Four tissues the body continually rebuilds — and four separate entries that the EU register carries on the subject.
To the notesSkin, mucous membranes, retina, and hair root are among the tissues the organism does not keep for a lifetime but continually rebuilds. The epidermis replaces its cell layers in roughly four weeks. The epithelium of the small intestine needs only three to five days for this. In the hair matrix, cells divide as often as almost nowhere else in the body, and the visual cells of the retina renew part of their outer segments daily.
Where new cells continually arise, the steps preceding them also run continually: the reading of genetic information, the duplication of DNA, the formation of structural proteins. A whole series of the proteins involved carries a zinc ion within its own structure — not as fuel, but as a fixed building block without which the spatial form does not come about.
The European Food Safety Authority has assessed the data on zinc in several opinions. On this basis the EU Commission has included statements in the Union list. Four of them concern exactly the tissues at issue here. Gneisader reproduces them in the wording in which they appear in the Official Journal — without addition and without rewording.
The four entries stand side by side in the same list but were assessed separately. None can be derived from another.
The epidermis consists of several cell layers that grow up from below and are shed at the top as horny flakes. Roughly 1.8 square meters of surface are constantly replaced in this way. About a sixth of all body zinc lies in the skin — a high share for an organ that is rather light measured by mass.
Pursuant to Regulation (EU) No 432/2012
Mucous membranes line the nose, mouth, esophagus, stomach, and intestine. The inner surface of the small intestine alone reaches an order of magnitude of several dozen square meters through folds, villi, and microvilli. This cell layer is replaced faster than any other tissue — and it is at the same time the place where zinc from food enters the blood.
Pursuant to Regulation (EU) No 432/2012
In the eye, zinc collects above all in the pigment epithelium of the retina and in the choroid beneath it. Measured against tissue mass, the values there lie considerably above the body's average. In these layers work enzymes that convert vitamin A into the form the visual process consumes; several of them carry zinc as a constituent.
Pursuant to Regulation (EU) No 432/2012
A hair grows because cells divide in the matrix at the base of the follicle, are pushed upward, and keratinize there. A head hair covers around one centimeter a month along this route. The finished shaft consists almost entirely of keratin and contains no living cells — everything that constitutes its character happens deep in the root.
Pursuant to Regulation (EU) No 432/2012
The four statements of the Union list name a tissue and a function, no more. Anyone wanting to know what the assessors based this on arrives at a manageable number of biochemical processes that can be described without turning into a promise of cure.
Some of the proteins that dock onto DNA and trigger the reading of individual genes there fold around a zinc ion. After the shape of this loop the construction is called a zinc finger. If the ion is missing, the loop stays open and the protein does not find its binding site. In tissues whose cells divide often, this construction is needed particularly frequently; the basal layer of the epidermis is one example.
The visual process begins with a light quantum changing the shape of a molecule: 11-cis-retinal stretches into all-trans-retinal. For resupply to arise, retinol must be oxidized from the vitamin A store. The retinol dehydrogenases responsible are among the zinc-dependent enzymes, and the transport protein that carries vitamin A in the blood also depends on zinc. This explains why the pigment epithelium of the retina is among the most zinc-rich tissues of the body.
At the base of each intestinal villus lies a crypt in which stem cells divide. The daughter cells migrate up the villus, mature along the way into brush border cells, and are shed at the tip — the whole passage takes three to five days. In the skin the same route from the basal layer to shedding takes about four weeks. Both tissues share the same construction of densely packed epithelial cells but work on entirely different time scales.
The cells of the hair matrix produce large quantities of keratin, a sulfur-rich structural protein. Because new amino acid chains must constantly be assembled and subsequently cross-linked for this, the follicle is among the sites with particularly brisk protein turnover. How much zinc is turned over in the process can be read from the finished hair only to a limited extent: hair analyses reflect the supply of the preceding months and are not a snapshot value.
In all four cases the word maintenance appears. It describes holding a normal state, not increasing it. And normal means the usual state of a healthy person, not a target value and not a promise. Claims about symptoms, about individual clinical pictures, or about an expected effect are not covered by the four entries and therefore do not appear on this page either.
The print edition sets out the four entries individually, with a portion table, an age scale, and a chapter on what the regulation expressly does not permit.
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