Twenty micrometers between inside and outside

Keeping water in, keeping foreign matter out

When the skin is described as a barrier, two separate tasks are meant. Outward, it holds water back: without this seal an adult would lose several times what they actually release across the surface. Inward, it limits the passage of substances from the environment, from dissolved salts to larger molecules that get caught in the fatty matrix between the cells.

Both tasks depend on the same thin layer. Technical texts like to describe its construction as masonry: the flattened cells are the bricks, the lipid layers between them the mortar. Unlike a wall, however, this structure is continuously renewed. In the lowest layer of the epidermis, cells divide, migrate upward, lose their nucleus and cell organelles, and are finally shed as flakes. The journey from bottom to top takes roughly four weeks in healthy skin.

The skin is thus a tissue with a high resupply requirement. It continually builds in new protein, above all keratins, and continuously divides cells to that end. It is precisely in this context that the statement stands which the European Commission has authorized for zinc: Zinc contributes to the maintenance of normal skin — Pursuant to Regulation (EU) No 432/2012. The formulation speaks of maintenance. It says nothing about skin conditions, about symptoms, or about the effect of a particular product.

From the small intestine to the actively dividing cell

Zinc is present in the organism on the order of two to three grams, distributed across nearly all tissues. There is no storage organ such as the liver is for iron. What is taken up today is available for tomorrow's turnover; what goes beyond that leaves the body again. This balance explains why the technical literature deals so extensively with absorption and excretion and so little with stores.

ZIP4 in the brush border membrane

The passage from the intestinal contents into the mucosal cell runs via transport proteins, one of which is particularly well described: ZIP4 sits in the membrane of the cells of the upper small intestine facing the intestinal contents. If supply is scarce, more of it appears in the membrane; if it is plentiful, it is broken down. The cell therefore adjusts the throughput to the situation instead of serving a fixed quota. For this reason, figures for absorption do not settle on a single value but lie in a range of about 15 to 40 percent of the amount supplied.

A sulfur-rich binding protein limits transfer

Within the cell, a small, sulfur-rich protein binds the absorbed ions. It is called metallothionein and is itself produced in greater quantities when a lot of zinc arrives. What remains bound to it does not pass into the blood but is lost with the shed intestinal cell. This protein thus regulates the balance from the other side: it limits how much is actually passed on when intake is high. In the blood, albumin then takes over most of the transport.

Excretion runs predominantly via the intestine

Unlike many other minerals, zinc does not leave the body mainly via the kidneys. The greater share is released into the digestive tract through secretions of the pancreas and the intestinal wall and excreted in the stool. Smaller amounts are lost via urine, sweat, and shed skin cells. Anyone wishing to derive the daily requirement adds these losses together and divides them by the share that crosses in the intestine — which is why every intake value turns out higher than the pure loss amount.

For the cell itself, the significance of this element is well documented: it is a constituent of numerous enzymes and is among the factors that hold the reading of gene segments in position. This is reflected in the Union list in an entry of its own: Zinc contributes to normal protein synthesis — Pursuant to Regulation (EU) No 432/2012. This statement too refers to a normal process in a healthy organism and not to a change that could be measured or aimed at.