Iron deficiency is the most common nutritional deficiency in the world, and women are disproportionately affected. Here is what to look for and what to do.
The most consequential fact about iron deficiency is procedural rather than biological: the blood test most people are given will not detect it until it is well advanced. Women are routinely told their blood count is normal, sent away with fatigue that has a straightforward physical cause, and never offered the one measurement that would have found it.
Deficiency happens in stages
Iron depletes in a sequence, and anaemia is the last step rather than the first.
First, stored iron falls. This is measured by ferritin, and it can decline substantially while everything else looks normal. Next, the supply of iron to the bone marrow becomes insufficient for efficient red blood cell production. Only after that does haemoglobin fall far enough to be classed as anaemia.
The stage in between — iron deficiency without anaemia — is common, produces real symptoms, and is invisible on a standard full blood count. Fatigue, breathlessness on exertion, poor concentration, reduced exercise tolerance, hair shedding, and restless legs can all appear well before haemoglobin moves.
A normal full blood count does not rule out iron deficiency. Ferritin falls long before haemoglobin does.
— WellnessLife editorial
The number to ask for
Ferritin is the test, and interpreting it is where things get contested. Laboratory reference ranges often flag deficiency only below 15 µg/L, a threshold derived from the point at which bone marrow iron stores are essentially absent. Many clinicians and several guidelines now treat below 30 µg/L as indicating deficiency, on the grounds that it identifies people with depleted stores considerably more sensitively, and symptoms are frequently reported at levels above that again.
This means a result can be inside the printed reference range and still be worth acting on. It also means the opposite caution applies: chasing an arbitrary “optimal” ferritin with supplements you were not prescribed is not supported either, and iron overload is a genuine harm.
One technical point worth knowing, because it causes missed diagnoses. Ferritin is an acute phase reactant — it rises with infection, inflammation, and chronic inflammatory conditions. Someone with both inflammation and iron deficiency can produce a falsely reassuring result, which is why CRP is often checked alongside it.
- Fatigue that rest does not resolve — the most common presentation and the most easily attributed to something else
- Breathlessness or a racing heart on exertion that used to be manageable
- Poor concentration and reduced mental stamina
- Hair shedding, brittle nails, and in some cases spoon-shaped nails
- Restless legs — an urge to move the legs at night, which has a well-recognised association with low iron stores
- Unusual cravings for ice, chalk, or other non-food substances, which is a specific and often-missed sign
- Heavy periods, a plant-based diet, pregnancy, or recent blood donation — reasons to test even without symptoms
Why women, specifically
Menstrual blood loss is the dominant cause in premenopausal women, and the difficulty is that heavy bleeding is so often normalised. Many women have no comparison point and assume their periods are typical. Clinically, periods that require changing protection hourly, last beyond seven days, involve large clots, or restrict daily activities warrant investigation — both for the bleeding itself and for the iron loss it causes.
Pregnancy increases requirements substantially, and plant-based diets increase them by roughly 1.8 times because of lower absorption from plant sources. Coeliac disease impairs absorption and frequently presents as unexplained iron deficiency.
One important exception to all of this: in postmenopausal women and in men, iron deficiency has no obvious menstrual explanation and requires investigation of the gastrointestinal tract, because slow blood loss from the gut is a possible cause that needs excluding. If you are past menopause and iron deficient, that is a reason for further tests rather than for supplements alone.
Food, and what interferes with it
Dietary iron comes in two forms that behave very differently. Haem iron, from meat, fish and poultry, is absorbed relatively efficiently and is largely unaffected by what else is on the plate. Non-haem iron, from pulses, leafy greens, fortified cereals, tofu and nuts, is absorbed far less efficiently and is strongly influenced by context.
Vitamin C improves non-haem absorption, which is the basis for the standard advice to pair plant iron sources with something like citrus or peppers — though a trial comparing iron supplements with and without vitamin C found no added benefit in treating established deficiency, so it matters more for dietary iron than for tablets.
Working against absorption: the polyphenols in tea and coffee, phytates in whole grains and legumes, and calcium. None of these needs eliminating. Moving tea and coffee an hour away from iron-containing meals is the change worth making, and it is a substantial one for anyone who drinks tea with dinner.
Supplementing properly
Here the research has produced something genuinely useful and slow to reach practice. Taking iron raises hepcidin, the hormone that regulates iron absorption, and that elevation persists for around 24 hours — which means a second dose taken the same day, or a large dose split across the day, is absorbed considerably less well.
Studies in iron-depleted women found that giving iron on alternate days produced higher fractional absorption than daily dosing, and that single daily doses outperformed split ones. Alternate-day dosing also tends to reduce the nausea and constipation that cause people to abandon supplements entirely.
Two further points. Repletion is slow: symptoms may improve within weeks, but rebuilding stores usually takes several months, and treatment is typically continued for around three months after levels normalise. And iron supplementation should follow a test, not a hunch — iron overload is harmful, haemochromatosis is more common than most people realise, and the symptoms that prompt people to self-supplement have many other causes.
The practical takeaway
If you are persistently tired and have heavy periods, follow a plant-based diet, or have recently been pregnant, ask specifically for a ferritin test rather than accepting a normal full blood count as an answer. Ask for the number rather than the interpretation, and ask whether CRP was checked alongside it.
If you are treating a confirmed deficiency, discuss single daily or alternate-day dosing with whoever prescribed it, keep tea and coffee away from meals, and expect the timeline to run in months rather than weeks. And if the cause is not obvious — particularly if you are postmenopausal — the deficiency itself is the finding that needs explaining, not just correcting.
Sources
- Pasricha SR, Tye-Din J, Muckenthaler MU, Swinkels DW. “Iron deficiency.” The Lancet, 2021.
- Stoffel NU, Cercamondi CI, Brittenham G, et al. “Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women.” The Lancet Haematology, 2017.
- Moretti D, Goede JS, Zeder C, et al. “Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women.” Blood, 2015.
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations, 2020.
- National Institute for Health and Care Excellence. Guidance on heavy menstrual bleeding and on iron deficiency anaemia.
