Iron deficiency is the most common nutritional deficiency in the world. It's also the one most women—even well-educated, health-conscious women—fail to diagnose in themselves.
You know why? Because the standard NHS blood test (full blood count) only shows haemoglobin. Haemoglobin only drops once you're in frank anaemia. By that point, your iron stores have been depleted for months. You've been living with fatigue, brain fog, cold intolerance, and restless legs the entire time, attributing it to age, stress, or poor sleep.
The actual marker you need—ferritin—is rarely ordered unless you specifically ask. And most GPs don't know to use a ferritin target (>50 ng/mL) that actually correlates with symptom resolution, rather than just "lab normal" (which is often 15 ng/mL and useless).
How Common Is This?
Pasricha SR et al. (2021) in The Lancet estimated that 25–30% of UK women of reproductive age are either iron deficient or iron depleted. That's roughly 1 in 3 to 1 in 4 women. For premenopausal women who train, vegetarians, or those with heavy periods, the number is closer to 50%.
You're likely not anaemic. Your haemoglobin is probably "normal." But your iron stores are shot.
What Iron Deficiency Actually Feels Like
Most women don't recognise the symptoms because they're gradual and non-specific:
- Fatigue that doesn't improve with sleep, often worse in afternoons
- Brain fog and difficulty concentrating (iron is essential for myelin synthesis and cognitive function)
- Cold intolerance: feeling chilled when others are comfortable
- Hair loss: iron is needed for the anagen (growth) phase of the hair cycle
- Breathlessness on exercise: even light exertion feels exhausting
- Restless legs, particularly in the evening
- Pale conjunctivas and nail beds
- Pica (craving ice, dirt, or starch): a sign of severe deficiency
You'll go to your GP, get told "your bloods are fine," and leave confused. You weren't wrong. You're just not being tested properly.
Ferritin vs Haemoglobin: Why This Matters
Haemoglobin is iron bound in your red blood cells, carrying oxygen. It doesn't drop until your iron stores are so depleted that your bone marrow can't make enough healthy RBCs. By that point—clinical anaemia—you've been iron deficient for a long time.
Ferritin is your iron storage protein. It drops first as stores deplete, long before haemoglobin moves. A low ferritin with normal haemoglobin means you're iron deficient but not yet anaemic. This is where most symptomatic women sit.
The target that matters: Ferritin >50 ng/mL is the threshold where most women feel well. Ferritin 15–50 is "normal range" by lab standards but often symptomatic. Below 15 is iron deficiency, and below 5 is severe.
If your GP orders ferritin and you get a result of 20 ng/mL with normal haemoglobin, you're not "fine." You need to address your iron status.
Why Women Get Iron Deficient
Menstruation. The biggest reason. Women lose 15–30mg iron per menstrual cycle through bleeding. If your periods are heavy (soaking through a tampon or pad every 1–2 hours on heavy days), you're losing even more. Combine this with inadequate dietary iron intake and you've got a deficit.
Vegetarian/vegan diet. Plant-based iron (non-haem iron) is 1–10% absorbed compared to 15–35% for haem iron from meat. If you don't eat meat but also don't strategically pair plant iron with vitamin C, your absorption will be poor.
Training. Endurance athletes (particularly women runners and cyclists) lose iron through:
- Sweat (small amount)
- Intravascular haemolysis (red cells breaking down from impact, especially in running)
- Increased hepcidin (the iron-regulating hormone) in response to inflammation
Even lifting can increase iron loss modestly through inflammation.
Digestive absorption issues. Coeliac disease, inflammatory bowel disease, or even low stomach acid (common with age) impairs iron absorption. If you're eating well but still deficient, get your digestion checked.
Absorption: The Real Bottleneck
You can eat iron-rich food and still be deficient if your absorption is poor.
Haem iron (from meat): 15–35% absorbed. Largely unaffected by dietary factors.
Non-haem iron (from plants, supplements): 1–10% absorbed normally, but this can be dramatically increased or decreased by what you eat it with.
Absorption boosters:
- Vitamin C (ascorbic acid): increases non-haem iron absorption 3–4 fold. Eat plant iron (spinach, lentils, beans) with citrus, tomatoes, peppers, or kiwifruit.
- Meat, poultry, fish: contain a "meat factor" that enhances non-haem iron absorption even when eaten alongside plant sources.
