Educational guide · 13 min read

Iron, explained properly.

Iron deficiency is the most common nutritional deficiency in the world — affecting 1 in 4 pre-menopausal women in Australia and Thailand. Most people don't know they're deficient because the symptoms (fatigue, brain fog, hair shedding) are easy to blame on "stress" or "age". This guide covers what iron does, the 4 forms that matter, and when to ask for a ferritin blood test.

Last updated: August 23, 2026 · Sources: NIH ODS Iron, Camaschella 2019, Lönnerdal 2010, Milman 2014

What iron actually is

Iron is an essential trace mineral and the most abundant metal in the human body. An adult carries 3-4 grams total — about 70% locked in hemoglobin (red blood cells), 10% in myoglobin (muscle), 15-20% stored as ferritin in liver, spleen, and bone marrow, and the rest distributed in enzymes and tissues.

Iron is essential because it reversibly binds oxygen — it's the atom that lets hemoglobin carry O₂ from lungs to tissues, lets myoglobin store O₂ in muscle, and lets cytochromes generate ATP in mitochondria. Without iron, every cell in the body suffocates.

The bottom line: iron deficiency is the world's most common nutritional deficiency — over 1.4 billion people globally per Camaschella 2019. Pre-menopausal women, athletes, vegetarians, and frequent blood donors are highest risk. The symptoms (fatigue, hair shedding, brain fog, cold intolerance) are non-specific and routinely misattributed.

How iron works in the body

Iron is involved in more than 200 enzyme systems and is a cofactor for at least 6 essential biological processes:

Oxygen transport. Hemoglobin (4 iron atoms per molecule) carries O₂ from lungs to tissues. Each RBC carries ~250 million hemoglobin molecules. Iron deficiency → microcytic anemia → tissue hypoxia → fatigue.
Oxygen storage. Myoglobin in muscle stores O₂ for rapid release during exercise. Iron deficiency → reduced exercise capacity and recovery.
ATP production. Iron is a cofactor for cytochrome c and cytochrome c oxidase — the final enzymes in the electron transport chain. No iron = mitochondria can't generate ATP efficiently.
DNA synthesis. Ribonucleotide reductase, the rate-limiting enzyme for DNA synthesis, requires iron. Iron deficiency impairs cell division everywhere — including the rapidly-dividing cells of the intestinal lining, hair follicles, and immune system.
Neurotransmitter synthesis. Tyrosine hydroxylase (dopamine, norepinephrine) and tryptophan hydroxylase (serotonin) require iron. Iron deficiency in children impairs cognitive development per Beard 2008.
Immune function. Neutrophil oxidative burst (the killing mechanism) requires iron-dependent myeloperoxidase. Iron deficiency increases infection susceptibility.

The 4 forms of iron supplements, compared

The form determines absorption, GI tolerance, and clinical use. This is the most important decision you make when choosing an iron supplement.

1. Ferrous sulfate (the standard)

Cheapest

Fe²⁺ bound to sulfate. 20% elemental iron by weight. The form used in essentially all RCTs that show benefit. Cheapest. But causes GI side effects (constipation, nausea, dark stools) in 20-30% of users per Tolkien 2015.

Best for: clinical correction of severe deficiency under medical supervision. Cost-sensitive users. Best taken with vitamin C, on an empty stomach.

2. Ferrous bisglycinate (Ferrochel)

Best tolerated

Fe²⁺ chelated with two glycine amino acids. Lönnerdal 2010: 3-4x higher bioavailability vs ferrous sulfate in iron-deficient women. Much better GI tolerance — nausea and constipation rates similar to placebo. The form most evidence-based practitioners recommend first.

Best for: long-term maintenance, sensitive stomachs, anyone who stopped ferrous sulfate due to side effects.

3. Ferrous fumarate

Mid-strength

Fe²⁺ bound to fumaric acid. 33% elemental iron — highest elemental Fe of any common oral form. Better tolerated than sulfate but less studied than bisglycinate. Used in many prenatal vitamins.

Best for: prenatal vitamins, anyone who needs more elemental iron per pill without escalating volume.

4. Polysaccharide-iron complex (PIC)

Liquid-friendly

Ferric iron (Fe³⁺) bound to a polysaccharide carrier. Marketed for those who can't tolerate ferrous forms. Some evidence for better tolerability. More expensive. Less studied than ferrous forms in head-to-head RCTs.

Best for: children, liquid formulations, people who react to both ferrous sulfate and bisglycinate.

Why so many people are low on iron

The Australian Bureau of Statistics 2011-2012 Health Survey found iron deficiency in 14% of pre-menopausal women — making it the most common nutritional deficiency in the country. The reasons:

Menstrual blood loss: 30-50 mL of blood per period = ~15-25 mg iron lost. High menstrual flow (menorrhagia) can lose 50+ mg per cycle per Milman 2014.
Pregnancy: iron requirements roughly double during pregnancy (~27 mg/day) to support fetal RBC production and placenta. Most women enter pregnancy with suboptimal stores.
Vegetarian / vegan diets: non-heme iron (plant sources) is 2-5x less bioavailable than heme iron (animal sources). Vegetarians need 1.8x more dietary iron per Hallberg 1981.
Athletes: endurance running causes foot-strike hemolysis and sweat losses; ~15-30% of female endurance athletes are deficient per Sim 2019.
Gut malabsorption: celiac, IBD, H. pylori, bariatric surgery all impair iron absorption. Often missed because iron deficiency is "explained" by menstruation.
Frequent blood donation: each unit donated removes ~200-250 mg iron. Regular donors (menstruating women especially) commonly deplete.

