toxicity depends on dose, exposure, frequency and health of the person. Symptoms vary by type of mercury and may include tremors, insomnia, memory loss, neuromuscular effects, headaches, and cognitive and motor dysfunction.

There are no consensus criteria for the diagnosis of mercury overload. Treatment includes removal of the source and, in some cases, chelation therapy, which binds toxins in the bloodstream.

Populations affected
Pregnant and breast-feeding women, infants and young children are most susceptible to the effects of mercury. Of greatest concern is how methylmercury crosses the blood-brain barrier and placenta. High levels of regular exposure can result in serious neurological effects to a fetus and may lead to mental retardation in children. There may be an association between elevated methylmercury and risk of cardiovascular disease in adults, but evidence is mixed.

Bottom line
Evidence shows the benefits of consuming low-mercury fish outweigh the risks of mercury overload. Young children and women who are of childbearing age, pregnant or breast-feeding should consume the recommended weekly servings of low-mercury fish and avoid eating high-mercury fish.

References

Alina M, Azrina A, Mohd Yunus AS, et al. Heavy metals (mercury, arsenic, cadmium, plumbum) in selected marine fish and shellfish along the Straits of Malacca. Int Food Res J. 2012;19(1):135–140.
Bernhoft R. Mercury Toxicity and Treatment: A Review of the Literature. J Environ Public Health. 2012;2012:1-10.
Budtz-Jørgensen E, Grandjean P, Weihe P. Separation of risks and benefits of seafood intake. Environ Health Perspect. 2007;115(3):323-327.
Choi AL, Cordier S, Weihe P, Grandjean P. Negative confounding in the evaluation of toxicity: the case of methylmercury in fish and seafood. Crit Rev Toxicol. 2008;38(10):877-893.
Davidson PW, Strain JJ, Myers GJ, et al. Neurodevelopmental effects of maternal nutritional status and exposure to methylmercury from eating fish during pregnancy. Neurotoxicology. 2008;29(5):767-775.
Eating Fish: What Pregnant Women and Parents Should Know. U.S. Food and Drug Administration website. Updated March 29, 2019. Accessed April 16, 2019.
EPA-FDA Fish Advice: Technical Information. U.S. Environmental Protection Agency website. Accessed April 16, 2019.
Health Effects of Exposures to Mercury. U.S. Environmental Protection Agency website. Accessed April 16, 2019.
How People are Exposed to Mercury. U.S. Environmental Protection Agency website. Accessed April 16, 2019.
Jaishankar M, Tseten T, Anbalagan N, Mathew BB, Beeregowda KN. Toxicity, mechanism and health effects of some heavy metals. Interdiscip Toxicol. 2014;7(2):60-72.
Karagas MR, Choi AL, Oken E, et al. Evidence on the human health effects of low-level methylmercury exposure. Environ Health Perspect. 2012;120(6):799-806.
Mercury and health. World Health Organization website. Published March 31, 2017. Accessed February 13, 2019.
Mercury Levels in Commercial Fish and Shellfish (1990-2012). U.S. Food and Drug Administration website. Updated October 25, 2017. Accessed April 16, 2019.
Mercury Poisoning Linked to Skin Products. U.S. Food and Drug Administration website. Updated March, 27, 2018. Accessed April 16, 2019.
Oken E, Wright RO, Kleinman KP, et al. Maternal fish consumption, hair mercury, and infant cognition in a U.S. cohort. Environ Health Perspect. 2005;113(10):1376-1380.
Omega-3 Fish Oil and Pregnancy. American Pregnancy Association website. Updated September 2, 2016. Accessed April 16, 2019.
Procter SB, Campbell CG. Position of the Academy of Nutrition and Dietetics: Nutrition and lifestyle for a healthy pregnancy outcome. J Acad Nutr Diet. 2014;114(7):1099-1103.
Questions & Answers from the FDA/EPA Advice on What Pregnant Women and Parents Should Know about Eating Fish. U.S. Food and Drug Administration website. Updated March 27, 2019. Accessed April 16, 2019.
Sagiv SK, Thurston SW, Bellinger DC, Amarasiriwardena C, Korrick SA. Prenatal exposure to mercury and fish consumption during pregnancy and attention-deficit/hyperactivity disorder-related behavior in children. Arch Pediatr Adolesc Med. 2012; 166: 1123–1131.
Strain JJ, Davidson PW, Bonham MP, et al. Associations of maternal long-chain polyunsaturated fatty acids, methyl mercury, and infant development in the Seychelles Child Development Nutrition Study. Neurotoxicology. 2008;29(5):776-782.
U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2015 – 2020 Dietary Guidelines for Americans. 8th Edition Published December 2015. Accessed April 16, 2019.
Virtanena J, Rissanen TH, Voutilainen S, Tuomainen TP. Mercury as a risk factor for cardiovascular diseases. J Nutr Biochem. 2007;18(2):75-85.
Ye BJ, Kim BG, Jeon MJ, et al. Evaluation of mercury exposure level, clinical diagnosis and treatment for mercury intoxication. Ann Occup Environ Med. 2016;28:5. 2016.

Kathleen Zelman
Kathleen Zelman, MPH, RDN, is the nutrition director of WebMD.