How does iron deficiency lead to anemia?
When iron stores run down, the bone marrow makes less hemoglobin and the blood carries less oxygen: that is iron-deficiency anemia. The WHO defines it as hemoglobin below 130 g/L in men and 120 g/L in non-pregnant women. The body absorbs only 1 to 2 mg of iron per day (Camaschella, 2015).
Schematic drawings, not to scale. Dotted lines are estimates or examples.
Where does iron go, and what happens when it runs short?
The path of iron, from balance to severe anemia. The organ involved blinks, from orange to red; tap an organ, slide along the curve or pick a row in the table.
Sources: Camaschella, 2015; WHO, 2024; Weiskopf, 1998 (32 volunteers, rapid anemia, at rest). Ferritin and hemoglobin values: an example, not measurements.
What do my hemoglobin and ferritin mean?
Set your two values and your situation: the page places them against the published cutoffs. This is a reference point to discuss with a doctor, not a diagnosis.
Sources: WHO, 2024; Powers et al., ASH, 2026 (conditional recommendations, low to very low certainty). Children: different cutoffs, not covered by this tool.
Which foods provide iron, and what blocks it?
Build a meal by tapping the foods. The daily need depends on your situation; the drink changes how much of the iron gets through the intestine.
Sources: NIH ODS (US servings, USDA); Morck, 1983; Hallberg, 1991; Cook and Monsen, 1977. Effects measured on a single meal.
Tablets: every day or every other day?
A tablet raises hepcidin, which holds back the next one. Compare the two schedules from the Stoffel study (40 women, ferritin of 25 or less, no anemia, 60 mg of iron), day by day.
On an empty stomach or between meals; taken with a meal, up to 75% less absorption. After returning to normal: about 3 more months; stores: 3 to 6 months. Sources: Stoffel, 2017; Moretti, 2015; Snook, BSG, 2021; Camaschella, 2015.
Frequently asked questions about iron and anemia
Is a ferritin of 25 normal?
The lab may mark it as normal, but since 2026 the ASH has considered a ferritin of 30 ng/mL or less to signal iron deficiency in adults. The WHO uses below 15 in healthy people. These new cutoffs rest on low-certainty evidence: discuss them with a doctor, along with your hemoglobin and your symptoms.
How long does recovery take?
Hemoglobin rises within a few weeks: about 10 g/L in 2 weeks is a very good sign, and 20 g/L or a return to normal is expected at 4 weeks (BSG, 2021). Stores take 3 to 6 months (Camaschella, 2015), which is why treatment continues after hemoglobin returns to normal.
When should you take the tablet, and what about tea or coffee?
On an empty stomach or between meals, if you can tolerate it: taken with a meal, it can lose up to 75% of its absorption (BSG, 2021). With a meal, tea reduces the iron absorbed by 64% and coffee by 39%; coffee taken 1 hour before the meal does not reduce it (Morck, 1983). Should you add vitamin C? In a trial of 440 patients, it changed nothing (Li, 2020).
Can low iron make you tired without anemia?
Possibly. In 198 tired women with ferritin below 50 and normal hemoglobin, fatigue fell by 47.7% with iron and by 28.8% with a placebo (Vaucher, 2012). The effect is real but modest, and fatigue has many other causes.
Should the cause be investigated?
Yes. About one third of men and postmenopausal women with iron-deficiency anemia have an underlying lesion, most often in the digestive tract (BSG, 2021). In a woman who has periods, they are a common cause, but not the only one.
True or false?
Spinach is the best source of iron.
False. Half a cup provides 3 mg, as much as lentils or tofu; 3 ounces (85 g) of oysters or a cup of white beans provide 8 (NIH).
Coffee taken one hour before a meal does not hinder iron absorption from that meal.
True. No drop was measured; taken one hour after, the drop is the same as during the meal (Morck, 1983).
Two iron tablets in one day are absorbed better than one.
False. In 40 women, the share absorbed was no better with two doses (11.8% versus 13.1%, Stoffel, 2017).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
What do you gain during treatment?
Average gain with iron alone, or with an added 200 mg of vitamin C: hemoglobin in g/L at 2 weeks, ferritin in ng/mL at 8 weeks (Li, 2020). Vitamin C changed nothing. Dotted lines: estimate between the start and the measured point.
