Is tuna safe to eat? Ever since in 1970 mercury was discovered in a can of tuna, the threat of heavy metal overdose has loomed over tuna lovers. Let’s put some facts into the discussion: how safe is tuna?
The other day I was at one of our local supermarkets and saw that they had my favorite brand of canned tuna on sale: first class albacore fillets, in water or olive oil. These usually go for €1.99, but they had them for €1.49 – almost a steal. I grabbed an entire box and, good son that I am, took out my cell to phone mom and ask if she wanted any, as I knew she liked to put the same brand on her salads or into sauces.
But things turned out differently. “No,” she said, with a bit of contempt in her voice, “they are so contaminated and besides, all those dolphins!” I played dumb: “What about the dolphins?” “They get caught in the nets with the tuna fish”.
My mother the eco warrior. The same woman who drives 20 miles to go shopping in the town she used to live in, because it’s “more familiar”, now worries about dolphins. I suppose the tuna being caught in the nets wasn’t a problem, as they had the bad luck of never having starred in a TV show. How about “Bluey the Tuna”? Then again I have no idea if you can get tuna fish to dance on their tails.
But I’m digressing. Fact is that tuna do tend to be contaminated with a heavy metal, mercury. Largely methylmercury, to be precise. And mercury is extremely toxic; being exposed to low doses over longer stretches of time can result in tremors and impair hearing, seeing and the rest of your senses, not to mention your ability to think. At higher doses you look at chest pain, limited lung function and inflammation of lung tissue, muscle spasms, hallucinations, depression and many more. It’s the stuff nightmares, literally, are made of.
This is certainly not what we want to eat and therefore have to look at a couple of numbers to find out if tuna is safe to eat or not: a) up to what dosis can humans avoid the above effects, b) how much mercury is in tuna, and c) taking both together, how much tuna can we safely eat over what stretches of time?
What Exposure To Mercury Is “Safe”?
Saying that exposure to mercury could be “safe” would be stretching it, so let’s say that up to a certain point the human body can cope with it without experiencing those unhappy effects above. This is called the “TDI” – the tolerable daily intake over a lifetime. For adults, the World Health Organization established this at a weekly 0.47 microgram per kg body weight (PDF), which is app. 0.22 per lb (1,000 micrograms = 1 milligram, 1,000 milligram = 1 g). Therefore, if you are a 160 lb adult (72.5 kg), you get away with 34.1 microgram (mcg) per week.
How Much Mercury Is In Tuna?
Now we get to where the meat, or rather, the fish is (depending on how you define “meat”). Different kinds of tuna have different mercury concentrations and this also varies with where the fish was caught. Interestingly, tuna caught in the Atlantic contains more mercury than those caught in the Pacific.
Nonetheless, if we average this and take the data the FDA has been accumulating in a monitoring program since 1990, we find that the contamination with mercury in “fresh / frozen” tuna is around 0.38 mg per kg, 0.35 in canned albacore and 0.12 in canned light tuna.
Let’s calculate this into cans, the most popular form in which tuna is bought in: The average, regular can of tuna contains 172 g (~6 ounces). As 1 kg has 1,000 g, we have to divide above mg numbers per kg by 5.81 (1000 / 172) and then convert that to micrograms. For albacore tuna cans this gives us 60 mcg and for light tuna 20 mcg of mercury per can. The comparable amount of fresh or frozen tuna steaks comes in at 65 mcg.
Putting It All Together
For our above average 160 lbs adult this means that the safety margin is:
- half a can / week of albacore tuna or
- 1 1/2 cans of light tuna per week or
- about 90 g / week (around three ounces) of fresh or frozen tuna
This of course provided that you don’t eat any other fish at all. Many fish are contaminated with mercury and some are way more than tuna (eg. swordfish who have 0.98 mg / kg, or tilefish, coming with 1.45 mg / kg). Therefore, if you eat fish in other forms than tuna, you would have to add that to your weekly mercury level.
Also keep in mind that for children and pregnant women the above tolerable levels are only half of what is stated here. If you are a pregnant woman, you may want to stay away entirely from albacore tuna and eat less than a can of light tuna per week.
If all this math is too much, check out the tuna calculator the Environmental Working Group put together, where you can enter your own data and then get a fairly accurate reading about how much tuna you should subject yourself to.
Mom Was Right
So, is tuna safe to eat? In a way, mom was right: tuna should be eaten with caution. You may not entirely have to say no to it, but overdoing your weekly tuna consumption may not be a good idea.
I know that especially many bodybuilders like their tuna as an affordable and low-fat protein source and some consume it daily. That indeed might be a bit too much and you may want to ease up on the tuna a bit and consider other cheap protein sources.