Vitamin K
Vitamin K is a family of fat-soluble compounds required for γ-carboxylation of specific proteins — clotting factors II, VII, IX, X, protein C, protein S, plus osteocalcin and matrix Gla protein. In practical terms this means vitamin K clots blood, mineralises bone, and keeps calcium out of arteries.
K1 vs K2
- Vitamin K1 (phylloquinone) — plant form; the primary dietary source. Mostly used by the liver for clotting factor synthesis.
- Vitamin K2 (menaquinones) — animal and fermented food form. Subtypes named by chain length: MK-4 (short, in meat and egg yolk) and MK-7 (long, in natto and cheese). MK-7 has a much longer half-life (~72 h vs ~1 h for K1) and reaches peripheral tissues more effectively — the basis of the bone / arterial argument for K2.
Daily requirements
- Adult men: 120 µg/day (AI).
- Adult women: 90 µg/day (AI).
- Tolerable Upper Intake Level: none established.
Deficiency
Rare in adults with normal diets. Bleeding tendency (elevated INR/PT) is the first clinical sign. Newborns routinely receive an intramuscular vitamin K injection at birth to prevent vitamin K deficiency bleeding (VKDB), which can otherwise cause fatal intracranial haemorrhage.
Bone health
Osteocalcin requires vitamin K–dependent γ-carboxylation to bind calcium to bone hydroxyapatite. Undercarboxylated osteocalcin (ucOC) is a marker of vitamin K insufficiency and correlates with fracture risk. Meta-analyses of K1 supplementation for fracture prevention are modestly positive; K2 (MK-7 45 µg/day) has stronger evidence for improving bone mineral density in postmenopausal women (Knapen 2013).
Arterial calcification
Matrix Gla protein (MGP), also vitamin K–dependent, actively inhibits arterial calcium deposition. The Rotterdam Study found high dietary K2 intake associated with lower coronary calcification and cardiovascular mortality (Geleijnse 2004). RCTs of MK-7 in kidney disease show slowed vascular calcification progression. This is the biological basis for pairing K2 with high-dose vitamin D.
Warfarin interaction
Vitamin K reverses warfarin. Patients on warfarin should keep vitamin K intake consistent, not necessarily low — sudden increases (starting a leafy-greens phase) or decreases (going low-vegetable) will destabilise INR. DOACs (apixaban, rivaroxaban, dabigatran, edoxaban) are not affected by vitamin K.
Food sources
- Vitamin K1: collard greens (530 µg/½ cup), spinach, kale, broccoli, brussels sprouts.
- Vitamin K2 (MK-7): natto (fermented soybeans — 850 µg/3 oz), aged cheeses, sauerkraut.
- Vitamin K2 (MK-4): egg yolk, beef liver, dark chicken meat.
Supplementation
- Not needed for most people eating leafy greens regularly.
- K2 (MK-7) 90–180 µg/day is common as an adjunct to high-dose vitamin D or in postmenopausal osteoporosis management.
- Do not add K2 while on warfarin without prescriber involvement.