What Is Iron? Functions, Iron-Rich Foods, Daily Intake, Deficiency & How to Choose Iron Supplements

“What does iron actually do in the body?” “Is eating liver or spinach enough?” “What is the difference between heme and non-heme iron?” “What are ferritin and ferritin iron?” “And what should you look for when choosing an iron supplement?”

Iron is one of the essential minerals required by the human body.

It is especially well known as a component of haemoglobin, the protein found in red blood cells.

Iron is also involved in myoglobin in muscle tissue and in a variety of enzymes and proteins throughout the body.

To understand iron properly, it helps to look beyond the amount alone. Important factors include heme and non-heme iron, your body's iron status, menstruation, pregnancy and breastfeeding, food combinations, and the form of iron used in supplements.

This guide explains what iron is, its functions in the body, iron-rich foods, recommended daily intake, heme vs non-heme iron, factors that influence absorption, the difference between iron deficiency and anaemia, ferritin and ferritin iron, iron needs for women and during pregnancy, and how to compare iron supplements.

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Contents

• What is iron? An essential trace mineral
• What does iron do in the body?
• Did you know? Iron is recycled again and again
• How much iron do you need each day?
• How much iron do people in Japan actually consume?
• Foods high in iron
• Did you know? The iron in hijiki can depend on the cooking pot
• Heme iron vs non-heme iron
• Food combinations that affect iron absorption
• Iron absorption is also regulated inside the body
• Iron deficiency and iron-deficiency anaemia
• What is ferritin? A marker related to stored iron
• What is ferritin iron?
• Why do menstruating women need more iron?
• Iron during pregnancy and breastfeeding
• What types of iron are used in supplements?
• What does “Iron 18 mg” actually mean?
• How to choose an iron supplement
• Important points when taking iron
• Summary

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What is iron? An essential trace mineral

Iron is a mineral represented by the chemical symbol Fe.

Because the amount present in the body is relatively small compared with major minerals such as calcium and magnesium, iron is classified as a trace mineral.

An adult body contains only a few grams of iron, and a large proportion of it is found in haemoglobin inside red blood cells.

Although the total amount is small, iron is essential for normal body function.

Did you know? Dietary iron and metallic iron are the same element

It may sound surprising, but the iron found in food and the iron used in metal products are the same chemical element: Fe.

In food, however, iron is not present as a lump of metallic iron. It exists bound to proteins and in a variety of chemical compounds.

One of the interesting things about iron is that the way the body handles it depends partly on the chemical form in which it exists.

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What does iron do in the body?

1. A component of haemoglobin

One of iron's best-known roles is as a component of haemoglobin in red blood cells.

Haemoglobin binds oxygen taken up in the lungs and carries it through the bloodstream to tissues throughout the body.

In Japan, the authorised nutrient-function statement for iron in Foods with Nutrient Function Claims includes:

“Iron is a nutrient necessary for the formation of red blood cells.”

2. Iron is also found in myoglobin

Iron is present in myoglobin, a protein found in muscle tissue.

Myoglobin is involved in oxygen handling within muscle.

3. Iron is involved in many enzymes and proteins

Iron is also used as a component of various enzymes and proteins throughout the body.

So iron is not only associated with red blood cells; it is used in many different tissues and biochemical processes.

Functional iron and stored iron

Iron in the body can broadly be considered in two categories:

• Functional iron: iron used in haemoglobin, myoglobin, enzymes and other functional proteins
• Stored iron: iron stored mainly in proteins such as ferritin and haemosiderin

The body does not simply rely on the iron eaten that day. It stores and recycles iron while maintaining overall balance.

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Did you know? Iron is recycled again and again

Iron has an interesting characteristic that sets it apart from many other nutrients:

the body recycles it very efficiently.

The average lifespan of a red blood cell is around 120 days.

Old red blood cells are mainly broken down by cells called macrophages.

The iron contained in their haemoglobin is then recovered and reused, including for the production of new red blood cells.

In other words, the body circulates and reuses existing iron while replacing daily losses through food.

