Mercury

A lifetime of sharing your catch enriches both health and community. To sustain this culture, and to be confident when preparing meals for family and friends, it is vital to acknowledge the lurking presence of heavy metals, especially methylmercury.

The main takeaways from this article are:

(1)    Recognize the real health benefits of regular fish consumption. A diet of unprocessed fish twice a week significantly reduces risk of heart disease, stroke, macular degeneration, and dementia progression.

(2)    Be accountable to yourself and family. Be aware of slow changes in your own mood and behavior, and intermittently test your own blood mercury levels.

(3)    The greatest impact of mercury exposure is in pregnant mothers and children.

What it is:

Due to both human pollution and deep-sea hydrothermal vents, elemental mercury (Hg) accumulates in ocean and fresh water sources. Briefly revisiting high school chemistry class, recall that elemental mercury has 2 valence electrons. This makes it very reactive and avidly bind to dissolved organic matter – primarily in plankton – which is then consumed and bioaccumulated through the food chain. The interaction of elemental mercury with organic matter makes methylmercury (a.k.a. Me-Hg or organic mercury). When we eat the fish, we eat Me-Hg. After eating a seared tuna steak, it takes approximately 70 days for a human to excrete half of the ingested MeHg, and then another 70 days to excrete half of what’s left. For this reason, over many closely spaced tuna meals, we accumulate huge amounts MeHg very fast with little excretion.

Figure 1: The rewards

The fish with highest potential for mercury accumulation can be split into 3 broad categories:

Following a meal, MeHg enters your blood and then distributes to most tissues in the human body – into fat cells, bone, lungs, brain, hair, urine, and if you are pregnant, into the placenta and fetus. It is important to be aware of the following concept: The majority of Me-Hg collects in fat, this hides symptomatology, and makes it difficult to measure in blood or urine.

It is a good idea to take a blood mercury test for the sake of concrete data. A $100 lab test measures MeHg in blood, urine, or hair, providing a window into total MeHg accumulation. These tests are valuable in tracking short-term spikes in MeHg consumption. It is recommended by the CDC to keep blood mercury levels lower than 4-5ug/L, and urine mercury between 1-8ug/L.  

That being said - between individuals, symptom presentation does not correlate with blood or urine Hg levels. Often, these tests are an underestimation of total MeHg burden. Physicians who specialize in mercury poisoning commonly see neurologic symptoms begin to appear in their patients at levels as low as 10ug/L in blood or not until levels as high as 200ug/L.

In spearfishermen, symptoms of mercury poisoning rarely occur after eating a huge meal of Me-Hg in striped bass or bluefin. Rather, symptoms “pop up” in the offseason during a weight loss event. Burning fat releases swaths of stored MeHg into the blood – this can happen if you suddenly join a gym, contract virus or bacterial infection, are a victim of cancer, have an autoimmune crisis, or are pregnant. Far and away, the largest impact of mercury poisoning is in pregnancy and awareness is critical if planning a family.

Geography:

Harvard Medical School did a study in 2006 which showed mercury levels in different fish species (Table 1). A study by UC Santa Cruz and Taiwan University reviewed mercury levels in each of the international tuna schools (Table 2). Freshwater fish, especially, striped bass, pike, walleye, drum, muskellunge, and bass, also contain mercury and should be eaten with this in mind. Our California fresh and brackish water ways are especially contaminated due legacy industrial contamination and to heavy mining using mercury during the Gold Rush. The Great Lakes are also a particular hot spot due to a long industrial history on the lakeshore.

Lake Huron Walleye and Freshwater Drum

 Table 1 – Mercury in Select Fish Species

Table 2 – Mercury in International Tuna Schools

Prevalence and fatalities:

Globally, seafood consumption contributes 56% to the total MeHg exposure. In the U.S., a 2013 report documented 1300 mercury exposures, with 24 cases of moderate to major effects. The annual death from the fatal heart attack secondary to Me-Hg exposure is estimated to be over 10,000 in China and 29,000 globally. In our spearfishing community, there are more people affected than you would think. Unfortunately, in the fishing community the classic neurologic symptoms of mercury toxicity are frequently confused with the symptoms of chronic alcohol use.

