NIH ODS + Karolinska iron absorption studies

Vegan vs Vegetarian Iron: Heme vs Non-Heme Bioavailability

Vegan and vegetarian diets contain plenty of iron by milligram count. The harder question is how much your gut actually absorbs. The chemistry of heme versus non-heme iron, the role of vitamin C, and the dramatic effect of tea and coffee with meals are what turn a plant-based diet into either an iron-replete diet or an iron-marginal one.

The short answer. The US Institute of Medicine recommends that vegans and vegetarians consume 1.8 times the standard iron RDA because non-heme plant iron is absorbed less efficiently than heme iron from meat. That works out to 14 mg per day for adult men and 33 mg per day for menstruating women. The practical levers are pairing iron-rich plants with vitamin C, avoiding tea and coffee with meals, and soaking or sprouting legumes to reduce phytate.

Heme versus non-heme iron, in plain terms

Dietary iron arrives in two forms. Heme iron is the iron bound inside the haemoglobin and myoglobin of meat, poultry, and fish. The heme molecule (a porphyrin ring with iron in the centre) is absorbed across the small intestine by a dedicated heme transporter, and absorption efficiency is high (15% to 35% depending on body iron status) and largely insensitive to other dietary components. Non-heme iron is the iron found in plants, dairy, and eggs. It exists as inorganic Fe2+ or Fe3+ and is absorbed by the divalent metal transporter DMT1, which is much fussier. Non-heme absorption efficiency ranges from 2% in a low-iron-status person eating a high-phytate vegan meal with tea, up to about 20% in an iron-deplete person eating a fortified cereal with orange juice.

That 5 to 10x range is what makes non-heme iron so context-dependent. Eat a lentil curry with brown rice and a glass of orange juice and you might absorb 8% of the iron in the meal. Eat the same curry with a cup of strong black tea and you might absorb 3%. The difference is not subtle, and it stacks across years of meals.

The NIH Office of Dietary Supplements iron fact sheet sets out the figures in detail; the BDA iron food fact sheet gives the practical pairing guidance for UK readers; and the Academy of Nutrition and Dietetics position paper on vegetarian diets (Melina, Craig, Levin, J Acad Nutr Diet 2016) summarises the same evidence base.

Why the IOM raises the vegan and vegetarian RDA by 1.8x

The standard iron RDA assumes a mixed Western diet with about 10% absorption averaged across heme and non-heme sources. A vegan diet is 100% non-heme, with an average absorption closer to 5 to 6% in real-world conditions. The Institute of Medicine resolves this by applying a 1.8x upward adjustment: vegan adult men should aim for 14 mg per day (8 mg standard x 1.8), vegan menstruating women for 33 mg per day (18 mg standard x 1.8), and vegan post-menopausal women for 14 mg per day (8 mg x 1.8). This is the official US position and is repeated by the Academy of Nutrition and Dietetics.

The UK SACN and the European Food Safety Authority do not apply the same 1.8x multiplier; they recommend the same baseline numbers and add guidance on absorption-enhancing meal patterns. The practical effect is the same: a vegan or vegetarian wanting to be iron-replete should eat iron-rich foods at most meals and pay attention to what they drink with them.

The 33 mg per day target for menstruating vegan women is high enough that it is worth thinking about in food terms. A typical day hitting that target might look like: half a cup of fortified bran flakes at breakfast (around 8 mg) with orange juice; a lentil and spinach soup at lunch (around 8 mg combined); roasted chickpeas as a snack (3 mg per 60 g); tofu stir-fry with broccoli and red pepper at dinner (around 8 mg from the tofu plus vitamin C boost); pumpkin seeds (2.5 mg per 30 g) sprinkled on a salad. That comes to roughly 30 mg, and the vitamin C and lack of tea-with-meals lifts the bioavailable portion comfortably above the absorbed equivalent of a typical mixed-diet 18 mg.

The absorption modifiers, with effect sizes

Modifier	Effect on non-heme absorption	Practical example
Vitamin C, 50 mg with meal	+150 to +200%	Half an orange or a small kiwi
Vitamin C, 100 mg with meal	+200 to +300%	A glass of orange juice or a red pepper
Citric acid (other organic acids)	+30 to +100%	Lemon juice over greens, tomato in stew
Heme iron (mixed diet)	Enhances co-eaten non-heme iron uptake	Not applicable for vegans
Phytate (legumes, grains, nuts)	-50 to -65%	Why soaking and sprouting matters
Polyphenols in tea (tannins)	-60 to -65%	Strong black tea with meal
Polyphenols in coffee	-35 to -50%	Coffee with breakfast cereal
Calcium 300 to 600 mg with meal	-30 to -50%	Milk-heavy meal or calcium supplement
Cooking acidic food in cast iron	+200 to +800% (added iron from pan)	Tomato sauce simmered 30 min in cast iron

Effect sizes are pulled from the Karolinska iron absorption studies (Hallberg L, Brune M, Rossander L et al.) and the WHO 2004 Iron and Health monograph. The single-largest practical win for a vegetarian or vegan is moving tea and coffee out of meal windows. The second-largest is making sure every iron-rich meal carries a vitamin C source. The third is reducing phytate by soaking beans overnight, using sourdough rather than industrial bread, and choosing sprouted grains where available.

