Tuna is an incredibly healthy source of protein and micronutrients and is especially useful for anyone looking to improve their body composition to build more lean muscle. Not only is tuna a fantastic lean source of protein, but it’s also high in omega 3 fatty acid which is great for your cellular energy and your brain as well as serving as an antioxidant. Best of all, it’s also cheap, delicious and versatile.
In short, tuna is something of wonder food that seems like the perfect choice for anyone trying to improve their general health.
But then there’s that whole ‘mercury poisoning’ thing that comes along and ruins it. So what’s the deal? Will eating tuna in high quantities really put you at risk of mercury poisoning?
The Tuna/Mercury Connection
Mercury, like zinc or iron, is a heavy metal. Unlike those two micronutrients though, mercury has no use in the body and thus isn’t something you should be seeking out in food. Unfortunately though, mercury isn’t just inert in the body either but actively damaging: causing damage to the brain and kidneys. It lasts about three days in the bloodstream and has a 90 day half-life in other tissues including the brain. When you consume mercury, it will be absorbed by your small intestine and then on to the liver where it will form a ‘complex’ with glutathione. This complex will then either be incorporated into the bile to be excreted, or into the blood. In the blood it will travel to the kidneys and from there onwards it gets excreted or stored.
Fortunately, our kidneys contain a protein known as ‘metallothionein’, which synthesizes mercury to store it in a nontoxic form. As long as you don’t overwhelm your body with mercury, it will remain in this form, but if you consume too much, it risks crossing the blood-brain barrier to get stored in the brain. Alternatively it might cause toxicity, damaging the kidneys. This is mercury poisoning.
Symptoms of Mercury Poisoning
Mercury poisoning can cause sufferers to act unusual. Because methylmercury is similar in structure to something called ‘methionine’, it is able to cross the blood-brain barrier. This can then end up affecting the brain in ways that might lead to ‘unusual’ behavior. Other symptoms include mood swings, nervousness, insomnia, headaches, ‘abnormal sensations’, tremors and muscular atrophy.
On the other hand, the transport of mercury across the blood-brain barrier can be inhibited by some amino acids including methionine, phenylalanine, leucine and other ‘large neutral’ amino acids.
How Much Tuna Is Safe?
Unfortunately, tuna contains very high amounts of mercury due to water pollution. The question is, is this content enough to cause problems? And how much would you have to eat in order to start seeing this issues arise?
The bad news, is that reports are very mixed when it comes to advice on tuna consumption. The CDC published a report called ‘Toxicological Profile for Mercury’ (1), and this report suggested that you could safely eat .003mg of tuna per kilogram of mercury every day without noticing any negative effects. This equates to just over one can of tuna per day for a 200lb man.
Other sources though, such as the Environmental Working Group (2), states that a 200lb man should only eat three cans a week – three to five cans less.
In another study, conducted by Sherlock et al in 1984, (reported in this study) it was found that after a year of consuming mercury-containing fish regularly, participants would reach ‘mercury saturation’ after which point chronic exposure would cause no increase in mercury accumulation. If this study is correct, then that would suggest there’s no read to worry. Another study by Cherian et al in 1978 supported this by showing an increase in mercury excretion during chronic exposure. Of these sources, the CDC offered the most comprehensive report of over 650 pages, but it’s still not clear what the best advice is.
So, How Should You Approach Tuna?
With all this in mind then, what’s the best way to approach tuna consumption?
Unfortunately, whatever advice we take at this point, it will have to be with no real certainty. Thus, the approach you take will have to ultimately depend on your attitudes. Some people will prefer to go the ‘better safe than sorry’ route and to avoid eating copious amounts of tuna until further information arises.
On the other hand, if you are seduced by the considerable benefits of tuna and its low price point, you may decide to throw caution to the wind. Eating up to 5 cans a week may well be perfectly safe and personally I have been eating about that much for the last 20+ years with no notable symptoms.
If you do decide to continue eating tuna despite warnings, you can reduce your chances of mercury poisoning by eating a diet high in amino acids and selenium, which has also been shown to be protective against mercury poisoning (3).