| Time | Ferritin, iron + vitamin C: +36 ng/mL (dotted) | Ferritin, iron alone: +34 ng/mL (dotted) | Hemoglobin, iron + vitamin C: +20 g/L (dotted) | Hemoglobin, iron alone: +18 g/L (dotted) | What happens |
|---|---|---|---|---|---|
| Start | 0 | 0 | 0 | 0 | At the start |
| 2 wk | 9 | 8 | 20 | 18 | Week 2: measured point |
| 4 wk | 18 | 17 | 20 | 18 | Week 4: estimate between 2 points |
| 6 wk | 27 | 26 | 20 | 18 | Week 6: estimate between 2 points |
| 8 wk | 36 | 34 | 20 | 18 | Week 8: measured point |
Key points
- A trial of 440 patients (Throughout, Shown in humans)
96.8% women, 100 mg of iron three times a day. Hemoglobin: +18 g/L at 2 weeks with iron alone, +20 g/L with vitamin C. Ferritin: +34 ng/mL at 8 weeks with iron alone, +36 ng/mL with vitamin C. The result is equivalent. Source: Li N et al. JAMA Netw Open, 2020 - Hemoglobin at 2 weeks (Up to week 3, Shown in humans)
According to the BSG, about +10 g/L in 2 weeks is a very good sign; at 4 weeks, +20 g/L or a return to normal is expected. Source: Snook J et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut, 2021 - Stores take longer (From week 3, Shown in humans)
It takes 3 to 6 months to refill the stores, which is why treatment continues for about 3 months after hemoglobin returns to normal. Source: Camaschella C. Iron-deficiency anemia. N Engl J Med, 2015 - Reading these lines (Throughout, Hypothesis)
Only the start and end point of each line come from the study. In between, the line is an estimate. In these figures, hemoglobin was not reported after 2 weeks: the flat line does not mean it stops rising. The axis mixes two units, g/L and ng/mL.
How long does it take to recover from a blood donation?
A donation removes 200 to 250 mg of iron. Share of the lost hemoglobin that has come back, with or without 37.5 mg of iron per day (Kiss, 2015). Only the days on which 80% was reached are measured; the dotted lines connect the donation to that day. Follow-up ended at 168 days.
| Time | With iron, higher ferritin: 80% in 31 days (dotted) | With iron, low ferritin: 80% in 32 days (dotted) | No iron, higher ferritin: 80% in 78 days (dotted) | No iron, low ferritin: 80% in 158 days (dotted) | What happens |
|---|---|---|---|---|---|
| Donation | 0 | 0 | 0 | 0 | day |
| 32 d | 80 | 80 | 33 | 16 | Day 32 after donating |
| 78 d | 80 | 80 | 80 | 40 | Day 78 after donating |
| 158 d | 80 | 80 | 80 | 80 | Day 158 after donating |
Key points
- Hemoglobin: 80% back (Throughout, Shown in humans)
Low ferritin at the start: 32 days with iron, 158 days without. Higher ferritin at the start: 31 days with iron, 78 days without. Source: Kiss JE et al. JAMA, 2015 - Ferritin back to its starting level (Throughout, Shown in humans)
Low ferritin at the start: 21 days with iron, more than 168 days without. Higher ferritin at the start: 107 days with iron, more than 168 days without. Source: Kiss JE et al. JAMA, 2015 - Reading these lines (Throughout, Hypothesis)
Each line connects the donation day to the measured day on which 80% of the lost hemoglobin had come back. The shape in between is an estimate. After that day, the line stays flat: the return to 100% is not drawn.
How much iron do you need at each age?
Recommended milligrams of iron per day (NIH). Women aged 19 to 50 have the highest need, 18 mg, mainly because of periods; it rises to 27 mg during pregnancy.
| Time | Women and girls | Men and boys | What happens |
|---|---|---|---|
| 1 | 7 | 7 | year: 7 mg per day |
| 4 | 10 | 10 | years: 10 mg per day |
| 9 | 8 | 8 | years: 8 mg per day |
| 14 | 15 | 11 | years: men 11 mg, women 15 mg per day |
| 19 | 18 | 8 | years: men 8 mg, women 18 mg per day |
| 51 | 8 | 8 | years: men 8 mg, women 8 mg per day |
| 70 yr | 8 | 8 | 70 years: men 8 mg, women 8 mg per day |
Key points
- Children (Up to age 14, Shown in humans)
7 mg from ages 1 to 3, 10 mg from 4 to 8, 8 mg from 9 to 13. Source: NIH Office of Dietary Supplements. Iron: fact sheet for health professionals - Ages 14 to 18 (Ages 14 to 19, Shown in humans)
11 mg for boys, 15 mg for girls. Source: NIH Office of Dietary Supplements. Iron: fact sheet for health professionals - Adults (Ages 19 to 51, Shown in humans)
8 mg for men, 18 mg for women aged 19 to 50. Pregnancy: 27 mg. Breastfeeding: 9 mg. Source: NIH Office of Dietary Supplements. Iron: fact sheet for health professionals - From age 51 (From age 51, Shown in humans)
8 mg per day, for women and men alike. Source: NIH Office of Dietary Supplements. Iron: fact sheet for health professionals - The upper limit (Throughout, Shown in humans)
45 mg per day for adults, counting food and supplements together, unless a doctor advises otherwise. Vegetarians have an estimated need 1.8 times higher, because iron from plants is absorbed less well. Source: NIH Office of Dietary Supplements. Iron: fact sheet for health professionals
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