Since the beginning of recorded history, fish has always been an important source of protein for humans. Delicious, rich in nutrients, and high in unsaturated fatty acids, fish are beneficial for our health. However, seafood is minimal in the American diet, and on average only about 5 percent of Americans eat seafood more than once a week.
Besides the expensive price tags, many people don’t consume fish or shellfish due to the heavy metals found in them. Currently, there are several definitions for heavy metal. According to one definition, heavy metals are any metallic chemical elements that have a high atomic weight and a density at least five times greater than that of water. The heavy metals found in seafood include lead, arsenic, cadmium and mercury, with mercury in fish giving the most cause for concern for many people.
We will explain the ins and outs of mercury in fish and its risk factor, as well as other risks and benefits of both consuming and not consuming fish as part of your diet.
Mercury Presence in Fish and Its Harmful Effects on Human Health
Mercury is a reactive heavy metal and widespread environmental pollutant, which can cause a wide range of adverse health effects in humans. One thing unique about mercury is that it exists in nature in three forms, including elemental, inorganic, and organic. Elemental mercury is liquid at room temperature. If it’s accidentally drunk, it doesn’t readily cross tissue barriers, so it will most likely pass through the body with very little being absorbed. After ingestion, inorganic mercury (e.g. cinnabar) is also poorly absorbed. However, the most frequently encountered organic form of mercury, methylmercury, is readily absorbed by the human body.
Mercury can be emitted from natural sources (e.g. volcanoes) and human sources (e.g. coal-fired electric power plants and gold mining). After being emitted, mercury cycles from rainwater into lakes and oceans, where it is digested by plankton and converted into methylmercury.
Methylmercury bioaccumulates in aquatic food chains. Small shrimps eat the methylmercury-containing plankton and then accumulate mercury in their own bodies, and they are in turn eaten by small fish, which are eaten by bigger fish. Therefore, mercury is bioaccumulating in this food chain, until it gets into large fish, such as sharks, or humans.
Mercury is dangerous to humans, as it is highly toxic and can result in mercury poisoning. As mercury can damage the central nervous system, the symptoms of mercury poisoning and having too much mercury in the body include numb or tingling fingers, lips and toes, anxiety, depression, hearing and speech difficulties, trouble walking, muscle and joint pain, irritability, memory problems, paresthesias, ataxia, and tremors.
Mercury can also cause developmental delays in children, and unborn babies’ nervous systems are the most vulnerable to the effects of mercury. In the 1950s and 1960s, cases of severe methylmercury poisoning were discovered in the city of Minamata in Japan, as local residents consumed fish and shellfish contaminated by mercury discharged in waste water from a chemical plant. Typical symptoms of the Minamata disease include sensory disturbances, ataxia, dysarthria, auditory disturbances, and tremors. And in extremely severe cases, insanity, paralysis, and even death were observed. When some pregnant women ingested the contaminated seafood, their children eventually ended up with neurodevelopmental abnormalities.
In addition, in the early 1970s, a large methylmercury poisoning outbreak took place in Iraq, after people consumed some imported seeds dressed with a methylmercury fungicide, which were not intended for human consumption. The symptoms of the mercury poisoning were similar to those of the Minamata disease, and it caused central nervous system damage in infants, through their mothers’ consumption of the grains. Therefore, it’s recommended that pregnant mothers, women who are planning to conceive in the next six months, infants, and young children limit their exposure to mercury in fish.
Different fish species contain varying amounts of methylmercury, depending on levels of environmental contamination, their positions on the food chain and lifespan.
Species that are long-lived and high on the food chain, such as bigeye tuna, shark, swordfish, and king mackerel contain higher concentrations of mercury than others. For instance, as the lifespan of swordfish can be 15 to 20 years and due to its predatory nature, the average mercury concentration in swordfish is 0.995 PPM (parts per million), which is considered high. On the contrary, salmon’s lifespan is only 4 to 5 years, so its average mercury level is 0.022 PPM.
It’s Safe to Consume Fish With More Selenium Than Mercury
Once ingested, mercury enters the human body and deactivates special selenium-dependent enzymes, which are called selenoenzymes. Selenoenzymes play important roles in protecting our brain. While our brain functions, it constantly consumes oxygen and produces oxygen by-products that can damage the brain’s own lipids and proteins. However, selenoenzymes can prevent such oxidative damage or reverse it. So, selenoenzymes are extremely important to our health.
