As a kid, I used to love to play with mercury. It is the only metal that is liquid at room temperature, and as a junior chemist I enjoyed chasing little balls of glistening silver as they split into bits and then gathering them into a larger mass. This was more fun than any 10-year-old could imagine — at least any 10-year-old nerd chemist.
Mercury was used in those days in thermometers, blood pressure machines, vaccines and for a variety of industrial uses. It currently gets into the ocean from power plants and volcanic activity.
If you read “Alice in Wonderland,” you cannot help but remember the Mad Hatter. His madness, or neurological insanity, was based on an archetype of that era. Hatters used mercury to cure skins and make felt for hats. Over time, they inhaled or ingested enough mercury vapors to cause neurological damage creating a kind of dementia.
Despite its beauty and utility, mercury is dangerous, especially methyl mercury, which is highly absorbable by humans. According to the editor of The Daily News, a large number of people called in after my article recommending eating more fish or taking fish oil to reduce cardiac risk. The readers were concerned about the risk of mercury toxicity from eating fish. So after additional research, I am following up with some more details.
There is some reality to these concerns. Certain fish, especially those higher on the food chain, concentrate mercury. If eaten, they pass this toxic substance onto humans. These are generally larger predator species that eat smaller fish and thus have high concentrations of mercury in their flesh.
These are fish that should be eaten rarely and definitely avoided by pregnant or breast-feeding women and children. Neurological development of children is strongly affected by heavy metals such as mercury and the least dose possible is the safest choice. Though not proven and still controversial, some think that mercury is a contributing cause to the rise of autism and attention deficit disorder among children.
Current recommendations include limiting canned tuna intake to a couple times a month for pregnant women or children. For other adults, a higher intake is tolerable.
Canned tuna is likely the largest source of mercury for most of us. Canned light tuna from skipjack is much lower in mercury than canned white albacore tuna. Recommendations for children are only two or three times a month for canned white tuna, depending on their age. Canned salmon is a safer and affordable choice. Check out canned tuna and mercury recommendations at EDF.org.
I still enjoy an occasional ahi tuna on salad and Thai dressing, done very rare, with wasabi sauce. And though I have long loved swordfish as the “Angus filet of the sea,” I have retreated in the face of evidence about high mercury concentrations and eat it only on rare occasions. My sushi choices are also changing as I learn more about the mercury in tuna and mackerel, longtime favorites as well.
The good news is that our local favorite shrimp is low in mercury, but be a bit more cautious with snapper as it is intermediate and should be limited to six servings a month.
The following fish species are not of concern regarding mercury levels. For further detail including a listing of moderate mercury level fish, visit NRDC.org.
Farmed fish, though they may be low in mercury, may also contain dioxin, PCBs and other toxic chemical contaminants that are known to be harmful. Wild caught fish such as salmon, sardines, halibut, anchovies, certain tunas and other cold-water species are safer and have minimal toxic load. The smaller the fish, the safer it is likely to be.
Studies have shown that fish oil supplements in general contain very little mercury. Nonetheless, supplements of fish oil need to be labeled to give consumers a level of mercury and other contaminants. That is why I recommended Nordic Natural and Spectrum products as my preferred choices for fish oil.
Check the label on fish oil to see if they list the mercury level or other toxins such as dioxins or PCB’s or even the freshness level measures, peroxide or anisidine. These toxins are a bigger concern in farmed fish.
A third party testing agency is the best source of information. Consumer Reports or the Food Marketing Institute are reliable, unbiased sources of information for both fish and fish oil. Some markets now label their fish for levels of mercury and other toxicants.
So, if you are trying to reduce cardiac risk, cancer risk, and reduce inflammation, by all means continue to take in fish and fish oil supplements, but be selective about the species and the brands you choose.
Dr. Victor S. Sierpina is the WD and Laura Nell Nicholson Family Professor of Integrative Medicine and Professor of Family Medicine at UTMB.