Mercury is a heavy metal that 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 human action in numerous industrial activities.
Mercury, once released, undergoes a series of transformations and cycles in the atmosphere, oceans and soil, and can occur in three different forms: 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 Miguel Angel Lurueña explains in his book Que no te líen con la comida, methylmercury is present in sea water and accumulates in the adipose tissue of fish: especially if they are large species, because they are acquiring the one that was in the smaller species from which they feed.
And what happens if we consume too much mercury? Food technologist Beatriz Robles explains it well in her book Come seguro comiendo de todo: as mercury dissolves well in fat, it can cross the placenta and the blood-brain barrier, the protection we have between the circulating blood and the brain tissue, affecting the central system.
Adults 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. This is why it is recommended that pregnant women and infants avoid consuming certain fish.
Which fish to be careful with
There is a widespread belief that mercury accumulation is a problem specific to oily fish, but in fact, as the European Food Safety Agency (EFSA) explains, it is not associated with oily fish. It is true that the fish with the most mercury are oily fish, with more fat than white fish, but there are many other oily fish whose mercury levels are low, such as sardines or salmon.
What really distinguishes fish with more mercury from those with less is their size. As a general rule, the larger a fish is, the longer it lives and the higher up the food chain it tends to be, therefore, it feeds for a longer time on other species and, in the long run, accumulates more mercury.
The fish with more mercury, on which health recommendations are extended, are framed in four species:
- Swordfish or emperor
- Bluefin tuna (Thunnus thynnus)
- Shark, which includes fish such as dogfish, mako shark, spiny 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: the consumption of up to three or four servings of fish per week is advised, trying in all cases to vary the species between white and blue fish.
For the vulnerable population (pregnant women, women planning to become pregnant or breastfeeding and children up to 10 years of age), consumption of the four species with high mercury levels should be avoided.
Children between 10 and 14 years of age should limit consumption of these four species to 120 grams per month.