Tuna fish and mercury: this is a marriage uglier than Ike and Tina. How much tuna can you eat without suffering from mercury poisoning? Is one kind of tuna better than the other? What about Tuna Steaks? Lots of questions but nobody has any real answers - it's all hearsay... until now. So sit back, grab a can, and enjoy!
Turning Away From Tuna
As an undergraduate working towards a B.S. in chemistry, I spent 2 years working in a lab doing organic synthesis. The combination of being in college and working in a lab meant that both money and time were tight. So like many powerbuilders before me, I turned to canned tuna and MRP's to provide the bulk of my protein intake.
With a long shelf life and at $0.50 a can for chunk lite tuna how could I go wrong? I use to keep my lab desk drawers stock full of chunk light tuna (who needs file folders? - I needed protein). What could be better?
I would regularly eat 3 cans a day; but then some of my fellow researchers started giving me a hard time saying that I was poisoning myself because tuna was loaded with mercury.
They printed off charts and diagrams saying how I should only eat 1-2 cans of tuna per week. At first I blew them off. What did they know; they thought the USDA Food Pyramid was the way to good health! But then I got scared. Mercury poisoning could lead to brain damage!
I had no way of proving them wrong and if they were right the thoughts of mercury poisoning were not very pleasant. So I swore off tuna. Other than the occasional can when I'm in a pinch I hadn't eaten tuna in about 3 years.
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-glutatione 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
Remember how I said I had been eating 3 cans of tuna a day with no problems? Well... looking back on that summer a couple strange things did happen - like my training partner refusing to workout with me only 3 weeks into our training cycle.
He said that I was "going mad" and that he was embarrassed by my antics as I got psyched for big lifts (like slamming my head against the power rack).
Sure I wasn't my usual self and I did feel a little crazy but my lifts were great... little did I know it was the tuna that was making me crazy! Really... no I didn't go crazy like that in the gym but neurological problems and "symptoms of madness" are classic signs of methylmercury toxicity.
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.
Let's Talk Tuna
Now that we've laid the foundation for understanding mercury (and methylmercury) let's look at its relationship to tuna and tuna consumption.
Fortunately for us canned tuna fish has less mercury than tuna steaks and chunk light canned tuna has less mercury than chunk white canned tuna. This works out for the financially conscious because at $0.50 a can, chunk light is the cheapest form of tuna around.
Unfortunately there is a lot of contradictory information floating around about tuna consumption. In their report entitled "Toxicological Profile for Mercury", the CDC states the following:
TOXICOLOGICAL PROFILE FOR MERCURY
"No consumption advice is necessary for the top ten seafood species that make up about 80% of the seafood sold in the United States: canned tuna, shrimp, pollock, salmon, cod, catfish, clams, flatfish, crabs, and scallops.
The methylmercury in these species is generally less than 0.2 ppm, and few people eat more than the suggested weekly limit of fish (i.e., 2.2 pounds)." -CDC
*2.2 pound of fish is almost 6 can of tuna.
Later on in the report, the CDC states that a person can chronically (for >365 days) ingest .0003mg/kg of mercury per day with "no observed adverse effect."
For a 200 lb. man, this would be a little over 1 can of chunk light tuna each day. But the Environmental Working Group has a "Tuna Calculator" where you enter your weight and they tell you (according to the FDA) how many cans of tuna you can eat each week.
Their calculations state that a 200 lb. man can eat 3 cans of chunk light tuna a week. That's 3-5 cans less that the CDC says you can eat.
There is one more study that is important for answering the mercury/tuna question. Sherlock et al found that after 1 year of consuming fish containing mercury the subjects bodies reached a steady state (mercury saturation). Chronic exposure of mercury after that point did not lead to any great accumulation of mercury.2
This study suggests that chronic ingestion of fish containing mercury will not lead to an overabundance of mercury in the body. The body has a fixed capacity for mercury storage that is typically maxed out after one year (anything after that will just get excreted).
This is supported by two other studies that have shown during chronic mercury exposure, the urinary excretion of mercury can be increased by up to 53%.3,4
In the end I think that we should side with the CDC. There report on mercury was over 650 pages and impressive. The FDA has spent a lot of time monitoring the levels of mercury in foods but they have failed (in my opinion) to look at the data and research on the effects of chronic consumption of fish contain mercury.
Preventing Mercury Toxicity
Even with the science in our favor I think it is important to look at some ways that we can help our bodies deal with chronic consumption of mercury.
Despite what countless detoxifying toxic ads would like to have you believe, EDTA is not a very good chelator of mercury (mercury is about the only heavy metal EDTA won't chelate) and thus won't due much for the excess mercury in your system.
Selenium has been shown in various animals' models to prevent against the toxic effects of methylmercury5,6 and even increase the inorganic-to-methyl mercury ratio in tissues.7,8
But unfortunately, selenium also has been shown to increase the methylmercury concentration in the brain - which is just about the worst thing it could do.9,8
Since methylmercury binds and potentially depletes glutathione stores in the liver it would be a good idea for heavy tuna eaters to supplement with N-acetylcystine (a glutathione precursor) to insure that the liver maintains optimal antioxidant ability. The suggested recommended dosage for someone looking to supplement with NAC is 1500mg a day.
This Should Be Followed By Heavy Tuna Eaters
As stated at the beginning of the article the kidneys can do a good job of removing toxic mercury from the body and storing it in a safer form. The key is not to overwhelm your system. Don't decide one day that you are going to add tuna to your diet and start eating 1-2 cans a day.
Increase you tuna intake over the course of several weeks so that your kidneys can adjust and produce metallonthionein accordingly.
Take Home Messages
- The science shows that there is no reason bodybuilders should cut tuna out of their diets due to the current mercury scare.
- One can of chunk lite a day is a reasonable and safe intake for a 200lbs man without the risks of any health problems.
- If you want to eat more tuna now make sure to increase your consumption over the course of several weeks so your kidneys can adjust.
- Adding 1.5 gram of NAC to your diet is a good idea so you can keep your glutathione stores full and your liver healthy.
- Clarkson, 1990.
- Sherlock et al., 1984.
- Cherian et al., 1978.
- Hursh et al., 1976.
- Ganther et al., 1972.
- Iwata et al., 1973.
- Komsta-Szumska & Miller, 1984.
- Brzeznicka & Chmielnicka, 1985.
- Magos & Webb, 1977.