The Mercury Experiment

Despite there being scientific evidence that mercury in tuna does not cause any harm to human health, tuna consumption continues to be associated with dangerous side effects that overexposure to the toxin can lead to. Is this concern justified, or is the general public being misinformed?

To find out for ourselves, we conducted an experiment that involved eating daily meals containing canned tuna and analyzing the effect that it has on mercury levels in blood, and any changes to wellbeing. Before we share the details, it is important to have some background information.

First and most important, how much mercury, according to existing regulations, is a dangerous amount to be exposed to? Most countries permit a maximum of 1.0 mg/kg of mercury in tuna.

The tunas with the highest content of mercury are fresh bluefin and albacore, with around 0.48 mg/kg, but canned albacore has a lower ratio. Fresh skipjack and yellowfin have between 0.27mg and 0.43mg/kg, and overall, canned products have less.

How did tuna get to be associated with mercury?

In Minamata Bay in the 1950s and 60s, there was an extreme case of chemical poisoning that found its way into the food-chain via fish. Those that ingested polluted seafood had mercury levels of 705 mg/kg; nearly 1,500 times higher than one would ever be exposed to from eating tuna.

Stories like this have been used to propagate a link between tuna and mercury, despite this being tenuous. Scientific evidence offers an explanation that tuna contains negligible levels of mercury and that excessive exposure to most minerals and compounds is likely to cause problems for the human body.

Who or what is responsible for consumers not getting the above message?

There are numerous NGOs and organizations that are accused of spreading misinformation, or not telling the real facts where tuna is concerned.

The US’ National Fisheries Institute (NFI) has created the ‘Truth Squad’, which it dedicates a page to on its website, and contains categories of organizations it believes have misled the public.

Part of the list is reserved for organizations that have incorrectly reported on the link between tuna and mercury. For example, there is a category dedicated to the Sierra Club; an environmental NGO that associated tuna, mercury and coal emissions, to lobby for reform in the fossil fuel industry, with slogans such as: “Is there coal in your tuna? Hint: it’s in the mercury”.

In addition to this, the Sierra Club targeted pregnant women with posters that read: “This little bundle of joy is now a reservoir of mercury.”

The Truth Squad page reads that the Sierra Club “are not being truthful when they attribute trace amounts of mercury in commercially fished seafood like skipjack, yellowfin and albacore tuna to mercury emissions from coal-fired power plants.”

With pregnant women understandably on high alert for anything that might damage the development of their infants, campaigns such as the ones led by the Sierra Club, which despite having little or no accountability to prove their assertions, have been particularly effective in damaging the reputation of tuna.

Weariness over the side effects of overexposure to mercury has perhaps been solidified by governmental recommendations, such as those previously coming from the US Food and Drug Administration (FDA).

It had advised of a maximum intake of some species of tuna and other fish, but had failed to advise that dropping below two or three portions of seafood a week is dangerous for health. In 2014, it revised its recommendations to include a minimum amount of seafood that should be eaten; two or three portions each week.

There has been a lack of information from health standards agencies and members of the industry to adequately explain the nutritional contents of tuna. For example, there are high levels of omega-3 fatty acids like DHA present, which is essential for brain development, and cannot be synthesized in high enough quantities by the body.

With the FDA aware that its previous position had encouraged many, not just pregnant or nursing women, to avoid seafood containing mercury out of caution. Pregnant women and infants are still advised to avoid tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel; but tuna is considered to be safe.

The difference between the previous advice and the current advice is that now pregnant women are advised to eat a minimum of 225-340 grams of seafood per week. Further to this, the same agency informs that they can even eat 1.5kg of canned albacore or 4.6kg of canned skipjack per week before any adverse effects may arise.

The risk in avoiding tuna and other seafood can be evidenced in a study by Harvard University that estimated that there are around 84,000 preventable deaths per year as a consequence of low omega-3 intake.

Further to this, evidence that supports the claim that fish meals, which include tuna, are important for brain development can be found in a study that began in the UK in 1990, called the Avon Longitudinal Study of Parents and Children (Alspac). There were 14,000 pregnant mothers enrolled in the study, which were monitored post-pregnancy and into their children’s adulthood.

It was found that the mothers who ate more seafood than was considered safe according to US guidelines, had children that were more advanced in terms of social skills and verbal IQ. Conversely, it was found that the mothers that ate no fish had children that were 44 percent more likely to have poor social behavior aged seven, as well as 48 percent more likely to have a low verbal IQ aged eight.

