Hemoglobin is iron in circulation. Ferritin is iron in storage. The body can protect hemoglobin at all costs - cannibalising storage reserves for months before the circulating level drops. By the time your CBC flags low hemoglobin, iron deficiency may have been present for the better part of a year.
This is why 'your blood work is normal' and 'you are not iron deficient' are not the same statement (Pasricha et al., 2021). A complete blood count with normal hemoglobin can tell you that your body has not yet reached the stage of frank anaemia. It can't speak to your ferritin, which is where the story may start.
The storage-vs-circulating gap
Ferritin is the protein that stores iron inside cells - primarily in the liver, spleen, and bone marrow. When dietary iron is insufficient or losses exceed intake, ferritin can deplete first. Only once stores are substantially exhausted may the body begin compromising red blood cell production, which can eventually show up as low hemoglobin (Mei et al., 2021).
The standard lab lower reference for ferritin may be 10–15 ng/mL - the threshold at which you may be clinically iron deficient by the narrowest definition. The functional floor for symptom relief may be a different number entirely.
The 50 ng/mL functional threshold
A randomised trial in non-anaemic women with ferritin below 50 ng/mL found that iron supplementation can significantly reduce fatigue scores (Vaucher et al., 2012). The body can be depleted enough to impair energy production, hair growth, and cognitive function while remaining above the anaemia cut-off. Optimal may not be the same as not deficient.
What low ferritin can look like
- Fatigue that may not improve with adequate sleep - iron is essential for mitochondrial energy production.
- Hair shedding (telogen effluvium) - follicles are iron-hungry; deficiency can trigger the resting phase.
- Restless legs syndrome - dopamine synthesis in the brain requires iron; restless legs can be an early neurological sign.
- Reduced exercise tolerance - ferritin below 30 ng/mL can impair VO₂ max even in the absence of anaemia.
- Brain fog and difficulty concentrating - can be especially pronounced in the luteal phase when progesterone demands more oxygen.
Who may be most at risk
Menstruating women
A typical menstrual cycle involves a loss of 20–80 mL of blood - roughly 10–40 mg of iron. Heavy periods (menorrhagia) can easily double or triple this. The WHO estimates that 30% of non-pregnant women of reproductive age are anaemic worldwide; in the US, estimates of iron deficiency range from 10–20%, with significantly higher rates in women with heavy flow.
Plant-forward eaters
Non-haem iron (from plant sources) is absorbed at 1–10% efficiency. Haem iron (from meat) is absorbed at 15–35%. Vitamin C at the same meal can substantially improve non-haem absorption; coffee, tea, and calcium can inhibit it (Zijp et al., 2000). Vegetarian and vegan women may need roughly 1.8× the iron intake of omnivores to achieve equivalent absorption.
Endurance athletes
Foot-strike haemolysis (red blood cell destruction from repetitive impact), sweat losses, and GI microbleeding during intense training can all increase iron turnover (Peeling et al., 2008). Female runners may have some of the highest rates of iron depletion of any population group.
How to replete effectively
Ferrous bisglycinate can be better tolerated than ferrous sulfate with equivalent or superior absorption - fewer GI side effects can mean better adherence. Take on an empty stomach with vitamin C where possible; avoid within two hours of calcium, antacids, or thyroid medication. Retest ferritin at 90 days - not hemoglobin, which can look normal before stores are rebuilt.
References
- Pasricha, S. R., et al. (2021). Iron deficiency. Lancet, 397(10270), 233-248. PMID 33285139
- Mei, Z., et al. (2021). Physiologically based serum ferritin thresholds for iron deficiency in children and non-pregnant women: a US National Health and Nutrition Examination Surveys (NHANES) serial cross-sectional study. Lancet Haematology, 8(8), e572-e582. PMID 34329578
- Vaucher, P., et al. (2012). Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ, 184(11), 1247-1254. PMID 22777991
- Zijp, I. M., et al. (2000). Effect of tea and other dietary factors on iron absorption. Critical Reviews in Food Science and Nutrition, 40(5), 371-398. PMID 11029010
- Peeling, P., et al. (2008). Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. European Journal of Applied Physiology, 103(4), 381-391. PMID 18365240
