The average American eats 2.5 pounds of canned tuna per year, according to Cornell University’s Seafood Health Facts website. Canned tuna is the second most widely consumed seafood product in the United States, but it poses some health risks to people who eat it regularly. Pregnant women should avoid it. Young children, who may experience toxicity from its mercury and BPA contents, should not eat canned tuna.
There are two main kinds of canned tuna: chunk light and solid or chunk white (albacore). Most canned white tuna is albacore. Its mercury levels are almost three times higher than the smaller skipjack, used in most canned light tuna.
Where does mercury come from?
Air pollution. Specifically, it rains down on rivers, lakes and oceans after being emitted from power plants and other industrial sources that burn fossil fuels.
Before we dive into the mercury/tuna debate, a little background on mercury is necessary. Mercury, like zinc, iron, and Lead, is a heavy metal. But unlike zinc and iron, lead and mercury have no useful function in the human body.
The only functions that mercury has are adverse; negatively affecting the brain and kidneys. Once in the body mercury has a half-life of ~3 days in the blood stream and a 90 day half life in other tissues (e.g. brain, kidneys, etc).
Where Does It Go?
When you ingest mercury (via your daily can of tuna) it gets readily absorbed by the small intestine and shipped to the liver where it forms a complex with glutathione.
From there the mercury has two fates – bile or blood. It can get incorporated in bile and excreted back into the intestines where it can be either reabsorbed or excreted in your feces.
The other fate for the mercury-glutathione complex is the blood stream. Once in the blood stream mercury readily travels to the kidneys or the brain. In the kidneys, it can get filtered and excreted in the urine or stored. The kidneys contain a protein called metallothionein that binds mercury and stores it in a nontoxic form.
As long as the dosage of mercury does not overwhelm the system the kidneys will do a good job of synthesizing metallothionein and binding mercury as needed.
If it finds a way to the brain it gets transferred across the blood brain barrier (more on this later) and stored. The storage option is the one that leads to mercury toxicity causing damage to the brain or kidneys.
Mercury Messes With Your Mind
The brain is pretty picky about what it lets across the blood brain barrier but mercury has found a loophole to get through and drive you nuts (literally).
Methylmercury can bind to cysteine and to the brain this methylmercury-cysteine looks via methionine (essentially methylated cysteine).
So methylmercury sneaks across the blood brain barrier disguised as an amino acid. Luckily the transport of this methylmercury-cysteine complex is inhibited by, methionine, phenylalanine, leucine, and other large neutral amino acids1.
Having this transport inhibited by certain amino acids could possibly mean that high protein diet (and the protein found in tuna fish) will help prevent the transport of methylmercury into the brain.
Our recommendation when purchasing tuna is to stick with canned light versions, or if buying canned albacore, to purchase it from a reputable supplier who has had the mercury levels tested by an independent lab.
The mercury content of other types of fish will vary with the species. If you are looking for some other options that could replace your canned tuna, some varieties recommended by the Environmental Working Group and the U.S. Public Interest Group include wild Pacific salmon, flounder, haddock, shrimp, farm-raised trout and catfish; there was no mercury detected in these fish.