Fatigue in your forties gets blamed on everything at once — sleep, stress, hormones, the fact that you are running a household and a career at the same time. Iron is the one cause on that list that is cheap to measure and easy to act on, and it is the one most often skipped.
This is not an article about anaemia. It is about the wide grey band underneath it: iron stores drifting down while a blood count still reads normal, which is exactly the situation a lot of women in the perimenopause years are in.
Why iron slips after 40
Iron leaves the body in two ways that matter here: bleeding, and not much else. There is no regulated excretion route, so intake and menstrual loss set the balance almost entirely.
Perimenopause changes both sides of that equation. Cycles become erratic before they stop — for many women that means longer, heavier, closer-together bleeds for several years rather than a gentle taper. At the same time appetite and eating patterns shift, red meat intake often drops on purpose, and absorption gets harder as stomach acid production declines with age.
The result is a slow drawdown. Nothing dramatic happens in any single month, which is precisely why it goes unnoticed until you are tired in a way that sleep does not fix.
Heavy cycles and ferritin
Haemoglobin is the last thing to fall. Your body protects circulating red cells by raiding storage first, so ferritin — the storage marker — moves years before a full blood count does. A "normal" CBC tells you that you are not anaemic. It does not tell you that your stores are full.
The World Health Organization's 2020 ferritin guideline puts the threshold for depleted stores in non-pregnant women at 15 µg/L, and notes that ferritin rises during inflammation or infection — which is why a single low-ish result taken during a cold is worth repeating.
Many clinicians working with heavy menstrual bleeding use a considerably higher working floor, often somewhere between 30 and 50 µg/L, because symptoms show up long before 15. Munro made the case in the International Journal of Gynecology & Obstetrics in 2023 that heavy bleeding and iron depletion are routinely treated as separate problems when they are one problem.
Two randomised trials are worth knowing about, because they tested this exact group — menstruating women who were tired but not anaemic. Verdon's trial in the BMJ in 2003 and Vaucher's in the CMAJ in 2012 both found that iron reduced fatigue scores in women with low ferritin and a normal haemoglobin. The effect was real, modest, and concentrated in the women who were actually depleted. That last clause is the whole point: iron helped the women who needed iron.
The practical bit: if you are tired and your cycles are heavy, ask for ferritin specifically — not just "bloods". A normal haemoglobin does not answer the question.
Food first, then supplements
Two kinds of dietary iron behave very differently. Haem iron, from red meat, poultry and fish, is absorbed at roughly 15–35%. Non-haem iron, from lentils, beans, tofu, dark greens and fortified grains, comes in at nearer 2–20% and is heavily influenced by what is on the plate with it.
Three levers actually move the needle:
- Vitamin C alongside plant iron. Peppers, citrus, tomatoes or a squeeze of lemon in the same meal meaningfully increase non-haem absorption.
- Tea and coffee away from meals. Polyphenols and tannins bind iron in the gut. A half-hour gap either side of a meal is enough.
- Calcium separately. A large dairy serving or a calcium supplement competes with iron for uptake, so do not take them together.
Food fixes the trajectory for a lot of women. It is slow, though — you are refilling a store, not correcting a number overnight — and if your losses are genuinely heavy each month, diet alone may only hold the line.
That is where a low, daily, supplemental dose earns its place: not as treatment, but as a way to keep routine intake steady through the years when losses are highest. And if a clinician does find a real deficit, they will direct the dose and the follow-up — that is a conversation to have with them, not with a label.
Strips vs pills
The best iron regimen is the one you will still be doing in March. Iron tablets have a genuine adherence problem: the classic ferrous sulfate dose brings nausea, constipation and a metallic taste often enough that a meaningful share of people quietly stop.
Dose size is part of that. Moretti's work in Blood in 2015 showed that a large oral dose spikes hepcidin, the hormone that shuts absorption down, and that the spike blunts uptake from the next dose for roughly a day. Stoffel's follow-up in The Lancet Haematology in 2017 found that alternate-day dosing produced better fractional absorption than daily dosing of the same total. The practical read-through: bigger is not better with iron, and gentler, smaller, more consistent intake is a defensible strategy rather than a compromise.
Format matters for the same unglamorous reason. Top Up is a raspberry-flavoured iron strip — 19 mg of iron as ferric saccharate with 400 µg of folate, thirty strips in a pocket tin, no water and nothing to swallow. Ferric saccharate is a gentler iron form than ferrous sulfate, and a strip that dissolves on your tongue at the kitchen counter is harder to forget than a bottle in a cupboard.
If tablets sit fine with you, keep taking them. The strip exists for the large group of women for whom they do not.
When to test
Test if any of these are true: cycles that soak through protection, that last longer than seven days, or that come closer than three weeks apart; fatigue that rest does not touch; breathlessness on stairs you used to walk up talking; restless legs at night; hair shedding beyond the usual; a history of low iron; or a diet with little or no red meat.
Ask for a full blood count and ferritin together. If ferritin is low-normal and you feel rough, a repeat in three months tells you the direction of travel — which is more useful than one number in isolation. And if ferritin comes back high, that is also worth a conversation; iron is not a supplement to take blind.
Retest roughly three months after changing anything. Iron stores move slowly, and a test taken three weeks in will mostly measure your last few doses.
Should I just take iron to see if it helps?
No. Iron accumulates, and some people carry genes for iron overload. Measure first, then decide.
When should I take iron?
Away from tea, coffee, calcium and dairy. Morning on a lightly filled stomach suits most people; consistency beats the perfect slot.
Sources
- World Health Organization. “WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations.” World Health Organization, 2020.
- Munro MG. “Heavy menstrual bleeding, iron deficiency, and iron deficiency anemia: framing the issue.” International Journal of Gynecology & Obstetrics, 2023.
- Verdon F, et al. “Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial.” BMJ, 2003.
- Vaucher P, et al. “Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial.” Canadian Medical Association Journal (CMAJ), 2012.
- Moretti D, et al. “Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women.” Blood, 2015.
- Stoffel NU, 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: two open-label, randomised controlled trials.” The Lancet Haematology, 2017.