This recycling system is one reason why the body contains several grams of iron while daily dietary requirements are measured only in milligrams.

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How much iron do you need each day?

Iron requirements vary with age and sex, and for women they also differ substantially according to whether menstruation is present.

According to the Dietary Reference Intakes for Japanese (2025), recommended daily intakes for adults are as follows.

For example, among adults aged 30–49, the recommended intake is 7.5 mg/day for men and 10.5 mg/day for menstruating women.

Iron is therefore a nutrient whose requirements can vary considerably depending on life stage and menstrual status.

No tolerable upper intake level was set in the 2025 Japanese reference values

One notable change in the 2025 Japanese Dietary Reference Intakes relates to the tolerable upper intake level for iron.

The previous 2020 edition included upper intake levels for adults, while the 2025 edition does not set a numerical tolerable upper intake level.

This reflects the fact that the available scientific evidence was considered insufficient to establish a quantitative upper intake level for the Japanese population.

In everyday use, the practical approach is to understand your needs, use food as the foundation of intake, and check the recommended serving when using a supplement.

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How much iron do people in Japan actually consume?

According to Japan's 2024 National Health and Nutrition Survey, average daily iron intake among adults aged 20 and over was:

• All adults: 7.7 mg/day
• Men: 8.1 mg/day
• Women: 7.3 mg/day

The important point is that these are population averages.

For example, the recommended intake for menstruating women aged 30–49 is 10.5 mg/day.

When considering your own intake, it is more useful to compare your age, sex, menstrual status, pregnancy or breastfeeding stage with the appropriate dietary reference value rather than relying on the population average alone.

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Foods high in iron

Iron is found in many foods, including meat, seafood, legumes, vegetables, grains and seeds.

The following table shows examples based on typical serving sizes.

*Iron values are approximate serving-size calculations based on the Standard Tables of Food Composition in Japan, 2023 Supplement to the 8th Revised Edition. Values can vary according to the specific food, product, preparation and processing method.

When comparing foods, it is helpful to look not only at how many milligrams of iron they contain, but also at the difference between heme and non-heme iron.

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Did you know? The iron in hijiki can depend on the cooking pot

In Japan, hijiki seaweed has long been remembered as a food that is exceptionally rich in iron.

Modern Japanese food composition tables distinguish between hijiki processed using iron pots and stainless-steel pots.

This is because the material used during processing can substantially affect the measured iron content of the final food.

Some of the very high iron values historically associated with hijiki were influenced by iron transferred from traditional iron cooking pots during processing.

The nutrient composition of a food can change with its ingredients and production method, even when the food name itself stays the same.

It is a fascinating example of why food composition databases are updated over time.

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Heme iron vs non-heme iron

One of the most useful distinctions when understanding dietary iron is the difference between heme iron and non-heme iron.

Iron in plant foods is primarily non-heme iron.

Meat and fish contain both heme and non-heme iron, with heme iron generally being absorbed more efficiently.

At the same time, non-heme iron is an important source of dietary iron, especially in diets containing legumes, vegetables and grains.

For plant-based iron sources, the composition of the whole meal becomes particularly useful when planning iron-rich meals.

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Food combinations that affect iron absorption

The absorption of non-heme iron can vary depending on the body's iron status and the foods eaten with it.

Combine iron with vitamin C

Vitamin C is known to increase the absorption of non-heme iron.

Examples include:

• komatsuna with capsicum
• legumes with broccoli
• tofu dishes with fruit

This is a practical way to combine plant foods containing iron with foods rich in vitamin C.

Combine plant foods with meat, fish or poultry

Including meat, fish or poultry in a meal can also influence the utilisation of non-heme iron in that meal.

Combining animal and plant foods can also provide a wider variety of nutrients in the same meal.

Tea, coffee and phytate

Phytate found in foods such as grains and legumes, and certain polyphenols in tea and coffee, can influence the absorption of non-heme iron consumed in the same meal.

If you are paying particular attention to iron intake, one option is to have strong tea or coffee at a different time from an iron-rich meal, while combining plant iron sources with vitamin-C-rich foods.