Mechanism:

Me-Hg interrupts the scaffolding which supports the architecture of neurons in the brain and skin and muscles, causing neurons to degenerate all over your body. For the scientist, MeHg antagonizes GTP binding site of tubulin subunits, preventing polymerization of microtubules in neurons.

Presentation:

Symptoms appear after an individually unique threshold of MeHg has been surpassed in your tissues – often during a major weight loss event. Symptom onset is within 1-2 days and is frequently misinterpreted as the neurological symptoms of alcoholism. Commonly, adults report neurologic symptoms like tremors, difficulty with coordination (hand/foot/eye), slow reflexes, numbness, muscle weakness, issues with memory and concentration, mood changes, irritability, nausea, and vomiting. Impact on male fertility have also been reported, including decreased sperm count. Cardiovascular symptoms tend to develop later with very high exposures, these symptoms include chest pain and shortness of breath which culminate in heart attack. Once you stop eating contaminated fish, symptoms regress over the course of 2-3 months, however some tremor and cognitive affects are often permanent.

Fish and Mercury in Pregnancy:

The American College of Obstetrics and Gynecology (ACOOG) recommends:

1.     Eat 2-3 servings of fish or shellfish per week before getting pregnant, while pregnant, and while breastfeeding. A serving of fish is 4 ounces.

2.     Do not eat bigeye tuna, king mackerel, marlin, orange roughy, shark, swordfish, or tilefish. Limit white (albacore) tuna to only 6 oz a week. Also check advisories about fish caught in local waters.

Fish and Mercury in Children:

The American Academy of Pediatrics states:

1.     Children and women who are breastfeeding should eat 1-2 weekly servings of a variety of fish.

2.     Families who eat freshwater fish they catch locally should check advisories and limit servings to once a week if the body of water where they fish is not monitored. 

There is no safe level of mercury exposure for a fetus or child less than 5 years old. Blood cells in a fetus bind MeHg much tighter than adult red blood cells. For this reason, mercury levels in a fetus can be 4x the level in maternal blood. Additionally, mercury is passed at low levels through breast milk, at highest concentrations closest to pregnancy which then taper. Brain growth and myelination of neurons is still occurring during pregnancy and for years during infancy, and mercury will exert a devastating effect. There is decreased survivability of neonates exposed to high levels of MeHg in the womb. Children who survive, develop neurological defects, including microcephaly cerebral palsy, severe mental retardation, blindness, deafness, and other malformations.

Benefits of Fish in Prenatal Mothers:

With the prior warnings in mind, seafood consumption in children is beneficial for their cognitive development. The AAP has publicly stated that seafood consumption by children has declined every year since 2007 which is likely due to overprotective federal mercury advisories on fish consumption. A US-based study named Project Viva examined hundreds of pregnant mothers. They found that maternal prenatal fish consumption greater than 2 servings a week was associated with better cognition during infancy and early childhood. The highest cognitive test scores were seen among children of mothers who ate more fish with lower mercury levels. A follow up study 6-10 years later showed no harm in the same cohort (Oken et al., 2015).

Benefits of Fish in Heart Health:

The EPIC-Oxford study is one of the most extensive studies investigating ischemic heart disease and stoke with respect to diet type: carnivorous vs. pescatarian vs. vegetarian. This study was done in UK between 1993 and 2010. From a total of 48,188 total participants, there were 24,428 carnivores, 7,506 pescatarians, and 16,254 vegetarians. In an 18 year follow up there was a total of 2,820 cases of ischemic heart disease and 1,072 cases of stroke. After adjusting for sociodemographic and lifestyle variables, pescatarians and vegetarians had 13% and 22% lower rates of heart disease than carnivores respectively, but vegetarians had a 20% higher rate of total stoke than meat eaters and pescatarians(Tong et al., 2019).