Therefore, when mercury binds to selenium, it deactivates the selenoenzymes and interferes with these enzymes’ function.
This also means selenium has a protective effect: when there is much more selenium than mercury, the small amount of mercury will bind to selenium, leaving the more abundant selenoenzymes to continue to carry out their crucial function.
Also, since selenium has a high binding affinity for mercury, when these two elements meet, they bind to form a new substance, thus preventing the body from absorbing the mercury separately. As a result, the mercury can no longer bind to anything else, such as brain tissues. However, when the mercury level becomes too high and starts to inhibit a significant percentage of selenoenzyme activities, brain health will suffer.
So which types of food contain high levels of selenium?
As it turns out, 16 of the 25 highest dietary sources of selenium are ocean fish, such as yellowfin tuna, halibut, and sardines. To take the selenium’s effects into account, researchers have proposed a new measure of seafood safety called the “Selenium Health Benefit Value” (SeHBV).
A positive (above zero) SeHBV indicates that the seafood contains selenium which exceeds potential binding by mercury, and therefore this seafood is safe to consume. And a fish with a negative (below zero) SeHBV is considered unsafe. Using this new safety measurement, most species of commonly eaten fish in the United States contain more selenium than mercury, and thus are safe to consume.
In a study published in 2007 in the journal Biological Trace Element Research, absolute and molar concentrations of mercury and selenium were determined in 15 fish species collected from the ocean near Hawaii, and selenium’s protective effects against mercury toxicity were demonstrated in all fish species evaluated.
Benefits Versus Risks of Fish Consumption
Fish is rich in nutrients and high in protein, so it’s an important part of a healthy diet. Many studies have demonstrated that consuming fish brings many health benefits to our brain, eyes, heart and body. While at the same time, consuming fish has potential risks, due to the heavy metals and contaminants contained in the fish.
Benefits of Eating Fish
Fish is high in nutrients: including protein, vitamin D and other vitamins, iodine, omega-3 fatty acids, and minerals. For instance, a 4-ounce (113-gram) serving of cooked salmon offers nearly 100 percent of the daily recommended intake of vitamin D.
Fish consumption protects the heart: Fish is considered one of the best foods for heart health. In a meta-analysis published in 2007 in the Journal of Clinical Psychiatry, omega-3 polyunsaturated fatty acids, which are contained in fish, show significant antidepressant efficacy. According to another meta-analysis published in 2011 in the journal Public Health Nutrition, low (1 serving/week) and moderate (2 to 4 servings/week) fish consumption both can effectively prevent deaths from coronary heart diseases. The fish high in omega-3 fatty acids include salmon, tuna, sardines, and mackerel.
Fish is beneficial for pregnant mothers and their unborn babies: Docosahexaenoic acid (DHA), one important omega-3 fatty acid, exists in large quantities in the brain and retina. DHA is crucial for eye and brain development. An American study published in 2008 in the journal Environmental Health Perspectives found that seafood consumption during pregnancy resulted in significant benefits in motor development and verbal and total IQ of the babies.
Fish consumption can protect against stroke, depression, Alzheimer’s disease, and other chronic conditions: Another important omega-3 fatty acid is eicosapentaenoic acid (EPA), which is found in cold-water fatty fish, such as salmon. Along with DHA, EPA helps fight inflammation in the body, thus reducing the risk of heart diseases, cancers, and diabetes. In addition, both DHA and EPA play an important role in metabolism.
Fish consumption boosts brain health: Fish has long been considered a “brain food.” In a study published in 2014 in the American Journal of Preventive Medicine, researchers compared the brain volume of people who ate fish and people who didn’t eat fish, and found that the consumption of baked or broiled fish is associated with the volume of gray matter in the human brain, but not with the level of omega-3 fatty acids. According to one of the researchers, Cyrus Raji, M.D., Ph.D., from the University of Pittsburgh Medical Center and the University of Pittsburgh School of Medicine, people who eat baked or broiled fish on a weekly basis may improve their brain health and reduce their risk of developing cognitive problems and Alzheimer’s disease. However, eating fried fish didn’t show the same results.
Fish consumption may be part of a weight loss diet: Compared to other types of meat such as red meat and poultry, most fish species are lower in fat and calories, while offering high-quality lean protein. Therefore, many people who are trying to lose weight make fish part of their diet.
