Key Factors For Fish Contamination and MeHg Exposure

The dieticians and the nutritionists recommended the fish in the diet. Despite the several health benefits associated with the consumption of fish, concerns of health risks arise, particularly in the tribal population due to their unique cultural practices or subsistence fishing practices. The significant risk associated with the consumption of organic fish is vulnerability to methyl mercury (MeHg). The exposure to MeHg may cause impairment in the functioning of the central nervous system or may even lead to pulmonary and nephrotic damage. Japan (Minamata and Nigata) and Iraq people have been documented with severe health effects of mercury due to high-level fish consumption. The irreversibility of the toxic effects of MeHg subsides the health benefits of eating fish. The only way to detect MeHg poisoning is by performing a blood test or hair mercury test. The difficulty in the diagnosis increases the complications.^[1]

The article on avoiding mercury in fish will address the following questions:

What could be the critical factors for exposure to methylmercury (MeHg) through fish consumption?

How can tribes, as well as the general population, minimize exposures and potential health risks from MeHg contaminated fish, while maintaining current dietary practices?

Methyl mercury (MeHg) bio-accumulates in the food chain of aquatic animals. The bio-accumulation leads to the highest levels of MeHg in the larger fish (such as swordfish, shark, king mackerel, and tilefish) that eat the small fishes. Due to this, MeHg levels of 1ppm or more can be found in certain types of fish only.^[1]

The natural environment and human activities have led to the existence of mercury in the atmosphere. The highest result of mercury emission due to human activities is coal combustion for energy production. The two forms of mercury, i.e., elemental and inorganic forms, are released into the atmosphere upon the burning of coal. The mercury is carried long in the air before paving a path into water bodies. In aquatic organisms, the inorganic form of mercury is transformed by microorganisms into a toxic organic form, i.e., MeHg, where it gets bioaccumulated in the fish through the aquatic food chain.^[2]

The gastrointestinal tract can absorb more than 95% of the MeHg consumed through fish. After transportation into the bloodstream, the toxic organic form gets distributed within 30 to 40 hours to all the organs. 10% of the methylated mercury is found in the brain. Later, MeHg starts demethylating slowly into inorganic mercury. The converted form has an inferior capacity to cross the blood-brain barrier. Therefore, the elimination of mercury from the body takes place in the demethylated inorganic way. The primary mode of excretion is liver bile and feces. Urine, sweat, and breast milk are the other forms through which elimination takes place. The mercury also tends to get stored in hair and nails.

The half-life of the MeHg in the blood is approximately 50-70 days in adults while it is longer in neonates. Research accounts for genetic variations for these differences. MeHg can cross the placenta, and the mercury levels found in umbilical cor blood are about 1.7 times higher than the mother’s blood levels. For this reason, the mother’s blood level should not exceed the mercury concentration of 3.5µg/L. It will keep the fetal blood mercury below 5.8µg/L, i.e., below the EPA reference level.^[2]

The clinical manifestations vary according to the degree and length of exposure. Also, the sensitivity for the MeHg varies individually due to unique genetic makeup. Also, people who eat high-mercury seafood such as swordfish or tuna very often are more favorable for mercury poisoning.^[2]

The clinical manifestation of exposure to MeHg observed during pregnancy is lethal. The poisoning can cause fetal death, severe congenital disabilities, mental retardation, long-term disabilities, and even blindness. Also, the low-dose prenatal MeHg exposure has been related to these neurological effects in children.^[4]

Blood analysis and hair sample testing are the initial laboratory tests for the diagnosis of MeHg poisoning due to fish and shellfish consumption. The level of mercury in hair identifies the long-term exposure of organic (methyl- or ethyl-) mercury. The combination of the following ranges of different samples confirms the MeHg exposure from seafood consumption:

Urine mercury test: <10 µg/L

Blood mercury test: >5 µg/L

Hair mercury test: >1 µg/g

Blood test: It reflects the recent exposures as well as chronic accumulation in the results. Patients are advised not to eat seafood for three days before a blood test for the diagnosis of mercury poisoning.^[2]

Hair test: At the average mercury level of 4.2 ppm in the hair sample, patients start showing functional neuropsychological deficits. The maternal hair levels of 1.2 ppm or higher may reflect the prenatal neurodevelopmental effects.^[2]

The extensive studies on fish contamination took place when the first MeHg derived poisoning due to fish consumption, named Minamata disease, recorded in Japan during the 1950s. The following clinical study describes the relationship between MeHg exposure level and fish consumption in the Japanese population.^[4]

Method: The exposure assessment model proposed during the study for establishing the relation of MeHg poisoning with fish consumption is drawn below.