A practical approach is to focus on the overall balance and timing of foods rather than avoiding otherwise nutritious foods.

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Iron absorption is also regulated inside the body

Iron absorption is influenced by both the type of food and the body's current iron status.

A key hormone involved in this regulation is hepcidin.

Hepcidin is produced mainly by the liver and helps regulate iron absorption from the intestine, as well as the movement of stored and recycled iron into the bloodstream.

For this reason, it is more accurate to understand iron absorption as something that changes according to the form of iron, meal composition and the body's iron status rather than assigning one fixed absorption percentage to every situation.

Did you know? The body regulates iron largely by controlling absorption

Iron balance relies heavily on controlling how much iron enters the body through the intestine.

The body adjusts iron absorption according to its current iron status and physiological needs.

This is one reason why appropriate intake matters for iron.

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Iron deficiency and iron-deficiency anaemia

Iron deficiency and anaemia are closely related, but they describe different conditions and stages.

Anaemia generally refers to a situation in which haemoglobin concentration falls below defined criteria.

However, the body's stored iron can begin to decline while haemoglobin remains within the normal range.

For this reason, medical assessment of iron status may use several measures, including haemoglobin and ferritin.

What is iron-deficiency anaemia?

When iron stores become sufficiently depleted that the body can no longer produce adequate haemoglobin, one possible result is iron-deficiency anaemia.

From a medical perspective, iron deficiency and iron-deficiency anaemia may be associated with symptoms such as:

• headache
• dizziness
• palpitations
• shortness of breath
• changes in the nails
• pica

These symptoms can have many different causes. When symptoms persist, blood tests and medical assessment can help identify the underlying reason and guide appropriate care.

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What is ferritin? A marker related to stored iron

When learning about iron, you will often come across the word ferritin.

Ferritin is a protein capable of storing iron inside its structure.

The concentration of ferritin in the blood is widely used as one of the important markers for assessing the body's iron stores.

A low ferritin level can indicate that stored iron has become depleted.

Ferritin is interpreted together with other information

Ferritin is a useful marker of iron stores, but its concentration can also increase in response to infection or inflammation.

For this reason, clinicians may interpret ferritin alongside haemoglobin, inflammatory markers and other laboratory findings.

If you are concerned about blood test results, discussing the complete set of results with a healthcare professional can give a more accurate picture of your iron status.

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What is ferritin iron? How is it different from ferritin on a blood test?

The term “ferritin iron” is increasingly seen in the names and descriptions of iron supplements.

It is useful to distinguish between ferritin measured in a blood test and ferritin iron used as a food or supplement ingredient.

Ferritin is a protein capable of storing iron within its structure, and ferritin is found not only in humans and animals but also in plants such as soybeans.

In foods and supplements, “ferritin iron” generally refers to iron associated with this ferritin protein structure.

Is ferritin iron a completely separate “third type” of iron?

Some commercial descriptions present heme iron, non-heme iron and ferritin iron as three separate categories.

From a broader nutritional classification, however, iron stored within plant ferritin does not contain a heme structure and can therefore be considered a form of non-heme iron.

Iron naturally present in soy has been studied in human absorption research, and some of that iron is stored within plant ferritin, or phytoferritin.

Research also indicates that the absorption of iron from soy foods can be influenced by factors such as food form, iron status, phytate and vitamin C.

When comparing iron supplements, it is therefore useful to look beyond names such as “ferritin iron” or “heme iron” and check how much actual iron is provided per daily serving, what other ingredients are included and how the product is intended to be taken.

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Why do menstruating women need more iron?

One of the major reasons iron requirements differ between men and women is iron loss through menstruation.

In the 2025 Japanese Dietary Reference Intakes, recommended iron intake for women aged 30–49 is:

• No menstruation: 6.0 mg/day
• Menstruating: 10.5 mg/day

This means that when considering iron requirements for women, menstrual status is an important part of the picture.

Menstrual blood loss also varies substantially from person to person.

If menstrual bleeding is consistently heavy or you have concerns about your health, medical assessment can help determine the most appropriate next step.