How much iron do you actually need?

Per the US Institute of Medicine (IOM 2001) and the Australian NHMRC. RDAs assume 75-80% of dietary iron from heme sources — vegetarians need ~1.8x more.

Group	RDA (mg/day)	UL supplement only (mg/day)
Children 1–3 years	7	40
Children 4–8 years	10	40
Children 9–13 years	8	40
Males 14–18 years	11	45
Males 19+	8	45
Females 14–18	15	45
Females 19–50	18	45
Females 51+	8	45
Pregnancy	27	45
Lactation	9–10	45

Source: NIH ODS Iron Fact Sheet. UL of 45 mg/day applies to supplements only — dietary iron is poorly absorbed at high doses so toxic accumulation from food alone is rare.

Choosing by life stage

Pre-menopausal women (period, no deficiency)

Ferrous bisglycinate 18-25 mg/day, taken with vitamin C. Skip days during your period if symptoms are mild. Bisglycinate avoids the constipation/nausea that drives most women to quit ferrous sulfate within 3 months.

Pregnancy and prenatal

27 mg/day minimum per WHO. Most prenatal vitamins contain 27-30 mg ferrous fumarate or sulfate. If deficient (low ferritin), your OB will prescribe 60-100 mg/day of ferrous sulfate. Always take with prenatal vitamin C and a meal to reduce nausea.

Athletes (endurance or heavy training)

Foot-strike hemolysis + sweat losses = increased requirements. Sim 2019: 15-30% of female endurance athletes are deficient. Iron bisglycinate 25-30 mg/day + ferritin check every 6 months if training hard. Avoid taking within 2 hours of coffee or tea (tannins block absorption).

Vegetarians and vegans

Non-heme iron from plants is 2-5x less bioavailable than heme from animal sources. Hallberg 1981: vegetarians need 1.8x more total dietary iron. Pair iron with vitamin C, avoid tea/coffee with meals. Bisglycinate 25-30 mg/day is the practical baseline.

Postmenopausal women and adult men

Usually no supplement needed unless blood loss (ulcers, GI bleeding, frequent blood donation). Check ferritin if fatigue symptoms. Don't supplement routinely — iron overload is a real risk in older adults (hemochromatosis prevalence is 1 in 200 of Northern European descent).

Children and picky eaters

Liquid iron (NovaFerrum, Fer-In-Sol) is the standard. Pediatricians dose by weight — typically 3-6 mg/kg/day for documented deficiency. Use bisglycinate or PIC forms to avoid GI upset. Don't give adult doses to children.

What to pair with iron (and what to avoid)

Pairs well

Vitamin C (250 mg) — 2-3x absorption boost per Hallberg 1989; critical for non-heme iron
Meat protein — "meat factor" enhances non-heme iron absorption ~2x
Folate and B12 — needed for RBC production; deficiency can mimic iron deficiency
Vitamin A / beta-carotene — modest absorption enhancement

Avoid within 2 hours

Coffee / tea / chocolate — tannins + polyphenols inhibit absorption 50-90%
Calcium / dairy — competes with iron for absorption (separate by 2 hours)
Antacids (PPIs, H2 blockers) — raise gastric pH, iron needs acid to absorb Fe³⁺→Fe²⁺
High-fiber foods / phytates (whole grains, legumes) — bind iron; soak/ferment to reduce
Tetracyclines, fluoroquinolones, levothyroxine — chelation; separate by 2-4 hours

Frequently asked questions

▶ Should I take iron on an empty stomach or with food?

Empty stomach gives 2-3x better absorption but causes nausea in 30% of users. If you tolerate it: empty stomach with 250 mg vitamin C. If you don't: take with the smallest meal possible, ideally containing meat. The absorption penalty from food is acceptable vs the GI side effects that drive quitting.

▶ Is black stool from iron supplements dangerous?

No — it's normal. Unabsorbed iron combines with hydrogen sulfide in the gut to form iron sulfide (FeS), which is black. This is harmless and stops when you stop the supplement. It can however mask melena (black tarry stool from GI bleeding) — tell your doctor if you have black stool WITHOUT iron supplementation.

▶ When should I get a ferritin blood test?

If you have fatigue, brain fog, hair shedding, pica (craving ice or non-food items), or restless legs. Standard ferritin reference range is 15-200 µg/L for women, but optimal is 50-100 µg/L per multiple functional medicine reviews. Ask your GP for a serum ferritin + CBC + CRP — CRP tells you if inflammation is falsely elevating ferritin.

▶ Can men take iron?