The mean MeHg level (µg/g) found in the overall population of Japan from consumption of different fish species is tabulated below:

Results: The data corresponding to USEPA guidelines showed that 80% of the female population had hair mercury levels above 1µg/g. While as per Japanese guidelines, only 3% of women reported levels are exceeding 5µg/g. The above table proved the particular species of fish, i.e., bigeye tuna, sea bream, yellowfin tuna are the most significant contributors to MeHg poisoning.^[4]

The primary step for MeHg poisoned patients is to stop eating fish until the situation is under control. Else, they can shift to low-mercury fish. Once the blood mercury declines to <5 µg/L or symptoms get resolved, low-mercury fish and shellfish can be reintroduced

As fish is a good source of protein and low in saturated fat, avoiding consuming fish cannot be the solution. The healthy fish consumption choices should include the fish most moderate in contaminants and higher in omega-3 fatty acids, as mentioned in the below table. The advice of limiting mercury consumption is for especially pregnant women, breastfeeding mothers, and children less than 12 years old as the effects in these cases can be worse.^[2]

Fish which are a good source of omega-3 fatty acids

Cooking does not reduce the mercury levels as it gets accumulated in the fish muscle. The fish become more contaminated due to the accumulation of persistent organic pollutants like PCBs in the far. Exposures of mercury in fish can be reduced by peeling the skin and removing the fatty tissue before cooking.^[2]

Scientific study has revealed that Se and Hg can individually be toxic at high exposures, but in combination, they antagonize each other’s toxic effects. One such study stated that tuna containing ~1:1 molar ratio could decrease the toxic effects associated with MeHg. There are several theories proposed until now explaining the Se protective mechanism. One such comprehensive approach claims the formation of highly stable organic MeHg-selenocysteine (MeHg- SeCys) in the brain. The sequestration of Se by the high concentration of MeHg makes Se biologically unavailable. Initially, it may result in the inhibition of selenoenzyme antioxidant activities, which may show a few adverse effects. Later, redistribution of Se from somatic cells reduces the toxic effects. Ultimately, Se antagonized the harmful effects of MeHg.

Adults have more capacity to protect the brains from Hg-stress due to the molar surpluses of Se over Hg, i.e., Se: Hg molar ratio>1. While the rapid rate of cell division in the fetal and young population increases the requirement of a steady supply of Se, hence, these age groups are a higher risk of toxicity.^[3]

Safe fish intake can be estimated based on the consumer’s body weight and type of fish species. There are several online calculators available that help for accurate estimation of methyl mercury levels upon fish consumption. The application is designed in a way to guide the MeHg exposure levels below the EPA RfD. The online tools are suggested for the general guidance as the absolute index of MeHg content can not help in understanding the variability of the fish species.^[2]

GotMercury.org and the Natural Resources Defense Council are the primary sources that offer easy calculation for the estimation of safe intakes. iPhone application, i.e., Fish 4Health, is another option that can be used for healthy seafood choices. The app can tell you the mercury contamination levels of different kinds of seafood. Based on this, you can select the type as well as the quantity of fish. Also, it keeps a regular check on your daily mercury and associated omega-3 fatty acid intake levels. The application has an in-built feature for reminding the sufficient omega-3 fatty acids intake or exceeding of MeHg intake as per EPA recommendations. The app is safe to use, even by pregnant and nursing mothers.^[2]

The Environmental Defense Fund Seafood Selector provides guidance for the inclusion of specific fish in the number of meals of a month. It uses EPA RfD recommendations for suggesting the limit of mercury. The website can offer protection in a better way than FDA/EPA guidelines as it also considers persistent organic pollutants in the poisoning.

Sea Web Kid Safe Seafood gears the suggestions for MeHg and persistent organic pollutants poisoning, particularly for children.^[2]

MeHg exposures are highest from dietary fish consumption. The poisoning cases are about 3 to 10 times higher in tribes than the general population. Mercury poisoning is associated with potential health risks that may even produce lethal effects. Scientific studies have proved that ~50% of the MeHg dietary exposures can be reduced by replacing high MeHg concentration fish (such as walleye, bowfin) with lower concentration fish. Further, the exposure assessment tools can help in implementing actions for reducing dietary exposures.

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