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Iron during pregnancy and breastfeeding

Iron requirements change during pregnancy as maternal blood volume increases and the baby develops.

The 2025 Japanese Dietary Reference Intakes specify additional iron above the usual recommended amount for adult women:

• Early pregnancy: +2.5 mg/day
• Mid-pregnancy: +8.5 mg/day
• Late pregnancy: +8.5 mg/day
• Breastfeeding: +2.0 mg/day

Iron requirements rise particularly during the middle and later stages of pregnancy.

Pregnancy also involves regular health checks and blood testing, so dietary intake and any need for supplemental iron can be considered together with advice from a doctor, midwife or other qualified healthcare professional.

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What types of iron are used in supplements?

Iron supplements can use a variety of iron sources and ingredient names.

Examples include:

• heme iron
• ferrous fumarate
• ferrous sulfate
• ferrous gluconate
• iron citrate
• chelated iron
• ferritin iron

Different iron sources vary in chemical form, proportion of elemental iron and how they are incorporated into product formulations.

Ferrous and ferric iron

You may also see the terms ferrous iron and ferric iron.

These refer to different oxidation states of iron.

Ferrous forms are widely used in oral iron preparations and iron-containing products.

When comparing products, it is useful to consider not only the chemical form, but also the actual iron content, other ingredients, directions for use and how practical the product is for regular use.

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What does “Iron 18 mg” actually mean? Understanding elemental iron

An important concept when comparing iron supplements is elemental iron.

For example:

• ferrous fumarate
• ferrous sulfate
• ferrous gluconate

contain different proportions of actual iron within the total weight of the compound.

As a general reference, elemental iron accounts for approximately:

• Ferrous fumarate: about 33%
• Ferrous sulfate: about 20%
• Ferrous gluconate: about 12%

For this reason, when comparing supplements, the most useful number is usually how many milligrams of actual iron are provided, rather than the total weight of the iron compound.

On New Zealand and other international supplement labels, you may see wording such as:

Iron 18 mg (as ferrous fumarate)

In this example, “Iron 18 mg” is the key figure for comparing the amount of iron provided.

Understanding this distinction can make international supplement labels much easier to compare.

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How to choose an iron supplement

1. Check how much actual iron is provided

Products may list iron per tablet, capsule, sachet or daily serving.

For comparison, look at how many milligrams of iron are provided by the recommended daily serving.

2. Check the iron source

Products may use heme iron, iron salts, chelated iron, ferritin iron or other forms.

Looking at the source can help you understand how the product has been formulated.

3. Look at the other ingredients

Some products contain iron alone, while others combine it with nutrients such as vitamin C, folate or vitamin B12.

For multi-ingredient products, consider the complete formula rather than looking at iron in isolation.

If you already take a multivitamin or other supplements, it is also useful to check for overlapping ingredients.

4. Match the amount to your needs

Iron requirements differ according to age, sex, menstruation and pregnancy.

Rather than choosing the product with the largest number, compare your likely requirement with the amount supplied in a daily serving.

Iron medicines used to treat anaemia have different purposes and doses from ordinary foods and supplements.

5. Consider how easy the product is to use regularly

Higher-dose iron products can cause gastrointestinal discomfort, nausea or constipation in some people.

Along with the form and amount of iron, consider directions for use, tablet size and how often it needs to be taken.

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Important points when taking iron

For everyday nutrition, a varied diet is the foundation for meeting iron needs.

With concentrated iron supplements and medical iron preparations, the dose and directions for use become especially important.

Gastrointestinal effects

Higher amounts of iron may cause:

• stomach discomfort
• nausea
• abdominal pain
• constipation
• diarrhoea

When using a supplement, follow the manufacturer's recommended daily serving and directions.

Interactions with medicines

Iron can affect the absorption of certain medicines.

For example, people taking the thyroid hormone medicine levothyroxine may be advised to separate it from iron by a period of time.

If you take prescription medicines or are receiving medical treatment, checking with a doctor or pharmacist before starting an iron supplement is a sensible step.

Keep iron products away from children

Accidental ingestion of a large amount of concentrated iron by a child can cause acute iron poisoning.

