Should we be worried about mercury in tinned tuna?
How much tuna is safe to eat and should we be concerned about eating other types of fish? By Sue Quinn French marine conservation group Bloom made headlines late last year with a report that found 100% of the 148 cans of tuna sampled from France, Germany, England, Spain and Italy were ‘contaminated with mercury’. The findings prompted one nursery group with sites in England and Wales to take tuna off the menu. But does tinned tuna really contain ‘toxic’ amounts of mercury as the report suggested? In response to the findings, the Food Standards Agency (FSA) published fresh guidance about tinned tuna to reassure the public it was safe to eat if consumed within recommended guidelines. Maximum safe levels of mercury in tuna were set out in law and kept under review, the FSA said. It pointed out that of the 30 UK products Bloom tested, only one ‘might’ have exceeded the legal mercury limit, suggesting that most UK supermarket tuna complies with safety standards. However, the report did not show individual results or how the mercury levels were calculated, so there was no way to know for sure whether the product did exceed the legal limit, the FSA said. Why is there mercury in tuna? Mercury is a heavy metal that enters the ocean through natural events like volcanic eruptions, and human activities such as coal burning and industrial waste. Once in the water, mercury is transformed by bacteria into methylmercury, the toxic form that builds up in fish. “All marine organisms will contain mercury,” explains Dr Andrew Turner, Associate Professor in Environmental Sciences at Plymouth University. "The further up the food chain you go, the more mercury is accumulated, so when you get to the top, you find the highest levels, including in tuna but also in species like swordfish and shark.” Larger, older fish accumulate more mercury over time, while smaller, younger fish tend to have lower levels. Predatory species such as swordfish and shark contain the highest concentrations due to their position at the top of the marine food chain. On Morning Live Dr Xand explored whether we should eat less tuna due to the mercury content. Watch Morning Live weekdays on BBC One at 9.30am. Or catch up on BBC iPlayer Mercury can be found in other foods but the World Health Organisation, which considers mercury one of the top ten chemicals of major public health concern, says humans are mainly exposed through fish and shellfish. To regulate the amount of mercury humans ingest, authorities have established legal limits for the mercury levels in fish. In the UK, tuna and other large predatory fish the limit is 1.0 mg/kg, while for smaller species like cod, it is 0.5 mg/kg. In Europe, some of the smaller species have a limit of 0.3mg/kg. Why is the mercury limit higher for tuna? “It may seem counterintuitive that tuna has a higher permitted mercury limit than cod,” says Alex Ruani, researcher in nutrition science education at University College London and chief science educator at The Health Sciences Academy. “But the EFSA and the FSA set these thresholds based on risk assessments, considering both the nutritional value of fish and the potential health risks.” Tuna provides nutrients including vitamin D, vitamin B12, iodine, and selenium, which contribute to overall health. “So, instead of imposing a limit so low that it would make tuna less available, authorities set a threshold that more realistically reflects its higher mercury levels while aiming to keep it a practical and nutritionally valuable option.” Some experts argue that the UK’s legal limits for mercury in fish need reviewing. “The current thresholds are based on scientific advice that is over two decades old,” says Professor Parvez Haris, a biomedical scientist at De Montfort University and a Fellow of the Royal Society for Public Health. “In light of the Bloom report and the concern it has created, I believe it is time to do more research and produce an updated guideline for mercury exposure from fish and other products.” Should we limit our tuna intake? In the UK, the NHS recommends that those who are pregnant or trying to conceive should limit their intake of tuna to no more than four cans (based on a medium can with a drained weight of 140g) or two tuna steaks (140g each) per week. (The recommended amounts are higher for tinned than for fresh, as tinned tuna can have lower levels of mercury. This is because it comes from smaller species and younger fish who have had less time for mercury to accumulate, according to the FSA.) In the UK, a 2018 report into mercury in the diets of very young children did not identify any concerns, according to the FSA. However, Haris believes recommended limits should be introduced for young children, as well as adults. “Ideally, we should reduce our exposure to mercury as it is a toxic substance which is not needed for healthy body functions. This will empower people to make informed decisions,” he says. In Europe, the EFSA has set tolerable weekly intakes (TWIs) of mercury for children due to concerns that those in high fish-consuming regions might ingest unsafe amounts. Ruani says the EFSA’s limits for children are equivalent to: Ages 1-3 years: No more than one small can (70g drained weight) per week.Ages 4-6 years: No more than one medium can (140g drained weight) per week.Ages 7-12 years: Up to two medium cans (140g each) per week, while also including lower-mercury fish in their diet. Ages 1-3 years: No more than one small can (70g drained weight) per week. Ages 4-6 years: No more than one medium can (140g drained weight) per week. Ages 7-12 years: Up to two medium cans (140g each) per week, while also including lower-mercury fish in their diet. Fish pie is a great way to eat a variety of different fish Should we worry about tinned tuna? The FSA regularly monitors mercury levels in food in the UK, while businesses are responsible for ensuring their products meet legal requirements. Local authorities also carry out sampling and testing, with any products found to exceed the permitted mercury limits subject to investigation and removal from sale. To help consumers make informed choices, Haris advocates clearer labelling on tuna tins. “Ideally, it would be good to have labels that provide information on mercury content so consumers are aware of what they are consuming,” he says. Mercury levels in tuna vary according to the species, where it was caught, and its diet, so Dr Turner recommends looking out for skipjack. “It can contain half the level of mercury found in some other varieties of tuna,” he says. According to the Marine Stewardship Council (MSC), almost 90% of tinned tuna sold in the UK and Ireland that carries the MSC mark is skipjack. The bottom line “Canned tuna remains a nutritious and accessible food option,” Ruani says. “However, consumers who eat tuna frequently should be mindful of their mercury intake.” To reduce exposure, she recommends eating a variety of lower-mercury fish such as salmon, sardines, cod, and mackerel (but not King mackerel, as it has high mercury content, often comparable to tuna). “While tuna products are strictly regulated in the UK, consumers – especially those who are pregnant or looking to conceive, and children – should prioritise a mix of smaller fish species to reap the nutritional benefits while keeping mercury exposure in check.” Originally published February 2024 Want more? Visit BBC Food on Instagram, Facebook and Pinterest. Want to watch the latest Food TV programmes? Visit BBC iPlayer.