What Does Cyanocobalamin Break Down To in the Body?

January 7, 2026 •

4 min read

A 2024 review in StatPearls noted that once absorbed, cyanocobalamin is converted in tissues into a cofactor for various metabolic processes. Most widely used in supplements and fortified foods due to its stability, cyanocobalamin breaks down into the two metabolically active forms of vitamin B12: methylcobalamin and adenosylcobalamin.

Quick Summary

In the body, the synthetic vitamin B12, cyanocobalamin, is metabolized into its active coenzyme forms, methylcobalamin and adenosylcobalamin, after the removal of its cyano group. This conversion enables it to act as a crucial cofactor in essential cellular processes, including DNA synthesis and energy production.

Key Points

Conversion to Active Forms: In the body, cyanocobalamin is converted into the two active forms of vitamin B12, methylcobalamin and adenosylcobalamin, which are required for metabolic function.
Cyano Group Removal: The conversion process involves enzymatically removing the cyano group from the cobalamin molecule, after which the minimal and non-toxic amount of cyanide is detoxified and excreted.
Methylcobalamin's Role: The methylcobalamin produced helps the enzyme methionine synthase convert homocysteine to methionine, a critical step for DNA synthesis and methylation.
Adenosylcobalamin's Role: The adenosylcobalamin works in the mitochondria, where it acts as a cofactor for an enzyme involved in fat and amino acid metabolism for energy production.
Superior Stability: Cyanocobalamin is widely used in supplements and food fortification because it is highly stable and cost-effective compared to the active coenzyme forms.
No Toxicity Concerns: The amount of cyanide released during the breakdown of cyanocobalamin from typical supplement doses is minimal and poses no health risk to the body.

The Metabolic Journey of Cyanocobalamin

Cyanocobalamin is a synthetic form of vitamin B12 commonly found in dietary supplements and fortified foods due to its stability. Unlike natural forms of B12 found in animal products, it contains a cyano group attached to a cobalt ion. Once ingested, this cyano group is removed and the molecule is converted into the two active coenzyme forms that the human body can utilize: methylcobalamin and adenosylcobalamin. This metabolic conversion is a key process that allows the synthetic version of the vitamin to perform its vital biological functions.

The Breakdown Process Step-by-Step

Absorption and Transport: Cyanocobalamin is absorbed in the terminal ileum of the small intestine. It then enters the bloodstream bound to a transport protein called transcobalamin II (TCII). This complex is essential for delivering the vitamin to the body's cells.
Intracellular Release: The TCII-cobalamin complex binds to specific cell receptors and is internalized. Inside the cell, the complex is broken down in the lysosome, releasing free cobalamin into the cytoplasm.
Cyano Group Removal: In a process known as decyanation, the cyano group is removed from the cobalamin molecule. The product of this step is cob(II)alamin. The removed cyanide is then converted to thiocyanate and safely excreted by the kidneys. Concerns about cyanide toxicity from cyanocobalamin are unfounded for typical supplement doses.
Conversion to Active Forms: The newly formed cob(II)alamin is then converted into one of the two active coenzyme forms, depending on the cellular location and specific enzymatic needs. The MMACHC gene product and cobalamin reductases catalyze this interconversion.

The Active Coenzyme Forms

Once inside the cell, cyanocobalamin is converted into two primary active forms to fulfill different roles:

Methylcobalamin: This form is primarily active in the cell's cytoplasm. It acts as a crucial cofactor for the enzyme methionine synthase, which is responsible for converting the amino acid homocysteine into methionine. This reaction is a pivotal part of the methionine cycle, which is essential for DNA synthesis and various methylation reactions, including those involving lipids and proteins.
Adenosylcobalamin: This form operates in the mitochondria, the cell's powerhouse. Here, it is a cofactor for the enzyme L-methylmalonyl-CoA mutase, which helps convert methylmalonyl-CoA to succinyl-CoA. This process is crucial for the metabolism of fats and specific amino acids, ultimately contributing to energy production.

Comparison: Cyanocobalamin vs. Active Forms

While the active forms, methylcobalamin and adenosylcobalamin, are sometimes sold as supplements, cyanocobalamin remains the most common form due to its superior stability and cost-effectiveness. The body efficiently converts cyanocobalamin into the necessary active coenzymes, making it a reliable and effective source of B12.


