Methylmercury: Why pregnant and breastfeeding women should avoid certain types of fish

Inorganic mercury is widespread in nature. It is released, for example, during volcanic eruptions, forest fires or the burning of coal. Bacteria found in the oceans and inland waters absorb the metal from their environment and convert it into methylmercury. In this form, the mercury is also easily absorbed by other living organisms. It accumulates particularly in marine animals and the marine food chain. At each stage of the food chain – from phytoplankton through zooplankton to fish, marine mammals or birds – the methylmercury content in the respective organism increases. When fish and seafood are consumed, methylmercury also enters the human body.

Methylmercury enters the human body primarily through the consumption of fish and seafood and is readily absorbed into the blood via the gut. Once absorbed, it is distributed throughout the body, with the highest concentrations being found in the liver, kidneys and brain. Methylmercury has also been detected in human cord blood, the placenta and breast milk.

During human metabolism, methylmercury is partially converted into inorganic mercury. Following exposureClose dialogexposureExposureTo glossary to methylmercury, therefore, both methylmercury and inorganic mercury are present in tissues and excretions. Methylmercury has an approximate half-life of 80 days for excretion from the human body. It is mainly excreted via faeces, urine and hair.

Following ingestion, methylmercury is distributed throughout the body via the bloodstream from the gastrointestinal tract. Numerous epidemiological studies have shown that the developing nervous system is particularly sensitive to the effects of methylmercury. It is therefore assumed that unborn children and infants are particularly vulnerable to health impairments caused by the ingestion of methylmercury.

Prolonged high exposure to methylmercury can cause nerve damage, resulting, for example, in impaired motor function, sensory perception and muscle weakness. However, such levels of methylmercury are not usually ingested through food.

The European Food Safety Authority (EFSAshort forEuropean Food Safety Authority) has established External Link:a tolerable weekly intake (TWI) for External Link:methylmercury of 1.3 micrograms (μg) of methylmercury per kilogram of body weight per week. The TWI specifies the quantity of a substance that can be consumed orally on a weekly basis over an entire lifetime without an appreciable health risk. The TWI is used to assess the health risk associated with chronic exposure.

After being absorbed through the gastrointestinal tract, methylmercury is distributed throughout the body via the bloodstream. In pregnant women, it passes through the placenta into the developing child’s body. In breastfeeding women, it can also be excreted into breast milk. As methylmercury is neurotoxic and can impair the development of the nervous system, the BfRshort forGerman Federal Institute for Risk Assessment also recommends that especially pregnant and breastfeeding women should limit their intake of methylmercury. Further information can be found in the statements “External Link:Fish consumption during pregnancy and breastfeeding: Some fish species have high levels of methylmercury” and External Link:“Methylmercury in fish and seafood – health assessment of new data from the BfR-MEAL study”, as well as in the External Link:consumer advice for pregnant and breastfeeding women.

People mainly ingest methylmercury by eating fish. Therefore, the amount ingested depends on how often fish is eaten and how much methylmercury it contains.

For the population in Germany, intake was assessed based on concentration data from the BfRshort forGerman Federal Institute for Risk Assessment-MEAL study. Specifically, methylmercury concentrations were measured in ready-to-eat fish and seafood, such as salmon or fish fingers. The concentration data was then linked to information on the consumption of these products, i.e. how often and how much fish and seafood or products made from them are eaten. This provides exposure estimates for the general population across all age groups. These estimates can then be incorporated into the health assessment of methylmercury intake.

In the BfRshort forGerman Federal Institute for Risk Assessment-MEAL study, the highest levels of methylmercury were found in smoked tuna, spiny dogfish and redfish. Although Alaska pollock had lower levels of this undesirable substance, it contributes most to methylmercury intake in the population because it is consumed in such large quantities. It is frequently found in fish fingers and many other frozen fish meals. The analysis showed that the average intake of methylmercury across all age groups is below the health-based guidance value of 1.3 µgshort formicrogram per kilogram of body weight per week. At exposure levels below this value, no adverse health effects are expected.

The highest weekly intake of methylmercury was found among adolescents and young adults aged between 14 and 25. As for all other age groups considered, the average methylmercury intake for this group is also below the health-based guidance value. However, the weekly intake of methylmercury for the high-exposed consumers (95th percentile) in this age group is at or above the health-based guidance value. Within this age group, especially people who eat tuna consume high levels of methylmercury.

People are mainly exposed to methylmercury through the consumption of fish. The amount of this undesirable substance found in fish depends on various factors inculding the species, size and age of the fish. Generally, the highest levels of methylmercury are found in large and older predatory fish that are high up in the food chain, such as sharks and tuna.

When considering fish consumption and health, it is important to note that fish and seafood are a valuable source of important nutrients. These include vitamins and trace elements, proteins, long-chain omega-3 fatty acids such as eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA), as well as vitamin D, iodine, selenium and vitamin B12. Studies indicate that regular fish consumption reduces the risk of cardiovascular disease and lipid metabolism disorders. These health benefits should be taken into account when considering reducing fish consumption and thus lowering methylmercury intake.

Various scientific studies have attempted to weigh up the benefits and risks of fish consumption. InExternal Link: 2015,External Link: for example,External Link: the European Food Safety Authority (EFSA) concluded that limiting the consumption of fish species with high methylmercury levels was the most effective way to achieve the health benefits of fish whilst minimising the risks posed by excessive exposure to methylmercury. This applies in particular to infants, children and women of childbearing age. Accordingly, the German Nutrition Society (DGE) recommends consumingExternal Link: 1 to 2 fish meals per week (in German only) due to the health benefits. However, pregnant and breastfeeding women in particular should avoid certain fish species with high levels of methylmercury.

The BfRshort forGerman Federal Institute for Risk Assessment also recommends that pregnant and breastfeeding women limit their consumption of fish with high levels of methylmercury, such as shark, swordfish, halibut or tuna. The recommendation can be found in the External Link:Federal Ministry’s consumer advice on health and food safety.