Major Causes of Iron Deficiency Explained

November 22, 2025 •

4 min read

According to the World Health Organization, iron deficiency is the most common single-nutrient deficiency globally, affecting billions. While poor diet is a frequent contributor, understanding what is the major cause of iron deficiency is crucial for effective prevention and treatment. This guide explores the primary reasons behind this widespread condition, including the often-overlooked role of blood loss.

Quick Summary

This article details the primary factors leading to insufficient iron levels in the body, primarily chronic blood loss, and provides information on other contributing elements such as diet, absorption issues, and increased physiological needs.

Key Points

Chronic Blood Loss: The most significant cause in many adults, especially from heavy menstruation in women and occult gastrointestinal bleeding in men and postmenopausal women.
Gastrointestinal Bleeding Sources: Common causes include ulcers, polyps, inflammatory bowel disease, cancer, and regular use of NSAID medications.
Inadequate Dietary Intake: A major factor in vegetarians, vegans, infants consuming too much cow's milk, and individuals on restrictive diets.
Impaired Absorption: Medical conditions like celiac disease, bariatric surgery, and H. pylori infection can prevent the body from absorbing iron effectively.
Increased Demand: Pregnancy, lactation, rapid growth spurts in children, and intense endurance athletics all increase the body's iron requirements significantly.
Investigation is Key: A proper medical evaluation is essential to determine the underlying cause, especially to rule out internal bleeding.

The Primary Culprit: Chronic Blood Loss

In many adults, particularly in developed countries, the single most significant reason for iron deficiency is chronic or hidden blood loss. The human body recycles iron efficiently from old red blood cells, but this process cannot keep up with persistent blood loss, regardless of the source.

Heavy Menstrual Periods

For premenopausal women, heavy or prolonged menstrual bleeding is the most common and frequent cause of chronic blood loss leading to iron deficiency. The iron lost each month can exceed the amount absorbed from even a healthy diet, leading to a gradual depletion of iron stores over time.

Gastrointestinal (GI) Bleeding

In men and postmenopausal women, the most common source of chronic blood loss is the gastrointestinal tract. This bleeding can be subtle and often goes unnoticed, a condition known as "occult bleeding." Potential causes of GI bleeding include:

Ulcers: Bleeding from peptic ulcers is a very common cause.
Polyps: Non-cancerous growths in the colon or stomach can bleed slowly.
Cancer: Cancers of the esophagus, stomach, or colon can cause chronic blood loss.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis cause chronic inflammation and bleeding in the digestive tract.
Medications: Regular use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen can cause irritation and bleeding in the stomach.

Other Sources of Blood Loss

Other less frequent but important causes of blood loss include:

Frequent Blood Donation: Repeated donations can deplete the body's iron reserves.
Urinary Tract Bleeding: Sometimes blood loss occurs in the urine, often associated with kidney conditions.
Trauma or Surgery: Significant blood loss from injury or surgical procedures can cause a rapid drop in iron levels.

Secondary Causes: Inadequate Intake and Malabsorption

While blood loss is often the primary driver, other factors related to diet and digestion play a critical role, especially when combined with increased physiological demand.

Inadequate Dietary Iron Intake

Although less common as a sole cause in adults in many developed countries, poor dietary intake remains a significant factor, especially in specific populations.

Vegetarian and Vegan Diets: The body absorbs heme iron, found in animal products, more readily than non-heme iron from plant sources. Vegetarians and vegans must be more deliberate about their iron intake and ensure they get enough enhancers like Vitamin C to improve absorption.
Picky Eating or Fad Diets: Restrictive or unbalanced diets can lead to insufficient iron consumption.
Infants and Toddlers: During rapid growth phases, children are at risk, particularly if they consume too much cow's milk, which is low in iron and can interfere with absorption.

Impaired Iron Absorption (Malabsorption)

Even with an iron-rich diet, certain medical conditions can prevent the body from properly absorbing the mineral.

