Why so many women are iron deficient, and how to actually fix it

Iron deficiency is the most common nutritional deficiency in the world, and menstruating women are the most affected group in high-income countries. The arithmetic is simple: an average period loses a modest amount of iron, absorption from a typical diet is limited, and a moderately heavy period every month can quietly outpace what you take in for years.

Ferritin is the test, and it is often not run

Iron depletion happens in stages. Stored iron falls first — that is ferritin. Then iron available for making red cells falls. Only last does haemoglobin drop, which is what a full blood count detects.

Symptoms start in the first stage. Iron is a cofactor for dopamine and serotonin synthesis, for thyroid hormone production and for mitochondrial function, so fatigue, brain fog, low mood, hair shedding, restless legs and reduced exercise tolerance can all be present with a completely normal full blood count.

This is why "my bloods were normal" so often means "nobody measured my iron". Ask for ferritin specifically, and ask for the number.

Interpreting it: below 15 µg/L is depletion in essentially everyone, but many labs flag anything above 12 or 15 as normal while symptoms are common well above that. A good deal of practice treats below 30 as deficient and below 50 as worth treating in a symptomatic person. Low ferritin and low mood goes further into the thresholds.

One caveat: ferritin rises with inflammation, so a normal value can mask deficiency in someone with an inflammatory condition. A CRP alongside helps interpret it, and transferrin saturation is more robust in that situation.

Where the losses come from

Menstruation is the dominant cause in people of reproductive age. If your periods are heavy, supplementation alone is bailing without patching the hull — the bleeding itself needs addressing, and it is usually treatable. How heavy is too heavy.

Pregnancy draws heavily on stores, and postpartum depletion is common and under-checked — the fourth trimester.

Coeliac disease impairs absorption and is substantially under-diagnosed. Unexplained iron deficiency should prompt coeliac serology.

Gastrointestinal bleeding must be considered in anyone not menstruating, in postmenopausal women and in men — iron deficiency in those groups warrants investigation of the gut rather than a supplement.

Endurance training increases losses through several routes, including foot-strike haemolysis and gut losses.

Diets low in bioavailable iron. Plant (non-haem) iron is absorbed several times less efficiently than haem iron from meat and fish.

How to replace it properly

This is where most people go wrong, and the errors are specific.

Take it on alternate days, not daily. A dose of oral iron raises hepcidin, the hormone that blocks iron absorption, and that suppression lasts roughly 24 hours. Trials have found that alternate-day single doses produce better total absorption than daily or twice-daily dosing, with fewer gastrointestinal side effects. If daily iron gave you constipation and no improvement, this alone may fix it.

Take it with vitamin C, which enhances non-haem absorption. A glass of orange juice works.

Take it away from tea, coffee, milk, calcium supplements, antacids and proton pump inhibitors, all of which impair absorption substantially. Tea with a meal can cut absorption dramatically.

On an empty stomach if you can tolerate it, though with food is better than not taking it.

Give it three to six months. Correcting haemoglobin is faster than refilling stores. Stopping when you feel better is how people end up back where they started within a year. Recheck ferritin after about three months, and aim to replenish rather than just normalise.

Different salts, similar efficacy. Ferrous sulphate, fumarate and gluconate all work; elemental iron content differs, so compare that rather than the tablet size. Iron bisglycinate is often better tolerated.

Intravenous iron exists, works quickly, and is appropriate for people who cannot absorb or tolerate oral iron or who need correction fast. It is under-offered.

Diet alone is usually not enough to correct a deficiency

Diet maintains iron status; it rarely corrects an established deficit at a useful rate. That said, for maintenance: red meat, liver, shellfish and fish for haem iron; lentils, beans, tofu, dark leafy greens, fortified cereals and pumpkin seeds for non-haem, paired with a vitamin C source. Cooking in cast iron adds a small amount.

Judge it by symptoms over months, not by how you feel next week

Iron replacement does not work quickly, and the symptoms it fixes — fatigue, mood, cognition — are exactly the ones that fluctuate for other reasons. That combination makes it very hard to tell whether it is working, which is why people stop.

The way through is a baseline and a trajectory: energy, mood and exercise tolerance rated before you start and tracked for three months. That is also what tells you whether to push for further investigation if nothing changes.

Naked is built to hold exactly that kind of slow-moving experiment, alongside the cycle data that usually explains why the deficiency happened in the first place. For a lot of people the record makes something obvious that was invisible: the worst week of the month is the one after the heaviest bleeding, every month, which points straight at iron.