Recent investigations conducted in Europe and the USA reveal widespread mercury contamination in canned tuna, which is widely consumed and appreciated – like all other fish species, algae and microalgae – as a source of essential Omega-3 fatty acids.
The levels of contamination allowed in tuna in the European Union, among other things, are much higher than those established for other fish species. Inexplicably, since tuna is the most consumed fish in the Old Continent. (1) What is the risk-benefit ratio?
1) Omega-3, introduction
Omega-3s are polyunsaturated fatty acids essential for the health of the cardiovascular and nervous systems, in addition to performing various other beneficial functions. (2) However, the human body is not able to synthesize them and must therefore take them through the diet. Their main sources are:
– nuts (e.g. hazelnuts) and seeds (e.g. flax, hemp, soybean and rapeseed oils), which contain alpha-linolenic acid (ALA), a precursor to Omega-3. However, the human body is unable to synthesize ALA into Omega-3, except in very small quantities;
– fish, crustaceans, molluscs, algae and microalgae. It is important to underline that marine sources of Omega-3 are the only ones to offer an adequate intake of EPA and DHA (eicosapentaenoic acid and docosahexaenoic acid).
Regular intake of fish, crustaceans, molluscs, algae and microalgae and/or food supplements derived from them is therefore essential to obtain an adequate intake of EPA and DHA, in all stages of life. (3)
2) Fish and mercury, benefits and risks
Fish is therefore a primary source of Omega-3 fatty acids (EPA and DHA), as well as high biological value proteins – that is, with a complete supply of essential amino acids – and micronutrients (vitamins and minerals). Its regular consumption is encouraged in all dietary recommendations and contributes on average to one fifth of the animal protein intake in 20% of the world’s population. And yet:
– fish consumption is simultaneously a primary cause of human exposure to methylmercury (MeHg)-a form of mercury, along with inorganic mercury-due to anthropogenic pollution of seas and surface waters;
– mercury (Hg) is a heavy metal, neurotoxic, which accumulates in tissues. MeHg, already classified by IARC as a potential carcinogen, is the subject of much research also in relation to the risks for the cardiovascular and endocrine systems. (3)
3) The fatty fish dilemma
Omega-3 intake in fish comes mainly from the algae and microalgae that they consume (also through other fish, in the case of predators). The levels of mercury contamination in turn depend on the levels of pollution in the seas and on the life, more or less long, of the fish. Thus:
– fatty fish are extraordinarily rich in Omega-3, up to 1.790 mg (EPA+EPA)/100 g for mackerel, 1.590 for salmon, 1.103-1.504 for fresh bluefin tuna (from the Atlantic and Mediterranean seas); 862 for canned white tuna (albacore), only 128-270 for slipjack (preserved in water and in oil, respectively). (4) And yet
– the predators are contaminated by mercury (Hg) at high levels. 0,24-0,48 mg/100g is the overall average for tuna and 0,35-037 for canned tuna (albacore), equal to three times that of slipjack (0,11/0,12). And it is even higher in other predators such as swordfish (0,98-1,03), migratory mackerel (king mackerel, 0,73-1,06), shark (0,75-0,99). (5)
4) Canned tuna, the survey in 5 European countries
The survey conducted by the non-profit association Bloom in 2024 on mercury levels in 148 canned tuna purchased in five European countries (England, Germany, Italy, France and Spain) revealed that:
- the 100% canned tuna tested by an independent laboratory revealed the presence of mercury;
- the 57% of the examined preserves has a mercury content higher than 0,3 mg/kg, confirming the average levels indicated in the literature; (5)
- the 10% of the packages exceeded the food safety threshold established in the EU for canned tuna (1 mg/kg);
- a sample of canned tuna, purchased in a Carrefour supermarket in Paris, reached the extraordinary level of 3,9 g mg/kg. (1)
5) ‘Canned tuna’, the investigation in the USA
Consumer Reports in turn analyzed the mercury levels in 30 samples of canned tuna from the five most popular brands in the USA, in 2023. (6) The analyses confirmed that:
– Albacore tuna contains three times more MeHg, on average, than skipjack tuna, regardless of brand. This is not surprising, since slipjack tuna is a larger fish, lives longer, and thus accumulates more contaminants in its tissues;
– the average levels of contamination of canned albacore tuna (the most common) are such as to advise against the consumption of more than one 4 oz. (113 g) serving of drained fish each week by adults. With total abstention for pregnant women and children;
– the unpredictable peaks of the contaminant contained in individual cans of albacore tuna, found in 20% of the samples analysed, ‘are such as to suggest the revision of the FDA recommendations on the frequency of consumption of that particular tuna’. (7)
6) Mercury in fish, the thresholds allowed in the EU
The investigation of the Bloom Association (2024), as we have seen, included the examination of a hundred official documents on mercury contamination and food safety, published by various international organizations (FAO-WHO, EFSA, European Commission). And it came to the conclusion that:
– risk analysis of food safety which must preside over the definition of maximum thresholds of contaminants ignores the levels of exposure and thus the health of adults and children (as well as pregnant women), taking into account that tuna is the most consumed fish species in Europe;
– it is unjustified, therefore, to set a contamination threshold (1 mg/kg) for tuna (and other large predators, i.e., swordfish) more than three times higher than that (0.3 mg/kg) set for other fish species (e.g., anchovies, cod, plaice, salmon, sardines, mackerel). (8,9)
7) Protection needs
‘It is necessary to review the scientific evidence that forms the basis of dietary and public health advice on how to balance the benefits and risks associated with fish consumption’ (Mahaffey et al.,2011).
