You Can Tune a Diet, but Can You Tuna Fish?
Mercury and fish
Posted Apr 29, 2018
The data is quite clear that including fish and shellfish as part of your diet correlates with superior health outcomes. This is not surprising, given the evidence that suggests that it was our ancestors harvesting and cooking of marine foods that allowed us to develop the brainpower to become the most dominant species on the planet.
However, the other day I received a great question on Twitter. Someone who often included canned tuna as part of their regimen was told to cut back and only eat it occasionally; due to health concerns. The question was, are there concerns; and if so, what does “occasionally” mean?
So let’s back up the boat a bit. If your mom was right (and we know she always ultimately is) and fish is a brain food that moved us from treetops to technology, why should we not be eating absolutely as much deliciousness as we can pull from Neptune’s net? Well, it turns out that we are the cause.
As detailed in my forthcoming book, Food Shaman: the Art of Quantum Food, although we often act in an independent and contrary manner without regard to the Earth’s ecosystems our individual well-being – whether we reclined in health and wellness or teeter into disease and disability – is critically dependent on our environment. It is an environment that we not only exist in, but put inside us several times a day in the form of the foods we eat.
And when it comes to fishy contaminants, the culprit we are concerned with is mercury. Specifically, we are concerned with methylmercury. Mercury occurs naturally in the environment, but of most concern is that which is the result of industry. The largest source of mercury emissions is coal combustion for energy production, although cement kilns and chlor-alkali plants are also important sources. When coal is burned, elemental and inorganic mercury compounds are released into the atmosphere.
Since natural laws always apply, what goes up must come down. Given that over 70% of Earth’s surface is covered by ocean, holding over 96% of all Earth’s water, statistics favor that these compounds wind up wet. Microorganisms then convert these pollutants into methylmercury (MeHg). Little fish eat these microorganisms and become contaminated with MeHg. Again following nature’s law, big fish eat little fish. This means that the larger predatory fish at the top of the food chain, fish like King mackerel, tuna (Bluefin, bigeye), shark, and swordfish, end up concentrating high levels of methylmercury.
When we eat such fish, more than 95% of the MeHg is absorbed in our gastrointestinal tract and enters the bloodstream. From there it circulates throughout the entire body, crossing both the blood brain barrier and the placenta. The brain accumulates roughly 10% of the absorbed dose. But once in the brain, the methylmercury is converted back to inorganic mercury, which does not cross the blood brain barrier very well. In essence, it is a one-way trip into the cerebrum were levels of mercury will continue to rise. It is likewise elevated in the placenta, and umbilical cord levels are over 1½ times as high as maternal blood levels; which makes it a significant risk for the unborn.
While the overall benefit of the diet partaking of the oceans bounty is widely recognized, the exact risks of our self-inflicted heavy metal woes are much less clear. It is difficult because the concentrations can vary by species, location, and even the season. The individual risk then varies by age, sex, weight, and a host of other idiosyncratic variables. Add in that many of the signs and symptoms at lower levels of exposure are nonspecific (sleep disturbances, headaches, fatigue, depression, loss of concentration and memory, abdominal pain, muscle and joint aches, loss of fine motor coordination, tremors, hair thinning heart rate abnormalities, and hypertension) and the diagnosis can become a shot in the dark. Even signs and symptoms associated with the highest levels of exposure (numbness of the hands or feet, clumsy gait, slurred speech, and visual changes) can often be attributed to other causes.
Finally, outside of women of childbearing age, there are no generally recognized guidelines for acceptable blood mercury levels for the rest of the population. The Centers for Disease Control and Prevention define excessive mercury exposure as a blood level above 10 μg/L. In a national survey carried out by the Centers for Disease Control in 1999-2004, 6 percent of women of childbearing age had blood mercury levels above the EPA’s definition of a safe level. Because of all the variables previously mentioned, an elevated blood mercury level merely confers an increased risk; not a certainty of pathology.
