Understanding Nutrition Diet: What is the number one cause of iron deficiency?

October 10, 2025 •

5 min read

Globally, iron deficiency is the most common nutritional deficiency, affecting an estimated two billion people. For many, the answer to what is the number one cause of iron deficiency is not merely a lack of iron in the diet but is primarily linked to chronic blood loss.

Quick Summary

Chronic blood loss is the most common reason for iron deficiency, which can stem from heavy menstrual bleeding in women or gastrointestinal issues in men and postmenopausal women. Other causes include dietary deficiencies and malabsorption problems.

Key Points

Chronic Blood Loss is Key: In adults, the number one cause of iron deficiency is chronic blood loss, not just poor diet.
Heavy Periods for Women: For reproductive-aged women, the most common source of chronic blood loss leading to deficiency is heavy menstrual bleeding.
GI Bleeding for Others: In men and postmenopausal women, bleeding in the gastrointestinal tract is the primary cause and warrants medical investigation.
Dietary Factors Play a Role: Poor diet, especially a vegetarian or vegan one, and increased demands during growth or pregnancy can contribute to low iron.
Absorption is Critical: Malabsorption issues from conditions like celiac disease or gastric surgery can prevent the body from utilizing dietary iron effectively.
Enhance Absorption: Pairing iron-rich foods with sources of vitamin C can significantly improve non-heme iron absorption.
Address the Root Cause: Effective treatment goes beyond supplements and requires identifying and resolving the underlying cause of iron loss.

The Dominant Cause: Chronic Blood Loss

While many people assume a poor diet is the primary culprit, the number one cause of iron deficiency in adults, particularly in high-resource countries, is chronic blood loss. This loss depletes the body's iron stores faster than they can be replenished through diet alone, even a well-balanced one. The source of this bleeding differs significantly based on demographics.

Heavy Menstrual Bleeding

For reproductive-aged women, heavy menstrual bleeding (menorrhagia) is the most frequent cause of iron deficiency. The amount of iron lost can be substantial, especially during consecutive cycles, making it difficult to maintain adequate iron levels. In fact, many women and even healthcare providers normalize heavy periods, leading to underdiagnosis. For these women, addressing the heavy bleeding is a critical step in managing iron deficiency, alongside dietary and supplemental approaches.

Gastrointestinal (GI) Bleeding

In men and postmenopausal women, the most common source of chronic blood loss is from the gastrointestinal tract. This bleeding is often slow and may not be obvious, sometimes only detectable through specific medical tests. It can be caused by a variety of conditions, including:

Peptic ulcers: Sores in the stomach lining or small intestine caused by stomach acid.
Hiatal hernias: A condition where part of the stomach pushes up through the diaphragm.
Colon polyps or cancer: Bleeding from the colon can be a sign of a serious condition like colorectal cancer, especially in middle-aged and older adults.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease and ulcerative colitis cause inflammation and can lead to bleeding.
Medications: Regular use of non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and ibuprofen, can irritate the stomach lining and cause bleeding.

Other Forms of Blood Loss

Besides the main culprits, other forms of blood loss can contribute to iron deficiency:

Frequent blood donation
Parasitic infections, such as hookworm, which are a major cause in developing countries
Internal bleeding from surgery or trauma

Other Major Contributors to Low Iron

While blood loss is the top cause for many, other factors play a significant role in developing iron deficiency. It is often a combination of causes that leads to a deficiency, rather than a single issue.

Inadequate Dietary Intake

Though less common as a sole cause in developed nations, poor dietary intake can certainly contribute, especially when combined with other risk factors. The body requires iron from food, and insufficient consumption can lead to a gradual depletion of iron stores. Certain populations are at higher risk:

Vegetarians and Vegans: Plant-based (non-heme) iron is less easily absorbed by the body than heme iron from animal products.
Infants and Children: Rapid growth requires increased iron, and inadequate iron in breast milk or formula can lead to deficiency.
Picky Eaters: Children with limited diets may not get a wide enough variety of iron-rich foods.

Malabsorption Disorders

Even with sufficient iron intake, the body may be unable to absorb it properly due to certain medical conditions or procedures. Iron is primarily absorbed in the small intestine, and issues here can significantly impact levels.

Celiac Disease: This autoimmune disorder damages the small intestine lining, hindering nutrient absorption, including iron.
Bariatric Surgery: Procedures that bypass or remove part of the small intestine or stomach can greatly reduce iron absorption.
Chronic Stomach Issues: Reduced stomach acid (hypochlorhydria/achlorhydria) can interfere with iron absorption.

