What Vitamin K Does
Vitamin K is essential for making proteins that allow blood to clot, and for proteins involved in bone metabolism. The NIH Office of Dietary Supplements sets the Adequate Intake at 120 mcg/day for men and 90 mcg/day for women [1].
K1 vs K2
The two main forms differ by source [1]:
- Vitamin K1 (phylloquinone) is 'present primarily in green leafy vegetables and is the main dietary form.'
- Vitamin K2 (menaquinones) are 'predominantly of bacterial origin' and 'present in modest amounts in various animal-based and fermented foods' (like natto and some cheeses).
Most dietary vitamin K is K1 from vegetables; gut bacteria also produce some K2.
No Upper Limit
NIH 'did not establish ULs for vitamin K because of its low potential for toxicity' [1]. As always, 'no UL' reflects low observed harm, not a reason to take unlimited amounts (see Upper Intake Levels).
The Warfarin Caution: Consistency, Not Avoidance
This is the most important practical point. Warfarin works by interfering with vitamin K-dependent clotting, so vitamin K intake directly affects the drug. NIH advises that 'people taking warfarin and similar anticoagulants need to maintain a consistent intake of vitamin K from food and supplements because sudden changes in vitamin K intakes can increase or decrease the anticoagulant effect' [1].
The goal for warfarin users is steady, predictable intake — not cutting out greens. Suddenly starting or stopping a vitamin K2 supplement, or dramatically changing how many leafy vegetables you eat, can throw off warfarin control. Discuss any vitamin K supplement with your prescriber (see Supplements and Medications), and mention it before procedures (see When to Stop a Supplement Before Surgery).
Practical Guidance
- Most people meet vitamin K needs from a normal diet, especially leafy greens.
- If you take warfarin, keep intake consistent and tell your prescriber before changing supplements or diet substantially.