Risks of Eating Fish
Some fish species contain high levels of mercury: Although there are many risks associated with mercury poisoning or having an excessive amount of mercury in the body, as aforementioned, as long as we choose the fish species with positive SeHBVs, it’s safe to consume them.
Fish may be infected with parasites and bacteria: Parasites cannot survive on their own, so they have to find hosts to infect for survival. Common types of fish parasites include roundworms (nematodes), flukes (trematodes), and tapeworms (cestodes). Fish get infected with parasites when they feed on their hosts. And humans may become accidental hosts when they consume fish infected with parasites. After entering the human body, roundworms can lead to anisakiasis, flukes can cause trematodiasis, and tapeworms can cause diphyllobothriasis. However, unlike contamination, parasites are a natural occurrence. As long as we consume thoroughly cooked fish, parasites shouldn’t be a concern. However, people that eat raw fish, such as sashimi and sushi, are at risk of infection by parasites or bacteria.
Some fish contain highly toxic PCBs and Dioxins: Polychlorinated biphenyls (PCBs) are synthetic organochlorine compounds previously used in industrial and commercial processes, and they have been banned in the United States since 1977. Being very slow to degrade, PCBs accumulate in the sediment at the bottom of lakes, rivers, and coastal areas, as well as in fish. Dioxins are organochlorine by-products of waste incineration, paper bleaching, pesticide and polyvinyl chloride plastic production.
Both PCBs and dioxins can cause neurological problems and may increase the risk of some cancers, including melanoma, liver cancer, and gallbladder cancer. Although not consuming fish can avoid ingesting these chemicals to a certain extent, the highest dietary sources of PCBs and dioxins are not fish. According to an article published in August 2020 in the journal Environmental Science & Technology, our dietary sources of PCBs and dioxins are meats other than fish (50 percent), dairy products (25 percent) and other types of food (e.g. vegetables). Fish constitute only 15 percent of our dietary intake of these chemicals.
Furthermore, the levels of PCBs and dioxins in our food are expected to decline over time, as the manufacturing and processing of PCBs was banned in 1977, and dioxin emissions have been reduced by more than 90 percent since 1987.
Conclusion: The comparison of the benefits and risks of fish consumption seems to suggest that eating fish will do more good than harm to our health. Also, although fish consumption has risks, overall, fish is still a healthier food than red meat and some other alternatives.
Benefits Versus Risks of Not Eating Fish
As aforementioned, fish can take in harmful chemicals, such as mercury and PCBs, from the water and the food they eat. So avoiding eating fish has its benefits. However, does avoiding fish consumption do more good to our body than harm?
Benefits of Not Eating Fish
Not consuming fish can avoid ingesting mercury and other contaminants: Some of the fish species containing the most amount of mercury include tilefish, shark, swordfish, king mackerel, marlin, bigeye tuna, and cod. However, as aforementioned, scientists now believe that the Selenium Health Benefit Value can better measure seafood safety. For instance, as per Figure 2, when taking the amount of selenium in the fish into consideration, bigeye tuna and marlin both become safe to consume, as their SeHBVs are positive. Also, if we stop eating fish to avoid PCBs, unless we give up most of the other meats (e.g. red meat), we may end up with more PCBs than otherwise, from the increased meat consumption.
Avoiding fish allergies: Fish allergy is different from seafood allergy, as seafood also includes shellfish (e.g. lobster). Fish allergy can develop at any age, so some people who consumed fish in the past without any problem may suddenly develop an allergy to fish. And different from many other allergies, fish allergy may remain for life. The common symptoms of fish allergy include trouble breathing, coughing, vomiting, diarrhea, throat tightness, swelling, hives, and a decrease in blood pressure. In severe cases, someone allergic to fish may even faint. Therefore, avoiding fish becomes a must for anyone who develops an allergy.
Abstaining from fish promotes ocean and aquatic life sustainability: The number of individual wild fish killed each year is estimated to be 0.97 to 2.74 trillion. According to the World Wide Fund for Nature’s (WWF) 2018 Living Planet report, 6 billion tons of fish and other invertebrates have been taken out of the sea since 1950. Fish populations around the world have been almost depleted by the fishing industry, which also pollutes the oceans and destroys marine habitats during its operations. Not consuming fish may help prevent the oceans from being destroyed and promote aquatic life sustainability.