Celiac Disease: An autoimmune disorder where consuming gluten damages the small intestine lining, which is where iron is absorbed.
Bariatric Surgery: Surgical procedures that alter or remove parts of the stomach and small intestine can significantly reduce the body's ability to absorb iron.
Helicobacter pylori Infection: This bacterium, which causes stomach inflammation and ulcers, can reduce iron absorption.
Chronic Inflammation: Conditions like inflammatory bowel disease (IBD) can interfere with iron absorption and utilization by the body.

The Role of Increased Iron Demand

Certain physiological states place a higher demand on the body's iron stores, making deficiency more likely if dietary intake is not increased.

Pregnancy and Lactation

Pregnant and breastfeeding women have a significantly higher iron requirement to support both their own increased blood volume and the developing fetus or infant. Without supplementation, iron deficiency is very common.

Periods of Rapid Growth

Infants and teenagers undergoing growth spurts have an increased need for iron. This is a particular risk factor for adolescent girls who also experience menstrual bleeding.

Endurance Athletes

Athletes, especially those in endurance sports, lose iron through sweating and, in some cases, through the breakdown of red blood cells during intense exercise. This, combined with exercise-induced inflammation, can lead to iron deficiency.

How to Assess Your Risk: Blood Loss vs. Other Causes

If you suspect you have iron deficiency, it's crucial to understand the underlying cause. A medical evaluation is necessary to determine the correct course of action, which may involve addressing blood loss, adjusting your diet, or managing an absorption issue.


Assessment Factor	Primary Causes (Blood Loss)	Other Causes (Diet/Absorption)
Dietary History	Often adequate iron intake, but losses outpace absorption.	Diet consistently low in iron-rich foods, or vegetarian/vegan.
Symptom Onset	Gradual, worsening fatigue and pallor over time.	Can also be gradual, but often linked to dietary habits or known digestive issues.
Risk Factors	Heavy menstruation, GI ulcers, regular NSAID use, frequent blood donation, GI surgeries.	Celiac disease, Crohn's disease, bariatric surgery, certain medication use (e.g., antacids).
Laboratory Markers	Low ferritin (iron stores) and low hemoglobin, often with microcytic anemia.	Low ferritin and low hemoglobin, though iron deficiency can precede anemia.
Treatment Focus	Primarily stopping blood loss, in addition to iron supplementation.	Increased iron intake (diet/supplements), potentially addressing malabsorption.

Conclusion: Prioritizing Investigation

While inadequate diet and malabsorption contribute to iron deficiency, especially in at-risk groups, the most common single cause in adults remains chronic blood loss. For this reason, anyone with suspected iron deficiency, particularly men and postmenopausal women, should undergo a thorough medical investigation to rule out a source of occult bleeding, such as from the gastrointestinal tract. Identifying and treating the root cause is the only way to achieve a long-term resolution and prevent the fatigue, heart issues, and other complications associated with this prevalent condition.

Frequently Asked Questions

The most common cause of iron deficiency in adults is chronic blood loss. This includes heavy menstrual bleeding in premenopausal women and occult (hidden) gastrointestinal bleeding in men and postmenopausal women.

Yes, a diet low in iron-rich foods can cause iron deficiency, particularly in vegetarians and vegans. However, in many cases, it is a contributing factor rather than the sole cause, often alongside blood loss or increased physiological demands.

Groups most at risk include premenopausal and pregnant women, infants and children, teenagers, frequent blood donors, and individuals with chronic diseases or malabsorption issues.

When blood is lost, the iron contained within red blood cells is also lost. While the body can recycle some iron, chronic or significant bleeding depletes the body's iron stores faster than they can be replenished through diet.

Yes, regular use of non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen can cause gastrointestinal bleeding, which in turn leads to iron deficiency.

Conditions that cause poor iron absorption include celiac disease, inflammatory bowel disease (e.g., Crohn's disease), Helicobacter pylori infection, and complications following bariatric surgery.

Yes, iron deficiency is very common during pregnancy due to the increased iron requirements to support the mother's expanded blood volume and the growth of the fetus. Supplementation is often necessary to meet this demand.