They are newspapers appeals from the Spanish health and food authorities to moderate the consumption of tuna, emperor and to a lesser extent other oily fish. Also to refrain in the case of pregnant women, women breastfeeding and children under 10 years of age.
Ten simple tips to eat fish while avoiding heavy metals
The reason is none other than mercury –although also to a lesser extent other heavy metals–, a mineral that accumulates in our fatty tissues and once it reaches a certain amount, it can be toxic to the body. It’s the same reason mercury thermometers were banned.
Organic and inorganic mercury
Mercury can occur in two forms in fish. The first is like inorganic mercury, which mainly affects the kidneys; but also –when it accumulates in excess– it affects the liver, the nervous system, the immune system and normal development in the case of minors.
However, the inorganic form is not as present in the tissues as the second form is, that is associated with a methyl radical. This form has a great affinity for fats and therefore accumulates in oily fish, especially in those that come from cold climates, such as the North Atlantic, since it accumulates more fat in its meat.
Methylated mercury attacks the central nervous system during the development of both the fetus and children under 10 years of age: hence the AESAN recommendations. In addition, it can promote obesity, although it is true that its fat is highly recommended due to the good ratio of omega 3 and 6.
The problem with this methylated form is that it is absorbed very quickly and passes into our tissues. In the case of pregnant women, is able to cross the placenta and reach the fetus interfering with the development of their nervous system. Consequently, this is the most dangerous way.
The bigger the fish, the more mercury
There is an unwritten but no less real mnemonic rule, which says that “the larger the fish, the more mercury it contains in its meat”, obviously expressed in parts per million, or what is the same, milligrams per kilo of meat.
There is an explanation for this, and it is that mercury can come from industrial pollution and is found in large deposits in the bottom of the sea in suspension.
From there it passes to the marine seedling when it filters the water and when it is eaten by other higher organisms, it passes to them, such as molluscs, cephalopods, etc. These are devoured by older beingsespecially fish, which in turn are eaten by other larger fish.
And so on until you reach the sharks, the large tuna, and other beasts of the sea such as the grouper, etc. but above all we must look at the bluefin tuna and the emperor due to its high consumption.
Mercury passes from one to another, and especially if the fish eaten is oily fish, since being more fatty absorbs mercury better and accumulates it, a phenomenon known as bioaccumulation.
Thus, the bluefin tuna, the emperor, etc., which has eaten many sardines, for example, accumulates considerably more mercury than mackerelfrigate frigate birds or bonito, all of them smaller blue fish species.
The problem is that, despite the fact that normal intakes of mercury in humans are not usually toxic, since mercury can be expelledthis expulsion occurs slowly, so it is convenient to moderate the entrance of this metal in the body so that it is not greater than its exit.
We must bear in mind that significantly higher levels of mercury have been detected in various tissues and organs of the Spanish population than in others in the European Union, probably due to a higher frequency of consumption at all ages.
Tuna, fresh or canned?
Here we come to the question that gives title to this article: is it the same with respect to the amount of mercury ingested eating fresh tuna what to do from a can? Obviously at the level of qualities and textures it is not the same and fresh tuna is always better, of which we can also know and control the quality.
The can allows us to eat tuna without absorbing so much mercury for the simple reason that the pieces used to make preserves are the smallest
However, when it comes to mercury strictly, the can allows us to eat tuna without absorbing as much mercury for the simple reason that the pieces used to make preserves are the smallest, reserving the large ones for the fresh market.
Thus, the smaller tuna would have eaten fewer fish and therefore would have bioaccumulated fewer parts per million of mercury in the fats – which are otherwise very healthy – in their meats. On the other hand, the large pieces, having eaten more fish, would have accumulated their mercury.
And the same if the can is tuna, which in addition to being more expensive would contain less mercury because tuna is a smaller animal.
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