It would be remiss to not mention the REDUCE-IT study published in 2018, in which 8,179 patients on statin therapy were given either a placebo mineral oil or icosapent ethyl (an ethyl derivative of the main “heart-healthy” omega-3 EPA found in fish). It showed a significant reduction in major adverse cardiac events like stroke and heart attack after this fish oil supplement(Bhatt et al., 2019)[1].

Benefits of Fish in Cognitive Stamina:

A study in 3,718 Chicago residents by Rush University Medical Center showed that individuals who consume 2+ fish meals per week experience cognitive decline 13% slower than those who consume fish less than weekly(Morris et al., 2005). A Norwegian study of 2,031 subjects. Between 70-74 years old showed that individuals whose mean daily intake of fish (especially non-processed fish) >10 grams/day had better mean cognitive test scores on tests that assess for dementia. They showed these benefits were dose dependent(Nurk et al., 2007).

Benefits of Fish in Macular Degeneration

A study of over 4,200 cases showed a significant dose-response relationship between fish intake and AMD risk in a 10-year duration. Antioxidant effects of polyunsaturated fatty acids (PUFAs) are thought to be the primary protective driver(Zhu et al., 2016).

Treatment:

Mercury poisoning is managed in two steps: (1) Remove mercury exposure – in other words stop eating fish. (2) Increase excretion of mercury, with thiol-based chelators. Chelators cause increased urinary excretion of mercury, however, there has never been a controlled trial evaluating chelators clinical benefits versus its side effects.

Chelation therapy is a century old technique to mobilize toxic metals from the body into the urine. For the bug hunters out there: “Chelation” has its origin in the Greek word “chele” that means claw of the lobster, thus depicting the concept of a chelating drug gripping a toxic Me-Hg molecule. The union of a chelator with the toxic metal not only leads to excretion, but also shielding of biologic targets.  

The common chelators used to treat organic mercury poisoning are: DMSA (meso-2, 3-dimercaptosucccinic acid, aka succimer), and more recently, acetylcysteine. Both DMSA and acetylcysteine remove mercury via the kidneys. DMSA is less toxic than previous chelators, interfering with fewer beneficial minerals – like Cu, Mg, Fe – is available orally (better dose-control). Acetylcysteine is becoming popular because it is even less toxic and has increased Me-Hg removal from the brain.

What you should expect from chelation therapy is a unique protocol but approximately 3 days on, 11 days off, where you take the chelator a few times a day. Dose adjustment is common, and your progress will be measured by urine mercury concentrations.

There are notable side effects to these therapies. Firstly, a chelator may liberate tissue MeHg and redistribute it causing a resurgence of toxic symptoms. Secondarily, a chelator may bind and deplete non-toxic metals, especially Copper and Zinc. Some side effects one may notice when on chelation therapy include: Burning if the drug is injected, fever and chills, headache, nausea and vomiting, diarrhea, convulsions, fall in blood pressure, trouble breathing, low calcium, irregular heartbeat and exacerbation of existing arrhythmias, and more.

Because of how chelators interact with the rest of the body’s metals, vitamins, and antioxidants a pre-treatment or combination treatment regimen is often required. Listed below are common supplements:

Sulfur – It is common for an individual’s sulfur stores to be diminished at baseline.  Chelators have sulfhydryl groups on them which they use to bind mercury. In a sulfur depleted patient, these critical groups are ripped off the chelator to replenish natural body stores, rendering the therapy ineffective. Natural substances like garlic (1-2 cloves/day) or methylsulfonulmethan (MSM) found in legumes and eggs are good sulfur supplements to take prior and during chelator therapy to improve efficacy.

Vitamin E – Many studies in mouse and rat models have shown vitamin E to have protect against oxidative damage by Me-Hg especially in the kidneys and testis. For humans, 400 I.U has a protective effect during tuna season.

