By Sophie Cochez, biologist and founder of Biodami, with 15 years of experience in the pharmaceutical industry. Last updated: November 2026. Reading time: 7 minutes.
Key takeaways
- Iron deficiency and anaemia are not the same thing. Iron deficiency means your iron stores are low. Anaemia means your haemoglobin is low. You can be iron deficient, and already tired, without being anaemic.
- Ferritin is your iron stock. It is usually the first marker to drop, long before haemoglobin.
- Ferritin can be misleading. It rises with inflammation, so a "normal" ferritin can hide a deficiency. That is why CRP is often measured alongside it.
- Taking more iron is not always better. Each dose of iron raises hepcidin, the hormone that blocks iron absorption, for about 24 hours. Studies show alternate-day dosing can improve absorption.
- Your gut plays a central role. Iron is absorbed in the intestine, and iron that is not absorbed reaches the colon, where it can disturb the gut microbiota.
What is the difference between iron deficiency and anaemia?
Iron deficiency is a lack of iron in the body's stores, measured mainly by a low ferritin level. Anaemia is a low level of haemoglobin, the protein in red blood cells that carries oxygen. Iron deficiency is the most common cause of anaemia, but not the only one: vitamin B12 or folate deficiency and chronic diseases can also cause it.
The two happen in sequence:
- Iron stores empty: ferritin falls.
- Iron deficiency sets in: tiredness, low energy or hair shedding may already appear.
- Haemoglobin drops: this is iron-deficiency anaemia.
The key point: not being anaemic does not mean you are not iron deficient. Many people are told their blood count is "fine" while their iron stores are already low.
Iron blood markers explained: a glossary
| Marker | What it is | What it tells you |
|---|---|---|
| Ferritin | The protein that stores iron, mainly in the liver | The size of your iron stock. Usually the first marker to fall |
| Transferrin | The protein that transports iron in the blood | How much transport capacity is available |
| Transferrin saturation (TSAT) | The percentage of transferrin carrying iron | How full the "delivery truck" is. Below about 20%, iron supply to tissues is likely insufficient |
| Serum iron | The amount of iron circulating in the blood at the time of the test | A snapshot that varies with the time of day and meals. Not reliable on its own |
| Haemoglobin | The iron-containing protein in red blood cells that carries oxygen | Whether you are anaemic |
| Hepcidin | A hormone produced by the liver that controls iron entry from the gut and release from stores | The "lock" of iron metabolism. Rarely measured routinely |
| CRP (C-reactive protein) | A marker of inflammation | Helps interpret ferritin, which rises with inflammation |
Reference values: according to the World Health Organization (WHO), a ferritin below 15 µg/L indicates iron deficiency in apparently healthy adults, and below 70 µg/L in adults with infection or inflammation. In clinical practice, many doctors use higher thresholds, often around 30 µg/L. For anaemia, the WHO threshold for haemoglobin in non-pregnant women is 12 g/dL. Reference ranges vary between laboratories, so always interpret your results with your doctor.
How does iron move through the body?
Iron follows a circuit. A simple way to picture it:
- The lock (hepcidin) controls how much iron enters the body from the intestine.
- The reservoir (ferritin) stores iron for later use.
- The delivery truck (transferrin) carries iron through the blood to where it is needed.
- The factory (haemoglobin in red blood cells) uses iron to transport oxygen.
The body has no active way to excrete excess iron, so it regulates iron almost entirely at the entry point: absorption in the gut. That is why the intestine, and the hormone that controls it, matter so much.
Why is my ferritin not going up even though I take iron?
This is one of the most common frustrations. Several factors can explain it.
Only a small fraction of iron is absorbed
The iron you swallow is not the iron you absorb. Absorption of non-haem iron (from plants and most supplements) is limited, and in some settings more than 80% of supplemental iron passes unabsorbed into the colon.
Hepcidin closes the door after each dose
Hepcidin is the body's iron gatekeeper. When you take a dose of iron, hepcidin rises for roughly 24 hours and reduces the absorption of the next dose. In two randomised trials in iron-depleted young women (Stoffel et al., The Lancet Haematology, 2017), taking 60 mg of iron on alternate days led to higher absorption than taking it every day, and splitting the dose twice daily raised hepcidin without improving absorption. The authors noted these findings should be confirmed in people with iron-deficiency anaemia, and a later study by the same team found a similar benefit in anaemic women.
Inflammation raises hepcidin too
Inflammation, whether from infection, chronic disease or inflammatory bowel conditions, increases hepcidin. Iron then stays "locked": less is absorbed from the gut and less is released from stores. This mechanism is behind what doctors call anaemia of inflammation.
Something may be reducing absorption or increasing losses
Heavy periods, coeliac disease, inflammatory bowel disease, Helicobacter pylori infection, certain medications (such as acid-reducing drugs) and drinking tea or coffee with your iron can all play a role. Persistent low ferritin is a reason to look for the cause with your doctor, not just to increase the dose.