Per-food iron table

Food	Serving	Iron (mg)	Diet
Firm tofu	100 g	6.6	Vegan
Tempeh	100 g	2.7	Vegan
Lentils (cooked)	1 cup (198 g)	6.6	Vegan
White beans (cooked)	1 cup (179 g)	8.0	Vegan
Chickpeas (cooked)	1 cup (164 g)	4.7	Vegan
Kidney beans (cooked)	1 cup (177 g)	5.2	Vegan
Spinach (cooked)	1 cup (180 g)	6.4 (high oxalate)	Vegan
Quinoa (cooked)	1 cup (185 g)	2.8	Vegan
Pumpkin seeds	30 g	2.5	Vegan
Cashews	30 g	1.9	Vegan
Dark chocolate (70%)	30 g	3.4	Vegan (check butter)
Fortified bran flakes	40 g	4 to 18 (label)	Vegan
Blackstrap molasses	1 tbsp	3.5	Vegan
One large egg	50 g	0.9	Vegetarian
Cheddar cheese	30 g	0.2	Vegetarian
Cow milk	250 ml	0.0	Vegetarian

Values from USDA FoodData Central. The dairy column is a reminder that vegetarians do not gain iron from the dairy and egg additions: eggs contribute modest amounts, dairy contributes essentially none. The vegetarian advantage on iron is small and depends on whether the eggs and dairy displace plant iron sources from the plate.

Spinach is not the iron superfood it was sold as

The original 19th-century iron analyses of spinach contained a decimal error that exaggerated its iron content by a factor of ten, and the legend persisted into the Popeye era. The actual spinach iron content (around 3.5 mg per cup cooked) is respectable, but spinach also contains 750 mg of oxalate per cup, and that oxalate binds the iron tightly and reduces absorption to a few per cent of nominal content. The practical implication is not that spinach is bad, but that spinach should not be your only or main iron source. Pair it with vitamin C, alternate with other dark greens (kale, collards, Swiss chard contain far less oxalate), and treat it as a contributor rather than a load-bearer.

The same caveat applies in milder form to other high-oxalate plants such as rhubarb, beet greens, and Swiss chard. Cooking and discarding the cooking water removes around 50% of soluble oxalate. Soaking and sprouting helps with phytate but not with oxalate.

Testing and supplementation when needed

The standard iron status panel is full blood count (looking at haemoglobin, MCV, and red cell distribution width) plus serum ferritin. Ferritin below 30 ng/mL in adults indicates iron stores are running low, even if haemoglobin is still normal. Below 15 ng/mL is depletion. Pre-supplementation, transferrin saturation and serum iron add detail; post-supplementation, ferritin is the marker to track. Avoid taking iron tablets in the 24 hours before a ferritin draw because acute supplementation can elevate the reading transiently.

If supplementation is needed, ferrous sulfate, ferrous fumarate, and ferrous gluconate are the cheap effective options. Doses of 40 to 65 mg elemental iron per day are typical; higher single doses tend to produce GI side effects (constipation, dark stool, nausea) without proportionally higher absorption. Alternate-day dosing is now supported by the Stoffel and Zimmermann group's work showing hepcidin suppression on consecutive-day dosing reduces net absorption. Take with vitamin C; do not take with calcium, dairy, tea, or coffee.

For pregnancy, the NICE-aligned UK approach is to supplement on the basis of haemoglobin status rather than universally; the US ACOG approach is more uniformly to supplement. Either way, prenatal vitamins typically contain 17 to 27 mg elemental iron, which is below the IOM RDA for pregnant vegans but is usually adequate when combined with dietary iron and an absorption-friendly meal pattern. See vegan vs vegetarian pregnancy for the longer treatment.

Iron excess matters too. Unlike B12 or vitamin C, iron is genuinely toxic in excess. The IOM tolerable upper intake is 45 mg per day for adults from supplements; food iron is essentially impossible to overdose on. Hereditary haemochromatosis affects around 1 in 200 people of Northern European descent and causes progressive iron loading from normal dietary iron. Do not take iron supplements without testing.

