I

One mineral, two opposite situations.

Most of what sits inside a multivitamin behaves predictably across a population. Vitamin C, the B group, zinc and selenium all have reference intakes that move a little with age and sex, and a single sensible level serves nearly everyone who takes the formula. Iron does not work that way.

A woman of 32 who menstruates and a woman of 58 who does not are given reference intakes that differ by more than a factor of two. A woman who is pregnant is given a figure higher than either. A woman who eats no meat is advised by the same reference body to aim higher again, because iron from plant sources is absorbed less readily than iron from animal sources.

Those are four different answers to the same question, and all four appear under the word women. A daily capsule has to pick one of them in advance and give it to everybody who opens the bottle.

That is the whole of the design problem, and the table below is the clearest way to see it.

II

Why the figure moves with a life.

The reason the reference intake falls at 51 is mechanical rather than mysterious. Menstruation is a recurring route of iron loss, and the reference bodies set the higher figure for women of reproductive age to account for it. When that loss ends, the modelled requirement returns to the same 8 mg given to adult men.

Pregnancy moves in the other direction, and steeply, because maternal blood volume expands and the developing fetus and placenta draw on maternal stores. The 27 mg figure reflects that, and prenatal formulas are built around it deliberately. A general daily multivitamin is not a prenatal formula and is not designed to carry that number.

Diet shifts the figure again. Iron appears in food in two forms. Heme iron, found in meat, poultry and fish, is absorbed comparatively efficiently. Non-heme iron, the form in plants, fortified grains and supplements, is absorbed far less efficiently and its uptake is sensitive to what else is on the plate. For that reason the reference guidance suggests vegetarians aim at roughly 1.8 times the standard figure.

So the honest summary is that four facts about a person, age, menstrual status, pregnancy status and diet, sit between the word iron and any specific milligram figure. None of them is printed on the outside of a bottle.

III

A door that opens one way.

Here is the fact that separates iron from almost everything else in a capsule. The human body has no regulated mechanism for excreting iron once it has been absorbed. Small amounts leave through shed skin and intestinal cells and through blood loss, and that is close to the whole of it. Roughly a milligram a day, in the standard accounting.

Because there is no exit valve, the control sits at the entrance. A hormone called hepcidin, made in the liver, regulates how much dietary iron is allowed across the intestinal wall and how much stored iron is released into circulation. When stores are ample, absorption is turned down. When they are low, it is turned up.

Water soluble vitamins behave in the opposite way. Take more than the body has use for and a good deal of the surplus is excreted. Iron offers no such margin for error, which is why it carries a tolerable upper intake level of 45 mg for adults, a figure the reference bodies set on the basis of gastrointestinal effects rather than anything more distant.

There is also a genetic dimension. Hereditary hemochromatosis, in which iron is absorbed and retained at an unusually high rate, is among the more common inherited conditions in people of northern European ancestry, and many people carrying the relevant genotype are unaware of it. That possibility is one more reason a fixed daily dose given to an undifferentiated audience is a cruder instrument than it looks.

IV

What an iron-free formula is actually doing.

An omission is a decision, and it is worth stating plainly what the decision is. Leaving iron out of a daily multivitamin unbundles it. The twenty or so nutrients where one sensible level serves nearly everyone stay in the capsule. The one nutrient where the right level depends on facts about an individual is handed back to that individual and to their healthcare practitioner.

That has a second consequence worth knowing. Minerals compete. Iron and zinc are taken up through partly shared pathways and compete when given together in supplemental amounts. Calcium has been shown to reduce the absorption of non-heme iron in single meal studies. Iron supplements are also commonly advised to be separated in time from certain prescription medicines. A formula without iron removes that set of interactions from the daily capsule entirely, and allows anyone who does take iron to time it on its own terms.

Vitamin C runs the other way. It is the best documented enhancer of non-heme iron absorption, which is one reason it appears so consistently alongside iron in the literature, and one reason a formula that carries vitamin C is not a neutral companion to an iron tablet taken at the same moment.

None of this makes iron-containing formulas wrong. It makes the two kinds of formula answers to two different questions, and the useful move for a reader is to know which question applies to her.

V

The warning most people have never read.

United States labeling regulation treats iron differently from the other minerals in a multivitamin. Under 21 CFR 101.17, dietary supplements and drugs in solid oral dosage form that contain iron or iron salts for use as an iron source must carry a specific warning on the label, stating that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under six, that the product should be kept out of reach of children, and that a doctor or poison control center should be contacted immediately in the event of accidental overdose.

That requirement was introduced after a run of pediatric incidents, and it is the only nutrient in a typical multivitamin to attract a warning of that kind. It is worth reading as what it is: a regulator signalling that the margin between a useful amount and a harmful one is narrower here than elsewhere on the panel.

A formula that contains no iron carries no such statement, because there is nothing for it to describe.

VI

Settled, and unsettled.

What is settled is the structure. The reference intakes, the upper limit, the absence of a regulated excretion route, the difference between heme and non-heme absorption and the interaction with zinc and calcium are all long established and are not seriously disputed.

What is not settled is any individual's number. National survey data consistently shows that low iron stores are far more common among women of reproductive age than among women past menopause, but a population prevalence tells a reader nothing about herself. Serum ferritin is the measure most often used to describe stored iron, with the caveat that it rises during inflammation and is therefore read alongside other markers rather than alone.

The practical position follows from that. Anyone wondering whether she should be taking iron is asking a question that blood work and a qualified healthcare professional can answer directly, and that a capsule cannot answer at all. Studies referenced here were conducted independently and did not involve any specific Codeage product.

The useful question when reading a panel is therefore not whether iron is present, but whether its presence or absence matches the person holding the bottle.

VII

Where this fits.

A daily multivitamin sits in Daily Foundation, the first pillar of The Longevity Code. A pillar is a location within a framework. It describes a category, not an outcome for any reader.

The house position on iron is the one set out above. It is a nutrient whose correct amount is personal, measurable and time varying, and those three properties together make it a poor candidate for a fixed quantity inside a general daily formula. The Codeage Women's Daily Multivitamin+ is formulated without iron for that reason, and says so plainly rather than quietly. Other formulas in the range do include it, which is the same decision taken for a different reader.

Anyone who has been advised to take iron should take it as advised. That advice belongs to a qualified healthcare professional, and nothing on this page replaces it.