What role does the gut microbiota play in iron absorption?
The link between iron and the gut microbiota is one of the most overlooked aspects of iron deficiency. It works in both directions.
Unabsorbed iron can disturb the microbiota
Iron that is not absorbed travels to the colon, where it becomes available to gut bacteria. Many potentially harmful bacteria, such as certain Enterobacteriaceae, depend on iron to grow, while beneficial bacteria like bifidobacteria and lactobacilli need much less of it.
A controlled trial in Kenyan infants (Jaeggi et al., Gut, 2015) found that iron-fortified foods increased enterobacteria, including pathogenic E. coli, reduced bifidobacteria and raised faecal calprotectin, a marker of gut inflammation. These results come from a setting with a high burden of infection, so they cannot be directly applied to healthy adults in Europe. They do, however, show that iron reaching the colon is not neutral for the microbiota, which may also help explain why some people experience bloating, constipation or stomach pain with iron supplements.
A balanced microbiota may support absorption
In the other direction, some gut bacteria may help the body absorb iron. Lactobacilli produce acids that lower the pH in the gut, which helps convert iron into its more absorbable ferrous form.
Specific probiotic strains have been studied for this effect. In a double-isotope study in healthy women of reproductive age (Hoppe et al., British Journal of Nutrition, 2015), adding Lactobacillus plantarum 299v to an iron-fortified fruit drink increased iron absorption by about 50%. Research in this area is still developing, and effects are strain-specific: results from one strain cannot be assumed for another.
Which blood tests should I ask for to check my iron status?
If you suspect low iron, a complete picture usually includes:
- Ferritin, to assess iron stores
- Transferrin saturation, to assess iron supply to tissues
- Haemoglobin (complete blood count), to check for anaemia
- CRP, to detect inflammation that could falsely raise ferritin
Serum iron alone is not enough to diagnose iron deficiency, because it fluctuates throughout the day and after meals.
Practical tips to support iron absorption
- Combine iron with vitamin C, which enhances the absorption of non-haem iron.
- Keep tea, coffee and calcium away from your iron intake, as polyphenols and calcium reduce absorption.
- Ask your doctor whether alternate-day dosing suits you, based on the research on hepcidin.
- Choose a well-tolerated form of iron if supplements upset your stomach, and discuss options with a health professional.
- Look after your gut health, through a varied, fibre-rich diet that supports a diverse microbiota.
- Retest and investigate if your ferritin does not improve: the cause matters as much as the treatment.
Frequently asked questions
Can you have low ferritin without anaemia?
Yes. Ferritin falls first, while haemoglobin can stay normal for a long time. This stage is called iron deficiency without anaemia, and it can already cause tiredness.
What is hepcidin?
Hepcidin is a hormone made by the liver that regulates iron. It reduces iron absorption from the intestine and blocks the release of iron from stores. It rises when iron levels are high and during inflammation.
Why can ferritin be normal if I am iron deficient?
Ferritin is also an inflammation marker. During infection or chronic inflammation it rises, which can mask a real deficiency. Measuring CRP at the same time helps interpret the result.
Is it better to take iron every day or every other day?
Research in iron-depleted women suggests that taking iron on alternate days, as a single morning dose, can improve absorption compared with daily dosing, because daily doses keep hepcidin elevated. Always follow your doctor's advice for your situation.
How does the gut microbiota affect iron?
Iron that is not absorbed reaches the colon, where it can favour some unwanted bacteria and disturb the microbiota. Conversely, a balanced microbiota, and certain lactobacilli strains, may support iron absorption by lowering gut pH.
What is the difference between ferritin and serum iron?
Ferritin reflects your iron stores over time. Serum iron is the amount circulating at the moment of the test and changes with the time of day and meals, so it is much less reliable on its own.
References
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020.
- 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: two open-label, randomised controlled trials. The Lancet Haematology. 2017;4(11):e524-e533.
- Stoffel NU, Zeder C, Brittenham GM, et al. Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica. 2020.
- Jaeggi T, Kortman GAM, Moretti D, et al. Iron fortification adversely affects the gut microbiome, increases pathogen abundance and induces intestinal inflammation in Kenyan infants. Gut. 2015;64(5):731-742.
- Hoppe M, Önning G, Berggren A, Hulthén L. Probiotic strain Lactobacillus plantarum 299v increases iron absorption from an iron-supplemented fruit drink. British Journal of Nutrition. 2015;114(8):1195-1202.
- Ganz T. Anemia of inflammation. New England Journal of Medicine. 2019;381:1148-1157.
This article is for educational purposes only and does not replace medical advice. If you have symptoms or abnormal blood results, talk to your doctor before starting or changing any supplementation.
