Health Reference LibraryIron
Iron is the most common nutritional deficiency worldwide and the most common cause of fatigue without a clear clinical explanation. The 43 entries in this section cover what the published evidence shows about ferritin thresholds, iron forms compared, vitamin C interactions, the four-hour rule with thyroid medication, restless legs syndrome, repletion timelines, and what to investigate when iron supplementation does not appear to work.
Iron deficiency is common, addressable, and frequently mismanaged. UK estimates suggest around one in three women of reproductive age have suboptimal iron stores, and unexplained fatigue is the most common reason adults try iron supplementation. The published research on iron is unusually well-developed: there are clear NICE and BSG (British Society of Gastroenterology) guideline positions, a robust dose-response literature, and decades of randomised trials on absorption, forms, and timing.
What complicates iron in practice is that the laboratory picture rarely matches the symptom picture cleanly. A "normal" iron panel can hide functional iron deficiency. Ferritin can read as normal in the presence of inflammation, because ferritin is an acute-phase protein. Haemoglobin can be preserved while iron stores are still depleted, which is why iron deficiency without anaemia (IDWA) is a recognised clinical state. The entries in this section work through these distinctions one at a time, anchored to the published guideline positions and the underlying evidence.
The questions covered include:
- Forms compared: ferrous sulphate vs ferrous fumarate vs ferrous bisglycinate, and where the side-effect and absorption evidence actually lands
- Vitamin C and absorption: what the single-meal isotope studies show versus what happens over weeks of repletion
- Timing and interactions: the four-hour rule for iron and levothyroxine, why iron and zinc should not share a meal, and how calcium reduces non-haem iron absorption
- Ferritin thresholds: what level genuinely warrants supplementation, and where UK clinical guidance and integrative-style framings disagree
- Repletion timelines: realistic expectations for haemoglobin (weeks) versus ferritin (months) versus symptom resolution (variable)
- Restless legs syndrome: the brain iron deficiency mechanism that is distinct from systemic iron deficiency, and why a higher ferritin supplementation threshold is conventionally used
- Heavy menstrual bleeding as a recurrent cause, and the NICE NG88 position on when to investigate
- What to check when iron supplementation does not appear to be working after four to six weeks
Every entry cites its sources to PubMed (PMID) or DOI. Where guideline bodies disagree, the disagreement is shown explicitly rather than averaged into a confident-sounding consensus.
While most entries focus on iron deficiency, the section also covers two adjacent conditions at the other end of the iron-status spectrum: hereditary haemochromatosis (HFE-mediated iron overload, where phlebotomy is the mainstay and dietary measures are adjuncts) and chronic iron loss from haemorrhoid bleeding (where stool softening and iron replacement intersect). Both sit within the adult outpatient nutrition-relevant scope of the reference library.
This section does not cover intravenous iron, transfusion decisions, or paediatric iron deficiency, which sit outside the scope of a reference library.
Entries
- Can iron deficiency raise resting heart rate?
- Does iron compete with calcium for absorption?
- How do iron, sleep, and stress create a vicious cycle?
- How do iron, thyroid, and mitochondria determine energy levels?
- How does inflammation mask iron deficiency on ferritin?
- How does iron deficiency impair thyroid function?
- How does iron status specifically affect female fertility?
- How does low ferritin affect focus before anaemia develops?
- How does low ferritin affect sleep quality independently of fatigue?
- How does low ferritin affect thyroid, energy, and cognition?
- How does nutrition help prevent and manage haemorrhoids?
- How does vitamin C affect iron absorption when deficient?
- How is hereditary haemochromatosis managed nutritionally?
- How long does iron take to correct deficiency, and when to retest?
- How long does iron take to resolve symptoms and ferritin?
- How long does it take to fully replete iron stores?
- Iron panel normal but still exhausted: what should I check?
- Iron supplementation timeline: when to expect energy improvement?
- Iron supplementation timeline: when to expect sleep improvement?
- Is there a loading protocol for iron supplementation?
- Should I take iron and zinc at the same time?
- What does low ferritin with normal haemoglobin mean?
- What does the full iron panel tell us beyond ferritin alone?
- What ferritin level warrants iron supplementation?
- What nutritional support helps with heavy periods and iron loss?
- What's the iron-sleep-stress triad and how does it present?
- Why do iron, thyroid, and vitamin D deficiencies compound?
- Why does inflammation block iron use even with full stores?
Most-cited evidence in Iron
- New England Journal of Medicine (2015) — Iron-deficiency anemia PMID: 25946282 · DOI: 10.1056/nejmra1401038 (cited in 11 entries)
- Gut (2021) — British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults PMID: 34497146 · DOI: 10.1136/gutjnl-2021-325210 (cited in 11 entries)
- PLoS One (2015) — Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis PMID: 25700159 · DOI: 10.1371/journal.pone.0117383 (cited in 10 entries)
- NIH Office of Dietary Supplements (US government) — NIH Office of Dietary Supplements — Iron Fact Sheet for Health Professionals Source (cited in 9 entries)
- Blood (2015) — Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women PMID: 26289639 · DOI: 10.1182/blood-2015-05-642223 (cited in 9 entries)