I

A mineral the body does not excrete.

Iron is unusual among minerals. The body has no active route for getting rid of it. There is no iron equivalent of the kidney clearing excess sodium, which means the amount held in the body is controlled almost entirely at the point of entry, by how much the gut is willing to absorb.

In the absence of bleeding, daily losses are small, and an adult man needs to absorb only about a milligram a day to stay in balance. Menstruation adds a recurring loss that the male reference figure does not have to account for, and that single difference is what separates the two numbers.

This is why the reference intake drops back to the male figure after the menopausal transition. The number is not describing a different body. It is describing the arithmetic of a monthly loss that has stopped. The same logic runs in the other direction during pregnancy, when the reference figure rises again.

Everything else in this article follows from that arithmetic: what leaves, what the body holds in reserve, and what determines how much of what is eaten actually crosses into circulation. The cycle itself, and the four phases across which that loss occurs, is described in the guide to the four phases.

II

Ferritin, the reserve rather than the flow.

Around two thirds of the iron in the body is in hemoglobin, the protein in red blood cells that carries oxygen. A further quarter or so sits in a mobilisable store, and the rest is in myoglobin in muscle and in the enzymes of oxidative metabolism.

Ferritin is the protein that holds that store, and the amount circulating in blood broadly tracks how much iron is held in reserve. It is the marker most iron research reports, because it moves before hemoglobin does. A reserve can be drawn down considerably while the number that describes oxygen carriage still reads as unremarkable.

That sequencing is the single most useful thing to understand about the subject. Researchers describe a graded picture rather than a switch, and the published cut points used to define low ferritin vary between laboratories and between studies, which is one reason figures quoted in different papers are not always directly comparable.

Ferritin is measured with a blood test. It is not something that can be inferred from how a person feels, which is why the research literature consistently works from measured values rather than reported symptoms.

III

What the menstrual research describes.

The association between menstrual blood loss and iron reserves is among the better documented relationships in nutritional epidemiology, and it holds across quite different populations.

A 2024 study of female blood donors published in Acta Obstetricia et Gynecologica Scandinavica reported that the volume of menstrual blood loss was consistently and significantly negatively associated with both ferritin and hemoglobin, and identified blood donation and menstruation as the two most important determinants of those values in that population.

In adolescents, the Healthy Growth Study examined 1,222 girls aged 9 to 13 and found that those who had begun menstruating were 2.57 times more likely to show depleted iron reserves than those who had not. In female varsity athletes, researchers reported that each one-point rise on a pictorial blood loss assessment score corresponded to roughly one percent lower ferritin.

These are observational associations describing populations, not predictions about any individual. Studies referenced here were conducted independently and did not involve any specific Codeage product.

IV

Two forms, two doorways.

Dietary iron arrives in two chemically distinct pools that the gut handles by different routes. Heme iron comes from the hemoglobin and myoglobin of animal tissue, which is why red meat, poultry, seafood and organ meats are the concentrated sources. It is taken up intact by a dedicated carrier protein on the intestinal surface.

Non-heme iron is the form in plant foods, in fortified products and in most supplements. It has to be reduced from its ferric state to its ferrous state before it can cross, using a different transporter, and the literature consistently describes absorption from this pool as both lower and far more sensitive to what else is on the plate.

The practical consequence shows up in the reference figures themselves. The NIH Office of Dietary Supplements notes that people eating mostly plant-based diets are generally advised to aim for close to twice the listed amount, precisely because the non-heme pool is absorbed less readily.

Neither form is described in the literature as better in the abstract. They are absorbed differently, handled differently once inside, and studied differently, which is the more useful way to hold the distinction.

V

The ceiling, and who sets it.

Because there is no exit route, the gut enforces a limit. The hormone doing the enforcing is hepcidin, made by the liver. When hepcidin is high it removes the exporter that moves iron out of the intestinal cell and into the bloodstream, and absorbed iron simply stops crossing.

Hepcidin responds to iron status, falling when reserves are low and rising when they are replete, which is the mechanism that holds the system in balance without any conscious input. It also responds to a single oral dose, rising for roughly the following day.

That second behaviour is why the relationship between an amount taken and an amount absorbed is not linear, and why stable-isotope work from research groups in Zurich has examined alternate-day schedules against daily ones. It is also why the literature treats larger amounts with more caution rather than less.

Around it sit the dietary factors: ascorbic acid alongside a meal, and on the other side the polyphenols in tea and coffee, calcium, and the phytates in whole grains and legumes. These describe what happens within a meal. They are not a protocol, and iron taken without a measured reason is one of the few areas in nutrition where the literature is openly cautious.

VI

What the label is telling you.

Supplemental iron is always non-heme, and the word in brackets after it names the compound it is bound to. Ferrous sulfate is the most studied and the form most trial protocols use. Ferrous fumarate and ferrous gluconate are salts of the same ferrous iron bound to different acids.

Ferrous bisglycinate chelate is iron bound to two molecules of the amino acid glycine. The chelate is described in the literature as being handled somewhat differently from the simple salts in the gut, and it is the form that appears in formulas designed to sit inside a broader daily blend rather than to stand alone.

Two other things on a label are worth reading together with the iron. Vitamin C, for the reason set out above. And folate, which appears either as folic acid or as the methylated form, 5-MTHF. Which one is used is a formulation decision, and it is stated on the panel.

What no label can tell anyone is whether they need it. That is a measured value, and the next section says how it is measured.

VII

Where this fits.

Iron sits inside Daily Foundation, the first pillar of The Longevity Code, the pillar concerned with the foundational micronutrients a daily routine is built on rather than with any single mechanism of aging. A pillar is a location within a framework. It describes a category, not an outcome for any individual reader.

It belongs there for a specific reason. Iron is not a compound anyone takes up in midlife having read about it. It is one of the small set of minerals whose requirement is set by physiology and by life stage, and whose reference figure changes as that life stage changes.

Formulations across the pillars are built around ingredients researched in the context described here. Any research referenced was conducted independently and did not involve any specific Codeage product.