Increased Iron Demands

Some life stages naturally increase the body's iron needs, making individuals more susceptible to deficiency if their intake isn't adjusted.

Pregnancy: The body's blood volume increases, and iron is needed for the fetus and placenta, significantly raising the required intake.
Infancy and Adolescence: These periods of rapid growth necessitate a higher iron intake to support development.

Dietary Strategies for Managing Iron

For all individuals, optimizing diet is a crucial part of both preventing and treating iron deficiency. Eating a varied diet that includes both heme and non-heme iron sources is ideal.

Heme vs. Non-heme Iron


Feature	Heme Iron	Non-Heme Iron
Source	Animal products (red meat, poultry, seafood, organ meats)	Plant-based foods (legumes, leafy greens, fortified cereals)
Absorption Rate	Highly bioavailable; absorbed much more efficiently by the body (up to 30%)	Less bioavailable; absorption is lower and can be inhibited by other foods (2-10%)
Absorption Enhancers	N/A (Already well-absorbed)	Vitamin C dramatically improves absorption

Combining Foods for Better Absorption

To make the most of your iron intake, especially non-heme iron, strategic food pairing is essential.

Eat with Vitamin C: Combining iron-rich foods with sources of vitamin C, such as citrus fruits, bell peppers, strawberries, or tomatoes, can significantly enhance non-heme iron absorption.
Avoid Inhibitors: Certain foods and drinks can block iron absorption and should be consumed at different times than iron-rich meals. These include:
- Tea and coffee
- Calcium (dairy products)
- Foods with high levels of phytic acid (wholegrains, legumes, nuts)

Diagnosing and Treating Iron Deficiency

Diagnosing iron deficiency starts with a doctor's visit and blood tests. A serum ferritin test, which measures iron stores, is often the most accurate method. However, other blood count indicators may also be assessed.

Treatment depends on the severity and underlying cause.

Iron Supplements: For most people, oral iron supplements (such as ferrous sulfate) are prescribed to restore iron stores. Intravenous iron may be used in severe cases or for those with absorption issues.
Dietary Adjustments: Your doctor or a dietitian may recommend changes to your diet to increase iron intake and improve absorption.
Treating the Root Cause: The most critical step is identifying and addressing the underlying cause of the blood loss or malabsorption. This may involve medication for ulcers, management of heavy periods, or other interventions depending on the diagnosis. For example, if heavy menstrual bleeding is the cause, hormonal treatments may be explored.

Conclusion

Understanding what is the number one cause of iron deficiency is a crucial step toward effective treatment. While nutritional factors play a part, recognizing chronic blood loss as the primary driver in adults shifts the focus from simply adding more iron to the diet to investigating the root cause. For women, this often points to heavy menstrual bleeding, whereas for men and postmenopausal women, gastrointestinal bleeding is a top suspect. A proper diagnosis and treatment plan, which addresses the underlying issue alongside dietary and supplemental support, is essential for replenishing iron stores and restoring overall health.

For more in-depth information, the National Institutes of Health provides reliable resources on iron deficiency anemia.

Frequently Asked Questions

Yes, an inadequate diet can cause iron deficiency, especially in individuals with increased needs like infants, children, and pregnant women. However, in many adults, it is often a contributing factor alongside chronic blood loss.

For women of reproductive age, the most common cause of iron deficiency is heavy menstrual bleeding, also known as menorrhagia.

In men and postmenopausal women, the most frequent cause of iron deficiency is bleeding in the gastrointestinal tract, which can stem from ulcers, polyps, or other conditions.

To improve absorption of non-heme iron from plant sources, combine them with foods rich in vitamin C, such as citrus fruits, bell peppers, or broccoli. Avoid drinking coffee, tea, or consuming calcium-rich foods at the same time.

Symptoms of iron deficiency can include fatigue, weakness, pale skin, dizziness, shortness of breath, cold hands and feet, brittle nails, and headaches.

Conditions that can affect iron absorption include celiac disease, inflammatory bowel disease (Crohn's, ulcerative colitis), and bariatric surgery.

Treatment for iron deficiency usually involves taking oral iron supplements to replenish stores, making dietary adjustments to increase iron intake, and most importantly, treating the underlying cause, especially if it involves chronic bleeding.

Yes, pregnancy significantly increases a woman's iron needs to support the increased blood volume and the growth of the fetus and placenta. Without supplements, many pregnant women develop iron deficiency.