Feature	Cyanocobalamin	Methylcobalamin / Adenosylcobalamin
Molecular Form	Synthetic, contains a cyano group.	Natural, active coenzyme forms.
Stability	Highly stable and shelf-stable.	Less stable, can be degraded by heat and light.
Cost	More cost-effective for mass production.	Generally more expensive.
Metabolism	Must be converted in the body to become active.	Active upon cellular uptake.
Function	Used as a precursor; provides the raw cobalamin molecule.	Directly functions as cofactors for specific enzymes.

Cyanocobalamin's Biological Impact

The breakdown of cyanocobalamin and its subsequent conversion into the active forms are critical for overall health. The resulting coenzymes, methylcobalamin and adenosylcobalamin, are integral to key metabolic pathways that support the nervous system, red blood cell formation, and DNA synthesis. Without proper conversion, these pathways would fail, leading to B12 deficiency symptoms like megaloblastic anemia and neurological damage. This demonstrates that while cyanocobalamin is a synthetic starting material, its metabolic fate is entirely natural and purposeful.

The Role in Methionine and Folate Metabolism

As a cofactor for methionine synthase, methylcobalamin plays a key role in the remethylation of homocysteine to methionine. This process is deeply intertwined with folate metabolism. In B12 deficiency, the folate cycle is impaired, leading to a buildup of 5-methyltetrahydrofolate (the "methyl trap" hypothesis) and subsequent DNA synthesis issues. By providing the necessary cofactor, the breakdown products of cyanocobalamin help to regulate this essential cycle.

Supporting Cellular Energy Production

In the mitochondria, adenosylcobalamin's role in converting methylmalonyl-CoA to succinyl-CoA is vital for cellular energy production. A deficit in this coenzyme can lead to the accumulation of methylmalonic acid (MMA), a clinical marker of B12 deficiency. This process directly links the breakdown of cyanocobalamin to the efficient functioning of the body's energy-generating machinery.

Conclusion

In summary, cyanocobalamin serves as a robust and stable precursor to the body's two active coenzyme forms of vitamin B12: methylcobalamin and adenosylcobalamin. The breakdown process involves the removal of the cyano group and subsequent conversion, with the resulting active forms playing indispensable roles in vital metabolic pathways. The body's efficient conversion of cyanocobalamin makes it a highly effective and cost-efficient option for fortifying foods and supplements, ensuring adequate B12 levels for critical functions like nerve health, DNA synthesis, and red blood cell formation.

Understanding B12 Metabolism

How Your Body Processes Cyanocobalamin

Your body breaks down the synthetic B12, cyanocobalamin, by first releasing the cobalamin molecule from its food or supplement matrix during digestion.
The cobalamin then binds to transcobalamin II, a transport protein that carries it through the bloodstream.
After entering the cell, the cobalamin molecule undergoes a conversion process where its cyano group is removed.
This process yields the two biologically active forms: methylcobalamin and adenosylcobalamin.
These active coenzymes then act as cofactors for essential enzymes involved in DNA synthesis, methylation, and energy metabolism.
The minimal amount of cyanide released is converted to thiocyanate and safely excreted via the kidneys.

Frequently Asked Questions

Cyanocobalamin is a synthetic form of vitamin B12, not naturally found in foods. It is used in supplements and to fortify foods because it is stable and cost-effective.

No, the minute amount of cyanide released during the metabolic conversion of cyanocobalamin is not dangerous. The body naturally and efficiently converts it into harmless thiocyanate, which is then excreted by the kidneys.

No, while both are active forms of vitamin B12, they serve different functions. Methylcobalamin is primarily involved in the methionine cycle in the cell's cytoplasm, while adenosylcobalamin is crucial for energy metabolism in the mitochondria.

After absorption and transport, cellular enzymes first remove the cyano group from the cyanocobalamin molecule. The resulting molecule is then converted into the specific active forms—methylcobalamin or adenosylcobalamin—needed by the cell.

Yes, vitamin B12 can also be obtained from animal-derived foods (which contain natural forms like adenosylcobalamin and methylcobalamin), as well as from supplements containing different B12 forms.

Cyanocobalamin is often used because it is more stable and less expensive to produce and store than the active forms. The body can reliably convert it to the necessary active coenzymes.

Not necessarily. The body is highly efficient at converting cyanocobalamin into its active forms. Some studies even suggest cyanocobalamin may be absorbed slightly better initially, though retention may vary.