Like many of my patients, and readers, you probably know that there have been concerns about eating different types of tuna as they can contain methyl mercury. Some recent news stories regarding children eating tuna have suggested that tuna may be even less safe than previous guidelines advised. Several groups of researchers, though, have differing opinions on how much tuna is safe for you to eat and that’s what I’d like to tell you about.
USDA vs. Other Agencies – Selenium vs. Methyl Mercury
First, let me explain to you the concern about methyl mercury. Tuna, like other fish, absorb methyl mercury from the seas in their fat tissues. That methyl mercury is then passed on to you when you eat the fish. And, like fish, these toxins also accumulate in your fat tissues. Your body then goes about the process of trying to clear this heavy metal from your body through your liver. This can be a slow process, though, if you are consuming a lot of methyl mercury-containing food over a short period of time. This can result in a buildup of toxic heavy metals.
For a long time, the USDA has been telling us that eating about 12 ounces of fish a week is safe, and list on their website 5 of the most common, low mercury containing fish such as canned light tuna, catfish, pollock, salmon, and shrimp. It was advised to eat only 1 can of albacore tuna a week, if at all, due to the high content of methyl mercury. Some sources cite the USDA as recommending only 8 ounces a week as the top limit.
Another agency, the EWG (Environmental Working Group) came up with a tuna calculator to tell you how much tuna you can safely eat per week (see their website link in references) if you are a man or woman. You enter your weight, select your sex, and voila the EWG tells you how much tuna to eat.
Other agencies had different recommendations. One of these, PETA (People for the Ethical Treatment of Animals), says ½ can of albacore tuna, or 2-1/2 cans of light tuna, a week should be your limit. This opinion, however, may be based on PETA’s claim that tuna are inhumanely caught and processed. Limited consumer consumption then, limits the amount of tuna that are harmed.
Another, more controversial, opinion exists from other groups – the EERC (Energy and Environmental Research Center), CATM (Center for Air Toxic Metals) and NOAA (National Oceanic and Atmospheric Association). The conclusions from these groups are in stark contrast to both the USDA and PETA. They claim you don’t have to worry about methyl mercury in your fish at all if its selenium content is higher.
According to these agencies, even albacore tuna gets a pass. They point out that, even though albacore does contain more methyl mercury than light tuna, albacore also has a rather high selenium level that cancels out its harmful effects.
According to Nicholas Ralston, a Biomedical Scientist at EERC, in one of their short documentaries [Selenium and Mercury: The Story in Fish], selenium binds to methyl mercury in an unbreakable bond in the proteins of fish flesh. This selenium/mercury bonding prevents the methyl mercury from doing harm when you eat it. Dr. Ralston cites studies that show that lab animals given equal amounts of selenium and methyl mercury-containing fish showed no adverse effects from the methyl mercury. The lab animals that did not get the selenium showed toxic neurologic effects.
Dr. Ralston also pointed out that these selenium/mercury test results also translated to humans as well. He explains that both humans and rats are selenium-dependent creatures and need/use selenium in the same way. The researchers concluded that, on the contrary, ocean fish consumption prevented methyl mercury toxicity rather than created it.
Other research published decades ago noted in the magazine, Nature [The role of selenium against methylmercury toxicity, Nature 268], suggests the same more recent findings as the EERC and NOAA researchers. A separate study published in the Environmental Science and Technology journal [Dietary Selenium Reduces the Retention of Methylmercury in freshwater fish] reports, too, that selenium clears methyl mercury in fish.
The Western Pacific Regional Fishery Management Council, a NOAA agency, also state on their website, “regardless of the amount of mercury in fish, if the selenium level is higher, the fish is safe to eat”. Their chart shows the fish with the lowest amounts of methyl mercury and the highest amounts of selenium as: Yellowfin tuna, mahi-mahi, skipjack, spearfish, Wahoo, albacore, big eye, Monchong, and striped Marlin.
According to their chart, the only fish really unsafe to eat are swordfish and mako shark as they both have very high levels of methyl mercury and low levels of selenium.
The research conclusions and opinions presented by the EERC, NOAA, and WP Regional Fishery Management Council is noteworthy, yet contradicts current USDA guidelines. Whose right? I’ll let you make up your own mind, but the independently reported science behind selenium and methyl mercury toxicity may more accurately answer the question.
My recommendation would be that, if you are not a pregnant woman, you could likely loosen up the USDA’s 12 ounce per week recommendation to perhaps double that. Instead of two, 6-ounce cans of light tuna, you could likely eat 4 or even 2-3 cans of albacore. When dining out, I would also keep in mind the WP’s Fisheries chart of ‘safe’ fish to eat. Stay healthy my friends!
Fish Safety, http://www.fda.gov/food/resourcesforyou/consumers/ucm077331.htm#Video
Tuna Calculator, http://www.ewg.org/tunacalculator
Top 10 Reasons to Not Eat Tuna, http://www.peta.org/living/vegetarian-living/top-10-reasons-not-to-eat-tuna.a
Selenium and Mercury: The Story in Fish, http://www.undeerc.org/fish/videoclips.aspx
Selenium in Fish, http://www.wpcouncil.org/councilmtgs/145th/Selenium_Poster_final.pdf
A role of selenium against methylmercury toxicity, http://www.nature.com/nature/journal/v268/n5615/abs/268073a0.html
photo credit: weightwatchers.com