Related nutrient pages

Keep reading

Frequently asked questions about iron

Are vegans and vegetarians iron deficient more often than meat-eaters?

Serum ferritin (iron stores) is consistently lower in vegans and vegetarians than in omnivores across the published cohort data, but the rate of frank iron-deficiency anaemia is not consistently higher. The EPIC-Oxford cohort found vegans had lower ferritin but similar haemoglobin to meat-eaters. The Adventist Health Study cohort found similar patterns. The interpretation is that lower iron stores are not the same as deficiency, and a body running on lower stores is not necessarily compromised. The exception is women of menstruating age, where any diet group has elevated risk and the vegan and vegetarian groups particularly need to pay attention.

Why does vitamin C help iron absorption?

Non-heme iron in plant foods exists predominantly as ferric (Fe3+) iron, which is poorly absorbed. Vitamin C (ascorbic acid) reduces ferric iron to ferrous (Fe2+) iron at the brush border of the small intestine, and it also forms a soluble chelate that resists the phytate and polyphenol binding that would otherwise prevent uptake. The published studies (Hallberg, Hurrell, and colleagues at the Karolinska iron lab) show that 50 to 100 mg of vitamin C consumed with a meal can triple non-heme iron absorption from that meal. A glass of orange juice with a bowl of fortified cereal is genuinely impactful, not folk wisdom.

Should I take an iron supplement on a vegan or vegetarian diet?

Routine supplementation is not recommended for adults on plant-based diets unless blood work shows depleted ferritin or low haemoglobin. The NHS, NIH ODS, and BDA all warn against unprescribed iron supplementation because iron excess (haemochromatosis or simple over-supplementation) damages the liver and pancreas. Pregnant women on any diet typically receive iron through their prenatal vitamins. Menstruating women losing significant blood may benefit from a low-dose supplement after testing. Otherwise, dietary iron from legumes, tofu, fortified cereals, and dark leafy greens, combined with vitamin C and avoidance of tea or coffee with meals, is the first-line strategy.

How does tea and coffee reduce iron absorption?

Tea contains tannins (polyphenolic compounds) that bind to non-heme iron in the gut and form an insoluble complex that cannot be absorbed. The published reduction in non-heme iron absorption from a meal taken with a cup of strong black tea is around 60 to 65%. Coffee has a similar but smaller effect (about 35 to 50% reduction) via chlorogenic acid and other polyphenols. The mitigation is straightforward: drink tea and coffee one hour before or two hours after meals, not with them. This single behaviour change has a larger effect on practical iron status for vegetarians and vegans than most supplement choices.

What are the best plant iron sources?

By per-serving content the leaders are firm tofu (around 6.6 mg per 100 g), lentils (3.3 mg per cup cooked), pumpkin seeds (2.5 mg per 30 g), spinach (around 3.5 mg per cup cooked, but oxalate-bound), chickpeas (2.4 mg per cup), white beans (8 mg per cup), and quinoa (2.8 mg per cup cooked). Fortified breakfast cereals can deliver 4 to 18 mg per serving depending on brand. Blackstrap molasses delivers around 3.5 mg per tablespoon. Iron-fortified plant milks have started to appear but are not standard. Combine any of these with a vitamin C source at the same meal.

Does cooking in a cast iron pan add iron to food?

Yes, measurably. Acidic foods cooked in cast iron pick up meaningful iron from the pan surface. Studies have measured a tomato sauce simmered for 30 minutes in cast iron rising from about 0.6 mg iron per 100 g to 5 mg per 100 g. The effect is largest with acidic, water-rich foods and small with dry or fatty foods. The iron picked up is non-heme ferrous iron and is absorbed at the standard non-heme efficiency. It is a small but real contribution to daily iron intake, particularly useful for vegans cooking tomato-based curries, chillis, and stews.

Sources cited. NIH ODS Iron fact sheet for health professionals; BDA iron food fact sheet; Hallberg L, Brune M, Rossander L. Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate, Am J Clin Nutr 1989; 49: 140-144; Hurrell R, Egli I. Iron bioavailability and dietary reference values, Am J Clin Nutr 2010; 91: 1461S-1467S; Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: vegetarian diets, J Acad Nutr Diet 2016; 116: 1970-1980; WHO Iron Deficiency Anaemia: Assessment, Prevention, and Control monograph; Stoffel NU, Zimmermann MB et al. Iron absorption from oral iron supplements given on consecutive versus alternate days, Lancet Haematol 2017; 4: e524-e533. All values as of May 2026.