Iron supplements and medicines should therefore be stored securely out of reach of children.

Appropriate intake still matters even without a numerical upper limit

The 2025 Japanese Dietary Reference Intakes do not establish a numerical tolerable upper intake level for iron.

This reflects the fact that current scientific evidence was considered insufficient to set a quantitative upper intake level for the Japanese population.

The practical approach remains to distinguish between food, supplements and medical iron preparations, and to choose an amount appropriate to your needs and the product directions.

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Summary: with iron, look at the amount, form, absorption and your individual needs

Iron is best known for its role in red blood cells and haemoglobin, but it is also involved in myoglobin and a range of enzymes and proteins throughout the body.

The key points are:

• Iron is an essential trace mineral and a component of haemoglobin and other important proteins.
• Iron from ageing red blood cells is recovered and efficiently recycled within the body.
• Iron requirements vary considerably with age, sex, menstruation and pregnancy.
• Dietary iron occurs as heme and non-heme iron, which differ in their absorption characteristics.
• Vitamin C and the overall composition of a meal can influence the absorption of non-heme iron.
• Ferritin is one marker used to assess stored iron, while ferritin iron refers to an iron source used in foods and supplements.
• When comparing iron supplements, check the iron source and how many milligrams of actual iron are provided.
• Although the 2025 Japanese reference values do not set a numerical tolerable upper intake level, matching intake to your needs and product directions remains the practical approach.

When thinking about iron, it is more useful to look beyond simply finding the food with the highest iron value or the supplement with the largest number.

Considering your own iron needs, the form of dietary iron, food combinations, and the actual iron amount and source in supplements gives you a much better basis for comparing foods and products.

If iron deficiency or anaemia is a concern, blood tests such as haemoglobin and ferritin can provide useful information for deciding what kind of follow-up may be appropriate.

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Related Articles

• What Is Vitamin B Complex? B1, B2, B6, B12, Folate, Foods, Deficiency & Supplements
  Learn more about folate and vitamin B12, nutrients often discussed alongside iron and blood-cell formation.
• When Is the Best Time to Take Supplements?
  A practical guide to supplement timing and how to fit supplements into everyday routines.
• How to Combine Supplements: Ingredients, Overlap and Practical Tips
  Useful when checking ingredient overlap across multiple supplements.

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Main References

• Ministry of Health, Labour and Welfare, Japan. Dietary Reference Intakes for Japanese, 2025.
• Ministry of Health, Labour and Welfare, Japan. National Health and Nutrition Survey 2024.
• Ministry of Education, Culture, Sports, Science and Technology, Japan. Standard Tables of Food Composition in Japan, 2023 Supplement to the 8th Revised Edition.
• World Health Organization. Guideline on use of ferritin concentrations to assess iron status in individuals and populations.
• NIH Office of Dietary Supplements. Iron – Fact Sheet for Health Professionals.
• Hackl LS, Moretti D, Sabatier M. Absorption of Iron Naturally Present in Soy. Advances in Nutrition. 2025;16(4):100396.
• Lönnerdal B. Soybean ferritin: implications for iron status of vegetarians. American Journal of Clinical Nutrition. 2009;89(5):1680S–1685S.
• Ganz T. Systemic iron homeostasis. Physiological Reviews.
• Korolnek T, Hamza I. Macrophages and iron trafficking at the birth and death of red cells.

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Important Information

This article is intended to provide general educational information about iron, nutrition and health.

It is not intended to diagnose, treat, cure or prevent disease, and it does not claim that any particular food or supplement treats anaemia or any other medical condition.

The interpretation of iron deficiency, anaemia, ferritin and other blood test results can vary according to age, sex, pregnancy, inflammation and underlying medical conditions.

If you experience persistent fatigue, dizziness, shortness of breath, palpitations or have been told that your blood test results are abnormal, seek medical advice to determine the underlying cause.

If you are pregnant or breastfeeding, take medication or are receiving medical treatment, consult a doctor, pharmacist or other qualified healthcare professional before using an iron-containing supplement or health product.