Although the study was not specifically focused on tuna, the popularity of this fish suggests that it is highly likely the results can be used to support health benefit claims. For example, as tuna is the second most commonly consumed seafood in the US, there is a strong possibility that a seafood portion in a meal will be tuna.

Perhaps the most important aspect of the mercury/tuna association that is often overlooked is that tuna contains selenium. Selenium is a key mineral that not only attaches to mercury and neutralizes its poisonous effects, it is also vital to brain function. There are 25 selenium dependent enzymes that are important in brain tissues. As Nicholas Ralston’s graph shows, the ratio of selenium to mercury in tuna is very favorable.

As can be seen in this graph, warnings over pilot whale and mako sharks can be understood in two ways; first the mercury levels dwarf those that are in species of tuna, and also have far inferior levels of selenium. So while it is important to ask “how much mercury is in my food?” perhaps we should insist on asking “what is the mercury/selenium ratio in my food?” immediately after.

Considering the ratio of mercury to selenium in skipjack, and the other health benefits that are associated with tuna, we were quietly confident an experiment would support the hypothesis that canned tuna is not at all a risky food substance.

Jeff, our media design guy, volunteered himself for the experiment.

In an effort to make the test as robust as possible, we set the following conditions: a can of 185g net (130g drained) skipjack (chunk light) tuna must be consumed every day, for a period of at least a month.

If widespread opinion that eating large quantities of tuna will lead to mercury poisoning are to be believed, we would expect that this quantity would have adverse effects on Jeff. Even the Environmental Defense Fund (EDF) in the US advises of canned skipjack, “older children and adults can safely eat it once a week”.

Despite this advice, Jeff pledged to eat seven times the weekly recommended allowance of tuna, for more than five weeks (40 days). At the start of the experiment, we went together to the hospital to have his blood tested for the level of mercury.

The hospital’s initial blood test results revealed that Jeff’s blood contained less than 5.0nm/l (nanomole/liter) of mercury; this can be converted to less than 1.5ng/ml (nanogram/mililiter), or 1.5 micrograms/liter. It has been indicated in many regulations that significant exposure to mercury is reached when blood levels exceed 50ng/ml.

As the days wore on, there were no notable changes to Jeff’s mood, or ability to keep our office ticking over. He stayed on top of his other working responsibilities and was quite pleased to receive a free can of tuna per day, in return for offering himself as a guinea pig.

Each day it was most common for tuna to be eaten at lunch time, in either a baguette or as part of pasta salad, and personal preference meant that the can was always in brine. With no headaches, muscle problems, hair or memory loss, it was evident that there were no physical or mental side-effects that are associated with mercury poisoning.

Already a reasonably healthy eater, the addition of a daily portion of tuna did nothing to destabilize Jeff’s diet.

Finally after 40 days, the second and last blood test came around, and the results are shown in the picture above.

Again, the results of the blood test showed that there was less than 5.0 nm/l of mercury in Jeff’s blood.

A can of skipjack a day, for 40 days, had zero impact on the mercury levels in Jeff’s blood. There wasn’t even a negligible increase.

We suspect that selenium is the reason behind Jeff’s mercury levels remaining constant.

Unfortunately, the tuna industry hasn’t been effective enough at combatting the erroneous message that tuna contains a dangerous level of mercury, and has been even less effective at promoting the presence and benefits of selenium.

Instead, more effective has been the prevailing narrative described by Gavin Gibbons of the National Fisheries Institute. He told Atuna: “Despite this overwhelming evidence (regarding the health benefits of tuna), groups like the Mercury Policy Project, Consumers Union and Environmental Working Group, coupled with marginalized activists like Ned Groth, continue to push, a narrative that attacks tuna. They are not only out of step with mainstream science they’re clearly doing more harm than good.”

He went on to state that the reason that tuna suffers the worst from this association is because its popularity makes it easy to target.

Perhaps an example of just how ineffective the industry has been at fighting against the mistruth regarding mercury and tuna is the recently founded firm SafeCatch. Here is a company that centers product proposition solely on the testing of each tuna for levels of mercury and selling albacore tuna with a lower content of mercury than the industry average.

Its website makes no mention of the selenium factor, and while building a business that attempts to offer security to insecure consumers makes sense, could abiding by a false narrative rather than confronting it reinforce the idea that tuna has a mercury problem?

It seems important then, that not just for the industry, but for consumers who are potentially missing out on vital nutrients, that more effort is made to communicate the wider truth, and ensure the mercury myth is placed in the past. ■