So with the vagaries associated with determining how much mercury is “too much,” the difficulty in determining how much of certain types of fish in our diet is “too much,” borders on nigh impossible. However, the question as first posited is extremely important, because tuna is the most popular fish consumed in United States. And among the forms of tuna eaten, canned tuna is the most popular. Canned light tuna (the less expensive variety) contains about 0.118 parts per million of mercury. Canned albacore tuna contains roughly 0.353 parts per million of mercury, about three times as much as canned light tuna. In 2009, The Mercury Policy Project reported that tuna accounts for about 37 percent of all the mercury in the US seafood supply. Tuna is responsible for six times as much mercury exposure as the four very-high-mercury fish varieties the government advises pregnant women not to eat; swordfish, shark, king mackerel and Gulf (not Atlantic) tilefish. Because of the variabilities and uncertainties about accurate dose assessments, several consumer and public-health organizations have advised pregnant women to avoid all tuna, including canned light.
The solution? As again detailed in Food Shaman: the Art of Quantum Food, the key lies in proper sourcing of our food. Awareness of how we grow our food, how we harvest, and how we produce our foodstuffs is quickly becoming one of the, if not the, most important considerations we make before we eat. In short, quality trumps calories, RDAs, percent sat fat and a host of other outdated and near useless calculations of our comestibles.
Getting schooled in the proper method of sourcing fish, involves knowing your options and where they come from. For example, several smaller brands of canned light tuna that originate in Central and South America may contain mercury in excess of the previously described values. Seafood and fish are not only healthful, they are delicious part of any varied diet. There are many scrumptious options that are generally recognized as containing very low levels of contaminants. In all, 25 fish and shellfish categories (some of which, like crabs, clams, and flatfish, contain many separate individual seafood varieties) have lower average mercury levels than light tuna. There are choices like sardines, Atlantic mackerel, anchovies, and periwinkles that are also high in healthful omega-3 fatty acids. Other good choices include Arctic char, cod, attic, herring, oysters, scallops, shrimp, clams, and mussels.
But clearly we love tuna, the steak of the sea.
So how much can we eat, how much is “occasionally”? Well, as discussed at best what we have our rough guidelines. For young children, pregnant women, and nursing mothers there is evidence to err on the side of caution. For everyone else, the recommendations have to take into consideration how often you eat fish, the type of fish and its mercury content, the portion size, and body weight among others. As an example, a 150 pound adult could consume two 6 ounce cans of canned light tuna per week and be within the EPA recommended safe zone.
A great option for those who are concerned is the use of an apps that list seafood choices and identifies their levels of mercury contamination. There is a free app from Purdue University that perform such a function. In the end it comes down to the effort of awareness. Being aware that we are part of the ecosystem of this planet, not its master. We are truly connected to all living things. The story of methylmercury and seafood is a classic example. When we wantonly tossed poisons into the atmosphere it comes back with interest in the foods we love to eat most. Karma, after all, may not be a bitch – she may be a tuna sandwich.
Guéguen, M., Amiard, J., Arnich, N., Badot, P., Claisse, D., Guérin, T., & Vernoux, J. ( 2011). Shellfish and residual chemical contaminants: hazards, monitoring, and health risk assessment along French coasts. Rev Environ Contam Toxicol., 213:55-111. doi: 10.1007/978-1-4419-9860-6_3.
mercuryfactsandfish.org. (2016). Mercury and Fish: the Facts. Retrieved from mercuryfactsandfish.org: http://mercuryfactsandfish.org/mercury-facts/how-well-substantiated-are-mercury-risks/
NRDC. (2018). Mercury Guide. Retrieved from https://www.nrdc.org: https://www.nrdc.org/stories/mercury-guide
Silbernagel, S. M., Carpenter, D. O., Gilbert, S. G., Gochfeld, M., Groth III, E., Hightower, J. M., & Schiavone, F. M. (2011). recognizing and preventing overexposure to methylmercury from fish and seafood consumption: information for physicians. J Toxicol, 2011: 983072. doi: 10.1155/2011/983072.