Children and women of childbearing age are the most vulnerable subjects, where the results of numerous studies have demonstrated that:
– MeHg body burden levels in many populations around the world exceed thresholds previously identified as ‘tolerable’;
– there is a clear correlation between the mercury levels of pregnant women and children and their defective cognitive (IQ), language and motor skills development. (3)
8) What fish should be eaten?
The choice of canned fish with higher Omega-3 contents and lower contamination levels it’s simple:
– among canned fish, the podium goes to mackerel, followed by salmon and sardines (980 mg EPA+DHA/100 g). Skipjack tuna (often promoted for pole fishing) has lower Omega3 levels than the others, but the MeHg levels are low;
– farmed trout fillets, among the refrigerated preparations, are an ideal choice for their delicate flavour accompanied by the richness in Omega-3 (580 mg/100 g) and the low quantity of mercury;
– fresh and frozen or deep-frozen fish to be favored are those of medium-small size, preferably caught in seas not in front of areas of high industrial intensity. Among the many, we can mention anchovies, hake, plaice, etc. (10)
Dario Dongo
Credit cover Le dessin de Chaunu : le thon n’a plus la pêche. Western France. 31.10.24 https://tinyurl.com/mr78uyby
Footnote
(1) Dario Dongo. Mercury alert in canned tuna. GIFT (Great Italian Food Trade).
(2) Marta Strinati, Dario Dongo. Omega-3, anti-inflammatory action that also benefits the muscles. Scientific review. GIFT (Great Italian Food Trade).
(3) Mahaffey KR, Sunderland EM, Chan HM, Choi AL, Grandjean P, Mariën K, Oken E, Sakamoto M, Schoeny R, Weihe P, Yan CH, Yasutake A. Balancing the benefits of n-3 polyunsaturated fatty acids and the risks of methylmercury exposure from fish consumption. Nutr Rev. 2011 Sep;69(9):493-508. doi: 10.1111/j.1753-4887.2011.00415.x
(4) Mahaffey KR. Fish and shellfish as dietary sources of methylmercury and the omega-3 fatty acids, eicosahexaenoic acid and docosahexaenoic acid: risks and benefits. Environ Res. 2004;95:414–428. doi: 10.1016/j.envres.2004.02.006
(5) Sunderland EM. Mercury exposure from domestic and imported estuarine and marine fish in the US seafood market. Environ Health Perspect. 2007;115:235–242. doi: 10.1289/ehp.937
(6) FDA and EPA issue final fish consumption advice. FDA (Food and Drug Administration), EPA (Environmental Protection Agency). January 18, 2017 https://tinyurl.com/yacpt55z
(7) Lauren Kirchner. How Worried Should You Be About Mercury in Your Tuna? ConsumerReports. February 9, 2023 https://tinyurl.com/ywnaj6e4
(8) Dario Dongo, Alessandra Mei. Food Contaminants Regulation (EU) No 2023/915. FT (Food Times). July 28, 2023
(9) Regulation (EU) No 2023/915, Annex I, points 3.3.1.2, 3.3.1.3
(10) Loredana Spagnardi, Dario Dongo and Alessandra Mei. #poverimabuoni. Savings and health in blue fish. GIFT (Great Italian Food Trade). 14.5.20
Dario Dongo, lawyer and journalist, PhD in international food law, founder of WIISE (FARE - GIFT - Food Times) and Égalité.