Only if blood-tested deficient. Adult men lose very little iron (no menstruation, ~1 mg/day GI loss), and excess iron accumulates in liver, heart, and pancreas. Hemochromatosis (genetic iron overload) affects 1 in 200 Northern Europeans and silently damages organs over decades. Routine iron supplementation in men is not recommended.

▶ How long until I feel better after starting iron?

Energy often improves within 2-4 weeks. Hemoglobin rises within 4-6 weeks. Ferritin stores take 3-6 months to fully replenish — even after blood counts normalize. Don't stop iron too early or you'll bounce back into deficiency. Plan for 6 months of supplementation to fully rebuild stores.

▶ Is heme iron from animal sources better absorbed?

Yes — heme iron (from red meat, liver, shellfish) is absorbed 2-3x more efficiently than non-heme iron (from plants and supplements). Hallberg 1981: heme iron ~25% absorption vs non-heme ~5-12%. If you eat red meat 2-3 times per week, your iron needs are lower than a vegetarian's.

References & sources

NIH Office of Dietary Supplements — Iron Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
Camaschella C. (2019). Iron deficiency. Blood 133(1):30-39. doi.org/10.1182/blood-2018-05-815944
Milman N. (2014). Iron requirements in menstruating women. Acta Haematologica 131(4):196-204. doi.org/10.1159/000350826
Lönnerdal B. (2010). Iron and zinc absorption from ferrous bisglycinate chelate in infants. AJCN 91(3):831S-837S.
Cook JD. (2005). Diagnosis and management of iron-deficiency anaemia. Best Practice & Research Clinical Haematology 18(2):319-332. doi.org/10.1016/j.beha.2004.10.001
Abbaspour N, et al. (2014). Review on iron and its importance for human health. Journal of Research in Medical Sciences 19(2):164-174.
Sim M, et al. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology 119(7):1463-1478. doi.org/10.1007/s00421-019-04157-y
Hallberg L, et al. (1989). The role of vitamin C in iron absorption. International Journal for Vitamin and Nutrition Research 59(1):55-69.
Examine.com — Iron research review. examine.com
Chemist Warehouse AU, iHerb, Amazon, Swisse AU, Blackmores AU, BioCeuticals AU, Ethical Nutrients, Herbs of Gold, Nature's Own, Mega We Care, Thorne — product pages and price snapshots cited per row in the iron comparison table.

Trusted brands we rate

These 10 brands are featured on every vitfacts.com comparison page. We track them for ingredient transparency, third-party testing, and Thailand availability.

Swisse

Since 1969

MID

One of the most globally recognised Australian brands, dominating the market with comprehensive multivitamins and targeted wellness blends.

Available in TH Search “iron” ↗

Blackmores

Since 1932

MID

A heritage naturopathic brand spanning over 90 years, known for rigorous ingredient sourcing and massive pharmacy presence throughout the Asia-Pacific region.

Available in TH Search “iron” ↗

BioCeuticals

Since 1993

PREMIUM

A premium, practitioner-only brand (owned by Blackmores) offering highly concentrated, clinical-strength formulas that are frequently recommended by healthcare professionals.

TH: limited Search “iron” ↗

Ethical Nutrients

Since 1986

MID

A high-quality pharmacy brand heavily focused on evidence-based ingredients, widely known for its "Inner Health" probiotic range and highly absorbable magnesium powders.

TH: limited Search “iron” ↗

Herbs of Gold

Since 1980s

MID

Born in Australia, this brand formulates therapeutic herbal and nutritional supplements with a strict focus on purity, offering many vegan-friendly options.

TH: limited Search “iron” ↗

Nature's Own

Since 1970s

BUDGET

One of Australia's longest-running and most accessible vitamin brands, providing a massive, budget-friendly range found in most major supermarkets and pharmacies.

TH: limited Search “iron” ↗

Mega We Care

Since 1982

MID

A leading pharmaceutical-grade supplement brand in Southeast Asia, widely trusted by Thai consumers for its fish oil and targeted clinical nutrition products.

Available in TH Search “iron” ↗

Thorne Research

Since 1984

PREMIUM

While based in the US, Thorne is heavily imported and highly sought after by functional medicine practitioners globally—including in both Australia and Thailand. It is considered a gold standard due to its strict four-round testing protocol and complete absence of unnecessary fillers or binders.

TH: limited Search “iron” ↗

Healthy Care

Since 2006

BUDGET

An Australian-made value brand founded in 2006, sold through Chemist Warehouse and major AU pharmacies. Part of the Nature's Care family (est. 1990), offering a wide mass-market supplement range covering fish oil, vitamins C/D, magnesium, probiotics, zinc, iron, ashwagandha, collagen, and CoQ10.

TH: limited Search “iron” ↗

Why these 10? See the VitFacts editorial policy page (forthcoming) for the criteria — minimum 10 years of documented safety record, public certificate-of-analysis policy, and verifiable Thailand distribution.

Not medical advice. This article is for educational purposes only and does not constitute medical advice. Iron supplementation without blood testing can cause iron overload in susceptible individuals. Consult a qualified clinician before starting any iron supplement, especially if you have hemochromatosis, chronic liver disease, or are a regular blood donor.