Fish are also sentient beings and social animals: Scientific evidence has shown that fish are sentient animals, who can experience pain, emotions, and form friendships. Fish are much more intelligent than previously thought, and they show a rich array of sophisticated behaviors. Fish can even develop complex traditions, cooperate with one another, and are even capable of utilizing tools. Therefore, some scientists are actually in favor of lending fish the same level of protection as any other vertebrate. Not eating fish will spare them from being tortured in the fishing process and being slaughtered for food.
Risks of Not Eating Fish
Not consuming fish might increase the risk of hearing loss: An inverse association has been found between total omega-3 polyunsaturated fatty acid intake and prevalent age-related hearing loss. Furthermore, there also exists an inverse association between long-chain omega-3 polyunsaturated fatty acids and incident hearing loss. As omega-3 polyunsaturated fatty acids mainly exist in fish oil, not consuming fish deprives us the protection from such fatty acids, thus increasing our risk of developing hearing loss.
Not eating fish can increase the risk of death from heart disease: According to a study published in the Journal of the American Medical Association, even one or two servings of fish per week, especially the fish species high in EPA and DHA, reduces risk of coronary death by 36 percent. This is because the omega-3 fats in fish protect the heart against cardiac rhythm disturbances, lower blood pressure and heart rate and improve blood vessel function. Therefore, not eating fish can increase the risk of coronary death.
Not eating fish linked to harm in pregnant mothers and young children: An abundant amount of evidence suggests that consuming less than 12 ounces (340 grams) of fish per week can harm pregnant women and young children. In a study published in 2007 in The Lancet, 11,875 pregnant women were asked about their seafood consumption frequency at 32 weeks’ gestation.
Later, the developmental and cognitive outcomes of their children were compared, and it was discovered that consuming fish less than 340 grams per week during pregnancy is associated with significantly increased risk of lower communication skills and verbal IQ in the children than their peers, whose mothers consumed more than 340 grams of fish per week. Furthermore, worse developmental outcomes were found in children whose mothers didn’t eat seafood at all during pregnancy.
Not consuming fish can lead to increased risk of stroke, depression, Alzheimer’s disease, and other chronic conditions: As aforementioned, adequate fish consumption can help prevent many chronic diseases. Therefore, when no fish is consumed at all, it’s reasonable to expect that the risk of developing these diseases will increase.
Minimizing the Effects of Mercury, Heavy Metals Due to Fish Consumption
If eating fish, there are a few things to consider to make this a safer dietary choice.
Select the fish species with positive SeHBVs: As aforementioned, instead of the absolute amount of mercury, it’s the ratio of selenium to mercury in the fish (SeHBV) that determines whether a fish species is safe to eat or not. So based on SeHBVs, we should avoid consuming sharks, swordfish, tilefish and king mackerels. Also, since chemicals build up in fish as they age, within the same species, we should choose smaller ones. The parts of the fish we should avoid include the skin, fat, and guts, as they tend to have a higher concentration of chemicals and contaminants than the flesh.
Purchase the most sustainable fish species: As an individual, you can contribute to a healthy ecosystem, or seafood sustainability, by choosing the most sustainable fish species to buy. Many organizations, such as Ocean Conservancy, have put together guidelines to choosing healthy and sustainable fish species.
Prepare fish with the right cooking methods: Unfortunately, cooking methods have little impact on fish’s methylmercury content. However, as mentioned previously, consuming baked or broiled fish on a weekly basis may improve brain health and reduce the risk of cognitive problems and Alzheimer’s disease. However, eating fried fish doesn’t yield the same results. It is suggested that frying can seal harmful chemicals into the fish, thus negating the health benefits of consuming fish.
Cook fish thoroughly: Regardless which cooking method you choose, remember to cook the fish thoroughly, in order to kill harmful parasites, viruses, and/or bacteria, and avoid getting sick.
Take measures to prevent mercury poisoning: If you catch your own fish, be sure to check the latest fish advisories in your area, to avoid potential mercury poisoning. When ordering sushi in restaurants, make sure you know which fish you are getting, as many sushi rolls are prepared with mercury-containing fish. If you think you’ve been exposed to mercury, wash your hands thoroughly before touching anything else. You can also take a blood or urine mercury test, if needed, such as before getting pregnant.
Consuming certain foods to eliminate heavy metals in your body: You should eat fresh fruits and vegetables, which contain abundant antioxidants and fiber, to support your body’s natural detoxification processes. The food items and nutrients that can chelate heavy metals include coriander, zinc, selenium, milk thistle, and vitamin C.