I know I’m not a pro, but reading this is definitely useful…check it out.
The Naked Truth: Mercury & Tuna
by Mike Roussell
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. Testosterone Nation has been crushing nutrition and training myths for years. Now it’s time to examine the tuna/mercury controversy and finally reveal the naked truth!
Turning Away From Tuna
As an undergraduate working toward a B.S. in chemistry, I spent two years slaving 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 MRPs to provide the bulk of my protein intake.
With a long shelf life and a cost of about 50 cents a can for chunk lite tuna, how could I go wrong? I use to keep my lab desk drawers stocked full of chunk light tuna. (Who needs file folders? I needed protein!) I’d regularly eat three 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 one or two cans of tuna per week. At first I blew them off. What did they know? After all, they thought the USDA food pyramid was the way to good health!
But then I got scared. Mercury poisoning could lead to brain damage! And I had no real way of proving them wrong at the time. So I swore off tuna. Other than the occasional can when I was in a pinch, I didn’t eat tuna for three years.
Before we dive into the 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 and can adversely affect the brain and kidneys.
Where Does It Go?
Once in the body, mercury has a half-life of around three days in the bloodstream and a 90-day half-life in other tissues (e.g. brain, kidneys, etc). 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 ï¿½?? 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 mecury-glutatione complex is the bloodstream. Once in the bloodstream, 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 doesn’t overwhelm the system, the kidneys will do a good job of synthesizing metallothionein and binding mercury as needed. If it finds its way to the brain, it gets transferred across the blood-brain barrier 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
Can excess tuna ingestion drive you crazy? Well, 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 like 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 acids (Clarkson, 1990). Having this transport inhibited by certain amino acids could possibly mean that a 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 well for the financially conscious because at around 50 cents a can, chunk light is the cheapest form of tuna around.
Unfortunately, there’s a lot of contradictory information floating around about tuna consumption. In their report, “Toxicological Profile for Mercury,” the CDC states the following:
“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 (2.2 pounds).”
(Note: 2.2 pounds of fish is almost six cans of tuna.)
Later on in the report, the CDC states that a person can chronically (for about 365 days) ingest .0003mg/kg of mercury per day with “no observed adverse effect.” For a 200-pound man this would be a little over one 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-pound man can eat three cans of chunk light tuna per week. That’s three to five cans less that the CDC says you can eat.
There’s one more study that’s important for answering the mercury/tuna question. Sherlock et al. (1984) found that after a year of consuming fish containing mercury, the subjects’ bodies reached a steady state (mercury saturation). Chronic exposure of mercury after that point didn’t lead to any great accumulation of mercury.
This study suggests that chronic ingestion of fish containing mercury won’t lead to an overabundance of mercury in the body. The body has a fixed capacity for mercury storage that’s typically maxed-out after one year (anything after that will just get excreted). This is supported by two other studies that have shown that the urinary excretion of mercury can be increased by up to 53% during chronic mercury exposure (Cherian et al. 1978; Hursh et al. 1976).
In the end, I think we should side with the CDC. Their report on mercury was over 650 pages and quite impressive. The FDA has spent a lot of time monitoring the levels of mercury in foods but they’ve failed (in my opinion) to look at the data and research on the effects of chronic consumption of fish containing mercury. After you read the book Food Politics by Marion Nestle, you won’t be too quick to trust the FDA (or the USDA for that matter).
Preventing Mercury Toxicity ï¿½?? Playing It Safe
Even with the science in our favor, I think it’s important to look at some ways that we can help our bodies deal with chronic consumption of mercury. Despite what countless ads would like you to believe, EDTA (ethylene diamine tetraacetic acid) isn’t a very good chelator of mercury (mercury is about the only heavy metal EDTA won’t chelate) and thus won’t do much for the excess mercury in your system.
EDTA supplements: don’t buy them if excess mercury is your concern!
Selenium has been shown in various animal models to prevent the toxic effects of methylmercury (Ganther et al., 1972; Iwata et al., 1973) and even increase the inorganic-to-methyl mercury ratio in tissues (Komsta-Szumska & Miller, 1984; Brzeznicka & Chmielnicka, 1985). But unfortunately, selenium has also been shown to increase the methylmercury concentration in the brain, which is just about the worst thing it could do (Magos & Webb, 1977; Brzeznicka & Chmielnicka 1985).
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. Dr. Ryan Smith recommends 1500mg a day. This is a good recommendation and should be followed by heavy tuna eaters.
1500mg a day of NAC might be a good idea for those who eat a ton of tuna.
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 isn’t to overwhelm your system. Don’t decide one day that you’re going to add tuna to your diet and start eating a couple of 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
ï¿½?ï¿½ The science shows that there’s no reason bodybuilders should cut tuna out of their diets due to the current mercury scare.
ï¿½?ï¿½ One can of chunk lite a day is a reasonable and safe intake for a 200-pound man without risking any health problems.
ï¿½?ï¿½ 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.
ï¿½?ï¿½ Adding 1.5 grams of NAC to your diet is a good idea if you eat a lot of tuna. This will help keep your glutathione stores full and your liver healthy.
In short, don’t worry too much about tuna consumption. If you are worried, play it safe and adopt the recommendations above.