Experts recommend limiting tuna consumption, but it is not clear that the appropriate amount is two tins per week

You sent us an article claiming that tuna consumption should not exceed two tins per week due to the presence of mercury in oily fish. This claim is MISLEADING. It is true that experts recommend moderating tuna consumption, but it is not clear that we should limit ourselves to two tins given that they might have different net weights and, as a result, a differing quantity of methylmercury. People who are pregnant and children under 10 years old are advised against eating tuna because it can cause problems in the development of the nervous system.

Experts recommend limiting tuna consumption to two tins weekly (…). The presence of mercury requires us to moderate our consumption of tinned tuna.

Tins of tuna contain, on average, 0.26 mg of methylmercury – the chemical form of mercury that is most toxic and easily absorbed by living organisms – per kilogram of tuna, according to a study by the Spanish Agency for Food Safety and Nutrition (AESAN). An individual person’s weight is a key factor when considering how many grams of yellowfin tuna (the generic name given to tinned tuna) we can eat every week (the higher the weight, the higher the tolerance), but other conditions such as age have an influence as well.

Safe intake levels of methylmercury

The highest concentrations of methylmercury are concentrated in the species located at the highest trophic level of the food chain, i.e. in the large predators such as sharks, swordfish and tuna.

Most people have blood mercury levels below the minimum levels associated with possible effects on health, but methylmercury is a strong neurotoxin. To avoid possible methylmercury poisoning, the European Food Safety Authority (EFSA) established a report in 2018 stating that the maximum recommended weekly intake for the general population is 1.3 micrograms of methylmercury per kilo of body weight.

Most of the methylmercury ingested is eliminated by the body via the digestive system as inorganic mercury, while a small percentage of inorganic mercury is absorbed and distributed to the tissues. “What effects chronic exposure to a low concentration of methylmercury has on human health remains controversial”, as is indicated in a 2012 study published in the Journal of Preventive Medicine and Public Health, courtesy of the Korean Society for Preventive Medicine.

The danger of mercury in minors

However, this recommendation changes for people who are pregnant or breastfeeding and minors under the age of 10, who should avoid consuming fish that could contain high quantities of methylmercury (swordfish/emperor fish, bluefin tuna [Thunnus thynnus], shark and river hake). That’s because methylmercury mainly affects the growing central nervous system, so the population groups that are most sensitive to this metal are children and fetuses (due to indirect exposure via the placenta during pregnancy). “The highest risk is during pregnancy due to the effects on the developing nervous system – and, by extrapolation, it is assumed, in small children, who are also developing. Above all, it affects the ability to learn, the intellectual level and probably the senses (hearing, sight)”, Carlos Rodrigo Gonzalo de Llíria, Regional Clinical Director of the Pediatric Area of the Germans Trias y Colina University Hospital, explains to Verificat.

Nutritional recommendations

The AESAN recommends that the general public consume fish between three and four times a week. Oily fish usually have higher levels of contaminants such as methylmercury, as stated by the United Kingdom National Health Service (NHS). As a result, the recommendation would be to avoid eating more than 2 portions of oily fish each week. As far as white fish goes, the same website explains that it can be eaten safely without any specific recommended limit. The recommendation of eating fish 3-4 times a week would thus be more related to the variety of foods included in the diet.