Albitera · Panorama
Written out in full, the same route looks like this: which molecule holds the iodine atoms, where the daily amount comes from, which proteins hold them back in the plasma, which proteins carry them across a membrane — and at which point the official wording stops.
Iodine is a halogen and, measured against its relatives fluorine, chlorine and bromine, an unusually heavy element: atomic number 53, atomic mass around 127. Building an atom of that size into a biological signalling molecule is nothing to take for granted. The organism does it all the same, and only on one single kind of molecule.
The class of compounds in question is called the iodothyronines. Their backbone consists of two rings joined by an oxygen atom, derived from the amino acid tyrosine. The iodine atoms are tied to those rings covalently, not merely laid against them. The halogen’s share of the total mass is correspondingly high, even though only a few atoms are involved.
Two variants occur in the blood to any notable degree. Thyroxine, T4 for short, carries four iodine atoms and makes up by far the largest share. Triiodothyronine, T3 for short, carries three of them and is present in a much smaller quantity, yet holds on to its receiver in the cell far more tightly. Which of the two forms is present at a given place is settled in the tissue itself — more on that further down.
The body does not make this element itself. It arrives in the gut with food and drink, is taken up there as iodide and reaches, by way of the blood, the tissues that work with it further.
Annex XIII puts the EU reference intake of 150 µg for iodine on record, set by Regulation (EU) No 1169/2011. That European figure exists so that manufacturers can work out percentage statements on packaging. It is an arithmetical quantity for labelling in the European Union, not an amount cut to fit one person, and not the figure that appears on American packaging.
The largest amounts come from sea fish and shellfish, then from dairy products and iodized table salt. How much a single case delivers depends on the fishing grounds, on the animal feed and on the way the food is prepared; the spread between two portions of the same product is considerable.
| Food | Iodine per serving | Share of the EU reference intake |
|---|---|---|
| Pollock, 150 g | about 300 µg | about 200% |
| Cod, 150 g | about 250 µg | about 165% |
| Blue mussels, 100 g | about 130 µg | about 85% |
| Iodized table salt, 1 g | about 20 µg | about 13% |
| Cow’s milk, 200 ml | about 20 µg | about 13% |
| Plain yogurt, 150 g | about 15 µg | about 10% |
| Hen’s egg, 60 g | about 6 µg | about 4% |
Figures from nutrient tables and review articles, rounded. The table is there to place orders of magnitude and is not a meal plan.
As soon as the finished iodothyronines arrive in the blood, they enter a state seldom so pronounced with other minerals: almost everything is bound, and only a vanishingly small fraction stands at the immediate disposal of the cells.
Three proteins share this job, with very unequal properties. A specialized globulin binds firmly but has little capacity. Transthyretin lies in between. Albumin binds weakly and is present in large amounts instead. What is left at the end as the unbound fraction lies in the range of hundredths of a percent.
Bound molecules are neither broken down quickly nor sent out through the kidney. From this follows a residence time in the plasma of roughly a week for thyroxine, while the three-atom form is turned over within a day. The bound stock therefore acts like a buffer that evens out the swings of single meals — one reason why little can be drawn from any one day.
| Protein | Share of the bound stock | Binding behavior |
|---|---|---|
| Thyroxine-binding globulin | about 75% | firm binding, low capacity |
| Transthyretin | 10 to 15% | medium binding, rapid exchange |
| Albumin | 10 to 15% | weak binding, large capacity |
| unbound | under 0.05% | the fraction that reaches the cells |
Rounded textbook figures; the numbers vary with the measurement method and with circumstances of life. None of these terms appears in the legal text quoted here.
For a long time the notion held that these molecules got through the cell envelope purely because they dissolve in fat. Work of the past decades paints a different picture: the crossing is tied to dedicated membrane proteins, and which of them a tissue carries differs from tissue to tissue.
Several families have been described. One of them covers the monocarboxylate carriers, listed in the gene catalog under SLC16; a second belongs to the organic anion carriers. Other proteins take the molecules along without being specialized for them. Which cell produces which carrier protein has a say in how much actually arrives there.
Inside the cell the four-atom form is not used unchanged. Enzymes known as deiodinases take a single iodine atom off the ring. Depending on the position at which this happens, the result is the form that binds its receiver firmly, or a variant that does not. A change the size of one atom thus settles what becomes of the molecule.
| Protein | Gene name | Mainly described in |
|---|---|---|
| MCT8 | SLC16A2 | nerve tissue, liver, kidney |
| MCT10 | SLC16A10 | liver, intestinal wall, skeletal muscle |
| OATP1C1 | SLCO1C1 | vessel cells at the border to the brain |
| LAT1 and LAT2 | SLC7A5, SLC7A8 | widely distributed, less selective |
Put together from work in membrane physiology. The names are given so that the account can be checked and are no part of the authorized formulation.
Everything described so far comes from the physiological literature. The legal text does not travel that road. It records a reviewed result in one sentence whose wording may be neither widened nor rewritten in paraphrase.
Iodine contributes to normal energy-yielding metabolismEU-authorized wording · Regulation (EU) No 432/2012 · EFSA IDs 274 and 402
Named there are an element, a process and the word normal. The last of these stands for the physiologically orderly course of events where supply is otherwise adequate. The formulation holds for the adult population at large, for men and women alike, and it draws no distinction by tissue, by organ or by age.
Anything going beyond this normal case is left out. The sentence says nothing about an intake above the reference figure, nothing about mood, sleep or concentration, nothing about complaints and nothing about an amount that ought to be taken. The carrier proteins, the binding proteins and the deiodinases of the earlier sections do not turn up there either.
Residence time describes how long a single molecule stays in the blood, not how much is present overall. With a usual diet, uptake and excretion stand in a ratio that the body itself keeps adjusting. Anyone weighing up supplements does best to discuss the amount with a medical practice or a pharmacy.
No. It comes from the annex to the European labelling rules and serves to convert amounts on packaging into percentages. An individual requirement can differ from it and cannot be read off a table.
Because this Panorama follows one single formulation, and that one concerns iodine in connection with energy-yielding metabolism. For the organ that forms the molecules there is a separately reviewed wording, which Albitera does not take up. Setting two claims side by side would shift the frame of both.
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