Fish Taco Tuesday (with a side of Mercury)

Mercury in Fish

I’ll be honest, it’s been hard to meet my daily protein macro goal. If I’m lucky, I hit it 50% of the time. I’ve never been a huge fan of meat; it’s a texture thing. I also don’t tolerate protein shakes that well, and yes, I’ve tried all the varieties- whey, plant, egg, etc. They leave me feeling bloated and unwell. Egg yolks make me sick (quite literally), though I do eat egg whites. All of this to say that goals can be tough, and some days are better than others.

With that said, I’ve been looking for on-the-go options of whole food sources of protein that I can incorporate into my day. Some days, my life involves working 13-14 hours at the hospital, while other days I’m jetting from the hospital to clinic to the gym. One item I found that meets this goal is StarKist Tuna Creations packs. I know, I know-- tuna is not a sexy food. It’s challenging to eat at work without making enemies of your coworkers. Nonetheless, it works for me. It’s flavor-filled, quick & easy, and doesn’t require Tupperware (always a win in my book).

As I’m sure many of you are aware, fish consumption has been linked to mercury toxicity. Mercury toxicity comes in numerous forms, but the kind linked to fish is organic mercury toxicity. In the 1950s, there was a large outbreak and a number of deaths associated with industrial drainage into coastal waters and rivers in Minamata and Niigata, Japan. Though outbreaks are far less common today, the association between fish consumption and organic mercury toxicity still exits, but to what extent? With farm-raised products now readily available, are we safer than we once were?

Organic Mercury Toxicity in Fish – Why and How?

So how exactly, does mercury end up in the fish we eat? Simply put, it comes from the atmosphere and makes its way up through the food chain. Atmospheric mercury settles into the water and is converted to methyl- or ethyl-mercury (the organic forms of mercury) by microorganisms in the water ecosystem. Small creatures ingest these products, which are in return eaten by larger fish, and so on and so forth. 80-90% of the methylmercury in fish arises in this manner. The other 10-20% is directly absorbed across gills from the water itself. Once fish consume or absorb the methylmercury, it combines with red blood cells and hemoglobin and is redistributed throughout the fish’s body to well-perfused tissues, mainly the muscle (highest concentration), liver, spleen, and kidney.

Factors that Influence How Much Mercury We Get From Fish

All fish are not created equally. All water sources are not created equally. So what influences how much mercury we, as humans, ultimately consume? First, the type of fish. Numerous studies have shown that fish further up the food chain (such as Mackerel, Sharks, Swordfish), older fish, and larger fish contain higher levels of mercury. This makes intuitive sense if we think about the food chain above, as these fish consume a greater number of mercury-containing food sources. A study out of Iowa also found that high levels of sulfate and phosphorous and open water and grassland areas were associated with lower mercury concentrations in fish. Meanwhile, this study also found that variables of hard water, an environment of wetland and forest, increased nitrogen-ammonia levels, and a higher Human Threat Index were all associated with increased mercury concentrations in fish. The takeaway? The environment that surrounds water sources can affect mercury levels. Furthermore, the guidelines on the maximum safe consumption of fish generally assume that we absorb 95-100% of the mercury in that fish. A study examining the bioaccessibility of mercury (which is the percent of mercury ingested that is released to the gastrointestinal tract for absorption) may fall somewhere between 12-79%, rather than 95-100%. [With that said, I would NOT disregard the guidelines. They exist for a reason; follow them.] Another study found that farm-raised fish were safer than freshwater fish, and freshwater fish were safer than wild fish. This study, however, also found that fish from different farm-raised locations differed considerably in their mercury content. Last but certainly not least, the cooking methods we use may affect how much mercury we consume from fish.

This section took us a bit into the weeds, but the big picture is that a number of variables play into the equation of how much mercury we absorb from fish. More importantly, with all of these variable, it can be difficult to predict how much mercury we are consuming from fish.

Guidelines On Fish Consumption

First, please note that these guidelines are for non-pregnant adults ONLY. Children, pregnant women, nursing women, or women planning on becoming pregnant should consult their physician for further guidance (mercury can cross the placenta and has been found in breast milk). The National Resources Defense Council (NRDC) states that fish with 0.3mg/kg-0.49mg/kg (or ppm) of methylmercury can safely be consumed three times per month, and fish with > 0.5mg/kg should be avoided. The Environmental Protection Agency (EPA) states that maximum daily consumption should be no greater than 0.1mcg/kg/day. But let’s get real- who is actually calculating this on a daily or weekly basis? The Federal Drug Administration and EPA put together the following table to help guide fish consumption, and it is much more user friendly than the guidelines above:

