Mercury is a heavy metal which is part of the natural composition of the earth’s crust. It is released into the environment through natural processes as a consequence of volcanic activity and rock erosion, but also due to the action of man in numerous industrial activities.
Mercury, once released, undergoes a series of transformations and cycles between the atmosphere, oceans and soil, and can occur in three different ways: metallic or elemental mercury, inorganic mercury and as organic mercury or methylmercury. The latter is the most dangerous for human consumption and is precisely the one that accumulates in fish.
As the food technologist explains Miguel Angel Lurueña In his book Don’t mess with food, methylmercury is present in seawater and accumulates in the adipose tissue of fish: especially in large species, because they are acquiring what was in the species smaller from which they feed.
And what happens if we consume too much mercury? The food technologist explains it well Beatriz Robles in his book Eat Safe Eating Everything: As mercury dissolves well in fat, it can cross the placenta and the blood-brain barrier, the protection we have between circulating blood and brain tissue, affecting the central system.
The adults they have to consume a lot of mercury for this to happen (and for a long time), but fetuses and young children are more sensitive to the substance. That is why it is recommended that pregnant women and infants avoid eating certain fish.
What fish to watch out for
There is a general idea that the accumulation of mercury is a specific problem of fatty fish, but, in reality, as explained by the European Food Safety Agency (EFSA), it is not associated with these. It is true that the fish with the most mercury are blue fish, with higher fat than the white ones, but there are many other blue fish whose mercury levels are low, such as sardines or salmon.
What truly distinguishes fish with more mercury from those with less is their size. As a general rule, the larger a fish, the longer it lives and higher up the food chain it tends to be, therefore it feeds longer on other species and eventually accumulates more mercury.
The fish with more mercury, on which the sanitary recommendations extend, are framed in four species:
- Swordfish or emperor
- Bluefin tuna (Thunnus thynnus)
- Shark, which includes fish such as dogfish, porbeagle, dogfish, dogfish and blue shark
- Pike
How much fish can we eat without worrying
The latest recommendations of the Spanish Agency for Food Safety and Nutrition, published in 2019, urge to continue eating fish, avoiding only excessive consumption of the four species mentioned above in certain groups of individuals.
For the general population: It is advisable to consume up to three or four servings of fish per week, trying in all cases to vary the species between white and blue fish.
For the vulnerable population (pregnant women, women who plan to become or breastfeeding and children up to 10 years) should avoid the consumption of the four species with high levels of mercury.
The children between 10 and 14 years old they should limit the consumption of those four species to 120 grams per month.
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