Name a vitamin synthesized by gut bacteria and a consequence of its deficiency.

• A. Vitamin K; deficiency can lead to impaired gamma-carboxylation of clotting factors and increased bleeding risk
• B. Vitamin A; deficiency leads to night blindness
• C. Vitamin D; deficiency leads to rickets
• D. Vitamin C; deficiency leads to scurvy

Answer: (A)

Vitamin K is produced by gut bacteria, and it’s essential for activating certain clotting factors. The vitamin serves as a cofactor for gamma-glutamyl carboxylase, which carboxylates specific glutamate residues on clotting factors II, VII, IX, and X, as well as proteins C and S, enabling them to bind calcium and function in the coagulation cascade. When vitamin K is deficient, these clotting factors aren’t properly carboxylated, so clotting is impaired and the risk of bleeding increases. In healthy people this is typically prevented by diet and gut microbes, but can occur with fat malabsorption, prolonged antibiotic use that disrupts gut flora, or in newborns with a sterile gut. The other vitamins listed are not made by gut bacteria and have different deficiency symptoms: vitamin A deficiency leads to night blindness, vitamin D deficiency to rickets, and vitamin C deficiency to scurvy.