Where are iron, B12, and folate absorbed?

December 31, 2025 •

2 min read

The absorption of vital nutrients like iron, B12, and folate occurs at specific locations within the small intestine, a key part of the gastrointestinal tract. Understanding these distinct sites is crucial for diagnosing and treating malabsorption disorders that can lead to anemia and other health problems.

Quick Summary

Iron is absorbed in the duodenum and proximal jejunum, while folate absorption primarily takes place in the jejunum. Vitamin B12 absorption requires intrinsic factor and occurs exclusively in the terminal ileum. Various factors can influence the efficiency of this process.

Key Points

Iron's Location: Iron is predominantly absorbed in the duodenum and proximal jejunum of the small intestine, requiring conversion to its ferrous form for uptake.
Folate's Zone: The jejunum is the primary site for folate absorption, where the nutrient must be converted to a monoglutamate form before transport via the PCFT carrier.
B12's Crucial Site: Vitamin B12 absorption happens in the terminal ileum and is critically dependent on intrinsic factor, a protein produced in the stomach.
Specific Mechanisms: Each nutrient uses a distinct mechanism: iron uses DMT1, folate uses PCFT, and B12 relies on the intrinsic factor-cubilin complex.
Malabsorption Consequences: Damage or removal of specific sections of the small intestine can lead to targeted nutrient deficiencies and corresponding health issues, such as different types of anemia.
Factors Affecting Absorption: Dietary factors, such as vitamin C intake for iron, or systemic issues like autoimmune conditions affecting intrinsic factor, can significantly impact absorption efficiency.

A Journey Through the Small Intestine: The Absorption Sites

The small intestine is a key site for nutrient absorption and is divided into the duodenum, jejunum, and ileum. Each section plays a specific role in absorbing different nutrients, including iron, vitamin B12, and folate. Issues affecting these sections can lead to nutrient deficiencies.

Iron Absorption in the Duodenum

Iron is mainly absorbed in the duodenum. This process is regulated and utilizes specific proteins in an acidic environment. Dietary iron comes as heme or non-heme iron. Non-heme iron needs to be converted to its ferrous ($Fe^{2+}$) state for absorption, a process enhanced by Vitamin C. A protein called DMT1 transports ferrous iron into intestinal cells, where it is either stored or transported into the bloodstream via ferroportin.

The acidic environment helps release iron from food.
DMT1 facilitates iron uptake into cells.
Iron can be stored as ferritin or enter the bloodstream via ferroportin.

Folate Absorption in the Jejunum

Folate is primarily absorbed in the jejunum. Dietary folates are polyglutamates and are converted to monoglutamates for absorption. A key transporter for folate into intestinal cells is the Proton-Coupled Folate Transporter (PCFT).

This absorption is pH-dependent.
Absorbed folate enters the bloodstream.

Vitamin B12 Absorption in the Terminal Ileum

Vitamin B12 absorption takes place in the terminal ileum and requires intrinsic factor (IF), secreted by stomach cells. B12 binds to IF in the small intestine, forming a complex. The B12-IF complex binds to cubilin receptors in the terminal ileum and is absorbed.

Comparison Table: Absorption of Iron, B12, and Folate


Feature	Iron	Folate	Vitamin B12
Primary Absorption Site	Duodenum and proximal jejunum	Jejunum	Terminal ileum
Mechanism	Active, carrier-mediated transport via DMT1	Active, proton-coupled transport via PCFT	Active, intrinsic factor-dependent transport via cubilin receptors
Required Cofactors	Ascorbic acid (Vitamin C) for non-heme iron reduction	Enzymatic hydrolysis by glutamate carboxypeptidase II	Intrinsic factor from gastric parietal cells
Clinical Consequence of Malabsorption	Iron-deficiency anemia (microcytic anemia)	Megaloblastic anemia, potential for neural tube defects in fetus	Megaloblastic anemia, neurological damage

Implications of Absorption Issues

Deficiencies can result from poor intake or problems with absorption sites. For example, B12 malabsorption issues would involve assessing the terminal ileum and intrinsic factor presence. Iron or folate malabsorption concerns would focus on the duodenum and jejunum.

Conclusion

Iron is absorbed in the duodenum, folate in the jejunum, and B12 in the terminal ileum, dependent on intrinsic factor. Problems with these locations or mechanisms can cause health issues like anemia. More information on B12 absorption and deficiency is available from {Link: NCBI Bookshelf https://www.ncbi.nlm.nih.gov/books/NBK441923/}.

Frequently Asked Questions

Iron is primarily absorbed in the duodenum and the proximal (upper) part of the jejunum. Heme iron is more easily absorbed, while non-heme iron requires conversion to its ferrous form before uptake.

Folate is absorbed mainly in the jejunum, the middle section of the small intestine. It is converted from its polyglutamate form to its transportable monoglutamate form on the surface of the intestinal cells.

No, vitamin B12 is not absorbed in the stomach. While it binds to intrinsic factor (IF), which is produced in the stomach, the actual absorption of the B12-IF complex occurs much later, in the terminal ileum.

Intrinsic factor (IF) is a glycoprotein that binds to vitamin B12 in the stomach and protects it as it travels through the digestive tract. This binding is essential for the B12 to be recognized and absorbed by specific receptors in the terminal ileum.

Some individuals may have malabsorption issues in the terminal ileum or lack intrinsic factor due to conditions like pernicious anemia or gastric surgery. In these cases, oral supplements are ineffective, and B12 injections bypass the compromised absorption route to deliver the vitamin directly into the bloodstream.

Celiac disease causes damage to the lining of the small intestine, particularly the duodenum and jejunum, where iron and folate are absorbed. This damage can significantly impair the absorption of these and other nutrients.

Yes, vitamin C (ascorbic acid) significantly enhances the absorption of non-heme iron. It helps convert the iron into a form that can be more easily absorbed by the intestinal cells.