Dr. Jeff Hersh
Q: How does mercury get into fish, and how much fish is safe to eat?
A: There are many nutrients we need to get in order to stay healthy, including some metals (for example we need iron to make the hemoglobin that carries oxygen in our red blood cells). However, mercury is NOT needed for humans, and this heavy metal can cause toxicity and health problems (mercury poisoning), including neuropsychological disorders (for example tremors, emotional instability, insomnia, memory loss, others), kidney problems and/or thyroid issues, among other health issues.
Where does the mercury come from in the first place? Like most elements (that is, all elements except the specifically man-made ones), mercury occurs naturally in nature. Humans then foolishly increase airborne mercury by industrial activity like coal-fired electricity generation, smelting, burning of certain wastes, and others.
Mercury in its elemental form (just mercury, Hg, itself) is the only metal which is liquid at room temperature, and is used to make thermometers, to purify gold or silver, to make fluorescent lamps and for other industrial uses. Historically it was (mis)used to make certain red dyes and for other purposes. Elemental mercury is poorly absorbed (0.01%) if ingested.
However, airborne mercury from industrial pollution (for example coal-fired electrical plants) can settle into bodies of water where tiny microorganisms (for example plankton) help it combine with carbon to make methylmercury, a type of organic mercury. Organic mercury is readily absorbed when ingested (5% to 15%), and when fish/plants eat/absorb some of these microorganisms, mercury builds up inside them. Predator fish eat some of these smaller fish, and they then get larger amounts of mercury building up inside them. Therefore, long-lived fish, especially those who are predators and/or are bigger, may have large amounts of mercury build up in them over time.
If/when humans consume some of these mercury-laden fish, the mercury can build up inside of them, and if their mercury levels get too high, they can develop mercury poisoning. Up to 10% of American women of child-bearing age have mercury levels high enough to put a developing fetus at risk.
So why eat fish at all? Well, there are huge nutritional benefits from eating fish as many are a great source of high-quality protein, omega-3 fatty acids, calcium, phosphorous, iron, zinc, iodine, magnesium, potassium, vitamins D and B2, and many other important nutrients. In fact, the U.S. Department of Health and Human Services and the U.S. Department of Agriculture 2015-2020 Dietary Guidelines for Americans note that for the general population “consumption of about 8 ounces per week of a variety of seafood … is associated with reduced cardiac deaths …” and “consumption by women who are pregnant or breastfeeding of at least 8 ounces per week from seafood choices that are sources of DHA (docosahexaenoic acid) is associated with improved infant health outcomes.”
So, a good balance of eating the right types of fish is the key. How can you do this? The answer is to eat the right amounts/types of fish to maximize the benefits while minimizing the intake of the bad stuff like mercury. To do this there are some
mercury calculators” you can find on-line, and then you can follow the recommendations of how much of what types of fish are OK for you (based on your weight and other factors). You can also look at the types of fish that in general have lower and higher levels of mercury in them, and limit eating the higher mercury types.
For example, fish that in general have lower levels of mercury include shrimp, scallops, sardines, wild/Alaskan salmon, tilapia, many others. Those fish that in general may have higher levels of mercury include bluefish, shark, swordfish, wild sturgeon, bigeye tuna, bluefin tuna, marlin, and many others.
What else can be done? Well, we can clearly minimize the problem at its onset by minimizing the pollution that causes the mercury to get into our bodies of water to begin with. This includes better regulation of the Mercury and Air Toxic Standards established by the Environmental Protection Agency, as well as other measures (for example this can be another potential benefit of clean energy production).
Finally, for people that have developed high levels of mercury, the treatment goal is to a) stop further exposure and b) lower their mercury levels, specifically in those organs most affected including the brain, kidneys and thyroid. This can be done by treatment with certain chelating agents (compounds that bind tightly to certain metal ions and then can be excreted from the body, hence helping remove the excess mercury), such as dimercaprol, penicillamine, others.
Jeff Hersh, Ph.D., M.D., can be reached at [email protected] .