Where is vitamin K synthesized in the gastrointestinal tract?

Vitamin K Synthesis in the Gastrointestinal Tract

Vitamin K2 (menaquinones) is synthesized by intestinal bacteria, including E. coli, primarily in the colon and terminal ileum, where it contributes to meeting human vitamin K requirements. 1

Bacterial Production Sites

Colon (Primary Site)

• Many intestinal bacteria synthesize vitamin K2 (menaquinones), but not vitamin K1 (phylloquinone), with E. coli being a key producer. 1
• The colon harbors members of the genus Bacteroides that synthesize very long-chain menaquinones (MK-10 through MK-13). 2
• Colonic absorption of bacterially synthesized vitamin K2 occurs via passive, nonsaturable diffusion that is not energy-dependent or carrier-mediated. 3
• The vitamin accumulates in the mucosal and submucosal layers of the large bowel during absorption. 3

Terminal Ileum (Secondary Site)

• The terminal ileum contains reasonable concentrations of bacterial menaquinones where bile salt-mediated absorption is possible. 2
• Direct human studies demonstrate that menaquinones delivered to the ileum are absorbed and produce measurable improvements in coagulation factors within 24 hours. 4
• This distal small bowel location allows for efficient absorption of bacterially produced vitamin K2. 4

Clinical Significance of Bacterial Production

Contribution to Vitamin K Requirements

• Bacterially synthesized menaquinones play a significant role in contributing to vitamin K requirements, especially during periods of dietary vitamin K deficiency. 5
• The preponderance of evidence suggests that bacterial menaquinones can prevent clinically significant coagulopathy when dietary vitamin K1 is inadequate. 5
• Hepatic stores contain predominantly long-chain menaquinones (MK-7 through MK-13), with approximately 60-70% being of bacterial origin. 2

Evidence from Antibiotic Studies

• Patients receiving broad-spectrum antibiotics may develop coagulopathy (elevated prothrombin time), indirectly confirming that intestinal bacteria-produced menaquinones contribute to maintaining normal coagulation. 4
• This clinical observation supports measuring vitamin K status in at-risk patients, including those on prolonged broad-spectrum antibiotics. 1

Important Clinical Caveats

Absorption Considerations

• While bacterial synthesis occurs in the colon, the exact site and efficiency of menaquinone absorption from the gut microflora remains incompletely characterized. 2
• Long-chain menaquinones are less well absorbed than shorter-chain forms, despite being bioactive in hepatic gamma-carboxylation. 2
• The body requires continuous dietary supply of vitamin K1 (phylloquinone) because approximately 60-70% of daily intake is lost through excretion. 2

Dietary vs. Bacterial Sources

• Vitamin K1 (phylloquinone) from green leafy vegetables and certain oils remains the primary dietary source and cannot be synthesized by bacteria. 1, 2
• Fermented foods (especially cheeses) provide dietary menaquinones (MK-8 and MK-9) that are distinct from those produced by gut bacteria. 6, 2
• The gut microflora produces a wider spectrum of menaquinones than those available from dietary sources. 2