How selenium fits into the protection of the cell from oxidation

Oxidative stress — what the term describes in biochemistry

The expression comes from redox biology and denotes an imbalance between reactive oxygen species on the one hand and the organism's degradation systems on the other. Reactive oxygen species are not a foreign body: they form continuously in the respiratory chain of the mitochondria, when electrons are transferred prematurely to molecular oxygen. Estimates hold that a low single-digit percentage of the oxygen consumed takes this route.

The organism has a graduated system to match. Superoxide dismutases convert superoxide into hydrogen peroxide; catalase and the glutathione peroxidases handle the second step through to water. To these are added non-enzymatic components such as glutathione, ascorbate, and tocopherols. Only when the quantity produced exceeds the capacity of this system do specialists speak of oxidative stress.

Two of the enzyme groups mentioned work with trace elements. For the glutathione peroxidases this is selenium — and it is precisely to this that the claim authorized in the European Union relates. It describes a documented contribution to a normal physiological process, without effect beyond that.

A family of about twenty-five selenoproteins

In the human genome, about twenty-five genes are known whose products contain selenocysteine. Besides the glutathione peroxidases and the deiodinases, they include the thioredoxin reductases, which keep the cell's second major redox buffer in a reduced state, as well as selenoprotein P, which circulates in blood plasma as a transport form and helps determine the content of the brain and the testis.

This distribution explains an observation from nutrition research: when intake is limited, the content does not fall uniformly everywhere. The organism keeps certain selenoproteins topped up as a priority, while others give way first. The order of priority depends on the tissue and is described in the literature as a hierarchical supply.

Where the selenium on plates comes from

Unlike iron or calcium, the selenium content of a food depends heavily on the place of cultivation. Plants take up selenate and selenite from the soil and incorporate them non-specifically into sulfur-bearing amino acids — above all in the form of selenomethionine. Vast parts of Western and Northern Europe have selenium-poor soils, while the growing regions of North America show considerably higher contents.

For the consumer this means that the content of grain products varies considerably depending on the origin of the grain. Animal sources are more reliable, since livestock farming uses mineral feeds with a defined selenium content. Fish, offal, eggs, and meat therefore fall within a tighter range than bread or pasta.

The reference value of 55 micrograms and where it comes from

The European food information regulation sets a nutrient reference value of 55 micrograms per day for selenium. This order of magnitude was derived from studies in which the activity of glutathione peroxidase in blood plasma was measured as intake increased. Beyond a certain point it ceased to rise — this plateau served as the benchmark.

This value describes a reference figure for labeling, not an individual recommendation. The amount actually absorbed by a person depends on the composition of their diet. Anyone wishing to situate their own status should discuss it with a health professional; in the laboratory, selenoprotein P or the plasma concentration is most often used for this purpose.