Absorption blockers:
- Calcium: competes with iron for absorption. Don't take calcium supplements with iron-rich meals.
- Tannins: in tea and coffee. Drink these between meals, not with iron-rich ones.
- Phytates: in grains and legumes. Reduce phytate content by soaking, fermenting, or sprouting legumes. Cooking reduces phytate but doesn't eliminate it.
- Polyphenols: in some plant foods. Not as strong as tannins but worth considering.
This is why you can eat spinach salad (phytate-rich, low bioavailable iron) and remain deficient, but a small serving of beef with vitamin C-rich vegetables and your absorption skyrockets.
Testing in the UK
Via your GP: Ask specifically for "serum ferritin" and "iron studies panel" (which includes ferritin, serum iron, TIBC, transferrin saturation). Also request a full blood count to check haemoglobin and MCV (mean corpuscular volume—low MCV is microcytic anaemia, classic iron deficiency).
Many GPs will push back if your haemoglobin is normal. Push back. Say you're symptomatic and want ferritin checked. If they refuse, move on to:
Private testing: Medichecks, LetsGetChecked, or Everlywell offer "iron studies" panels (ferritin + serum iron + TIBC) for £30–50, often with GP interpretation included. This is money well spent.
Supplementation: Ferrous Sulphate vs Ferrous Bisglycinate
Ferrous sulphate: the NHS standard. Cheap, effective, but GI side effects are common: nausea, constipation, dark stools, abdominal cramping. Many women stop taking it because it makes them feel worse.
Ferrous bisglycinate (chelated iron): iron bound to the amino acid glycine. Better tolerated, fewer GI side effects, possibly better absorption. Moretti D et al. (2015) in Nutrients found bioavailability comparable to or slightly better than ferrous sulphate, with significantly fewer side effects.
My recommendation: If you tolerate ferrous sulphate fine, use it—it's cheap and effective. If you get GI symptoms, switch to ferrous bisglycinate. It costs more (£8–12 vs £2–4 per month) but if it means you actually take it consistently, the cost is worth it.
Dosing: Standard is 200mg elemental iron daily (ferrous sulphate 325mg contains 65mg elemental iron; ferrous bisglycinate 25mg contains 25mg elemental iron). Start low (100mg elemental) if you're sensitive, increase as tolerated.
Take iron on an empty stomach with orange juice (vitamin C boost) if you can tolerate it. If GI symptoms emerge, take with food (absorption drops but tolerability improves—still better than not taking it).
Who Needs Testing
- All premenopausal women, particularly if symptomatic (fatigue, brain fog, cold intolerance, restless legs, hair loss)
- Women who train hard, especially endurance athletes
- Vegetarians and vegans: unless you're eating iron deliberately and pairing it with vitamin C
- Women with heavy or prolonged periods: if soaking products every 1–2 hours on heavy days, you need testing
- Women with digestive issues: IBS, low stomach acid, coeliac disease risk
Practical Action Plan
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Get tested: Ask your GP for ferritin, or order a private iron studies panel. Request ferritin target of >50 ng/mL.
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If ferritin is low (< 50 ng/mL):
- Start supplementation: 100–200mg elemental iron daily
- Pair with vitamin C (orange juice, supplement, or food)
- If GI symptoms, switch to ferrous bisglycinate
- Retest ferritin after 8–12 weeks
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While supplementing, improve diet:
- Include haem iron (meat, fish) 3–4 times per week if possible
- Pair plant iron with vitamin C (spinach + tomatoes, lentils + peppers)
- Reduce tea and coffee around iron-rich meals
- Soak and cook legumes to reduce phytates
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Once ferritin reaches 50 ng/mL:
- Drop to maintenance supplementation (50–100mg iron 2–3 times per week)
- Or maintain through diet if absorption is good and losses are low
- Retest annually if premenopausal
Why This Matters
Iron deficiency in women is preventable and treatable. But it's only treated when it's diagnosed. You're not failing if you feel exhausted. You're probably just iron deficient, and nobody tested for it properly.
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Seb covers nutrition and metabolic health for active adults over 35. He writes for fuel-optimal.co.uk based on peer-reviewed research and works with clients across the UK to build sustainable eating strategies for long-term health and lean mass.