Selenium – 200-400mcg/day facilitates the function of glutathione, a natural molecule which detoxifies waste products in the body. This supplement is found to be especially useful in neurologically compromised from Me-Hg toxicity.

Antioxidants – Mercury causes oxidative damage to tissues, and loss of natural antioxidants like Zinc. Supplementing with antioxidants during tuna season is protective to a certain extent. Productive antioxidants include, carrots, berries, Zinc, and Vitamin C.

Chlorella – A popular supplement during bluefin tuna season is the Chlorella tablet. These are made from C. vulgaris algae and have shown some degree of reduction in inorganic and organic mercury accumulation in soft tissues like the gut, muscles, and bone. Many of these studies were done by dentists when investigating mercury accumulation in patients with dental amalgams (fillings).

Cilantro is a common anecdotal remedy for heavy metal toxicity, with potential to help remove mercury from the brain and central nervous system. Consider adding a bushel of cilantro to your homecooked fish meals.

Fun Facts:

Ocean mercury is derived from both natural and human pollutive sources. In a study of the yellowfin tuna fishery from 1971 to 1998, while atmospheric mercury levels increased threefold, the mercury content in YFT flesh did not change. For this reason, it is believed that the dominant source of mercury in our pelagic species likely comes from hydrothermal vents (essentially underwater volcanos) and deep ocean sediment.

Scientists estimate a loss of $16 billion due to decreased IQ points from mercury in the US and EU.

Citations:

Bhatt, D. L., Steg, P. G., Miller, M., Brinton, E. A., Jacobson, T. A., Ketchum, S. B., Doyle, R. T., Juliano, R. A., Jiao, L., Granowitz, C., Tardif, J.-C., & Ballantyne, C. M. (2019). Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. New England Journal of Medicine, 380(1), 11–22. https://doi.org/10.1056/NEJMoa1812792

Morris, M. C., Evans, D. A., Tangney, C. C., Bienias, J. L., & Wilson, R. S. (2005). Fish Consumption and Cognitive Decline With Age in a Large Community Study. Archives of Neurology, 62(12), 1849. https://doi.org/10.1001/archneur.62.12.noc50161

Nurk, E., Drevon, C. A., Refsum, H., Solvoll, K., Vollset, S. E., Nygård, O., Nygaard, H. A., Engedal, K., Tell, G. S., & Smith, A. D. (2007). Cognitive performance among the elderly and dietary fish intake: the Hordaland Health Study. The American Journal of Clinical Nutrition, 86(5), 1470–1478. https://doi.org/10.1093/ajcn/86.5.1470

Oken, E., Baccarelli, A. A., Gold, D. R., Kleinman, K. P., Litonjua, A. A., De Meo, D., Rich-Edwards, J. W., Rifas-Shiman, S. L., Sagiv, S., Taveras, E. M., Weiss, S. T., Belfort, M. B., Burris, H. H., Camargo, C. A., Huh, S. Y., Mantzoros, C., Parker, M. G., & Gillman, M. W. (2015). Cohort Profile: Project Viva. International Journal of Epidemiology, 44(1), 37–48. https://doi.org/10.1093/ije/dyu008

Tong, T. Y. N., Appleby, P. N., Bradbury, K. E., Perez-Cornago, A., Travis, R. C., Clarke, R., & Key, T. J. (2019). Risks of ischaemic heart disease and stroke in meat eaters, fish eaters, and vegetarians over 18 years of follow-up: results from the prospective EPIC-Oxford study. BMJ, l4897. https://doi.org/10.1136/bmj.l4897

Zhu, W., Wu, Y., Meng, Y.-F., Xing, Q., Tao, J.-J., & Lu, J. (2016). Fish Consumption and Age-Related Macular Degeneration Incidence: A Meta-Analysis and Systematic Review of Prospective Cohort Studies. Nutrients, 8(11), 743. https://doi.org/10.3390/nu8110743

[1] Recent criticism of this study states that the mineral oil placebo increased inflammatory markers which may have exacerbated the beneficial results.