Note: The data for this guide to mercury in fish comes from two federal agencies: the U.S. Food and Drug Administration, which tests fish for mercury, and the U.S. Environmental Protection Agency, which determines mercury levels that it considers safe for women of childbearing age. View the equations we used to make these serving recommendations.

https://www.nrdc.org/stories/smart-seafood-buying-guide?gclid=CjwKCAjw57b3BRBlEiwA1ImytpN5yZLJJVpXP3cyUyVYxCpiTuc4i1TvjQeSp_ljh_HzeytwDlaZjRoCl2QQAvD_BwE

Organic Mercury Toxicity: Pathophysiology, Symptoms, and Treatment

Mercury is theorized to interfere with cellular structure and interrupt DNA transcription and protein synthesis, especially in the developing brain. It is also felt to interrupt neurotransmitters within the nervous system and generate free radicals (in non-medical terms, this means that it can disrupt many body processes). If a person develops organic mercury toxicity, he or she may experience extreme stomach discomfort, breathing concerns, numbness or tingling around the lips, and generally just a feeling of illness and fatigue. Other potential manifestations include deafness, tunnel-type vision (decreased visual fields), or an unstable gait (called ataxia in medicine). Please note, mercury toxicity from fish in developed countries is quite rare. Though I don’t have exact statistics on organic toxicity to share, there were 1300 total cases of mercury toxicity in the US in 2013 from ALL sources. The diagnosis is made through a blood test. We have no treatment for organic mercury toxicity. Other forms of mercury can be treated with a chelating agent, a compound that combines with mercury to create a product that can be eliminated through urine. Unfortunately, we CANNOT treat organic mercury toxicity in this manner. In fact, we cannot treat organic mercury toxicity at all; we can only prevent it. The neurologic side effects of organic mercury toxicity are resistant to treatment and therefore usually permanent. This is why it is important to follow guidelines on fish consumption.

And on that uplifting note… Happy Taco Tuesday!!! For real though, enjoy fish tacos, just be mindful of your overall fish consumption.

The Takeaways:

· Numerous factors affect the mercury content of fish. Fish that are further up the food chain, larger, older, and wild in origin have higher mercury content than farm-raised, smaller fish.

· In general, adults > 18 years old who are not pregnant or nursing can consume most types of fish 2-3 times per week safely (total of 8-12 oz). See table above for specifics.

· Canned light tuna is safer than white or albacore tuna. Canned light tuna can be safely consumed 6 times per month. Limit albacore tuna to 6 oz. per week or 3 times per month.

· We can’t successfully treat mercury toxicity from fish, so we have to prevent it.

Resources

Beauchamp, G., MD, Kusin, S., MD, & Elinder, C., MD, PhD. (2018, July 5). Mercury toxicity. Retrieved June 21, 2020, from https://www.uptodate.com/contents/mercury-toxicity?search=mercury+toxicity

Bernhoft, R. A. (2012). Mercury Toxicity and Treatment: A Review of the Literature. Journal of Environmental and Public Health, 2012, 1-10. doi:10.1155/2012/460508

Bradley, M., Barst, B., & Basu, N. (2017). A Review of Mercury Bioavailability in Humans and Fish. International Journal of Environmental Research and Public Health, 14(2), 169. doi:10.3390/ijerph14020169

Greenfield, N. (2015, August 26). The Smart Seafood Buying Guide. Retrieved June 21, 2020, from https://www.nrdc.org/stories/smart-seafood-buying-guide?gclid=CjwKCAjw57b3BRBlEiwA1ImytpN5yZLJJVpXP3cyUyVYxCpiTuc4i1TvjQeSp_ljh_HzeytwDlaZjRoCl2QQAvD_BwE

Mercury poisoning. (2020, June 19). Retrieved June 21, 2020, from https://en.wikipedia.org/wiki/Mercury_poisoning

Mills, N., Weber, M. J., Pierce, C. L., & Cashatt, D. (2019). Factors influencing fish mercury concentrations in Iowa rivers. Ecotoxicology, 28(2), 229-241. doi:10.1007/s10646-019-02017-1

Olson, D., MD. (2019, December 05). Mercury Toxicity. Retrieved June 23, 2020, from https://emedicine.medscape.com/article/1175560-overview

Publishing, H. (n.d.). What to do about mercury in fish. Retrieved June 21, 2020, from https://www.health.harvard.edu/staying-healthy/what-to-do-about-mercury-in-fish

Zupo, V., Graber, G., Kamel, S., Plichta, V., Granitzer, S., Gundacker, C., & Wittmann, K. J. (2019). Mercury accumulation in freshwater and marine fish from the wild and from aquaculture ponds. Environmental Pollution, 255, 112975. doi:10.1016/j.envpol.2019.112975