Two natural forms
Vitamin K is a fat-soluble vitamin that exists in two main natural forms [1]:
- Vitamin K1 (phylloquinone) is the predominant dietary form, found mostly in leafy green vegetables (kale, spinach, broccoli) and vegetable oils.
- Vitamin K2 (menaquinones, e.g., MK-4 and MK-7) comes from fermented foods (like natto), some animal products and cheeses, and is also made by gut bacteria.
What vitamin K does
Both forms help the body make proteins needed for blood clotting and for bone metabolism [1]. The NIH Office of Dietary Supplements sets the adult Adequate Intake at 120 mcg/day for men and 90 mcg/day for women [1]. There is no separate official requirement for K2; the AI is based on total vitamin K, mostly from K1-rich foods.
K1 vs. K2: what the evidence supports
Marketing often presents K2 (especially MK-7) as far superior for bones and arteries. The forms do differ in how long they stay in the blood, but NIH's intake recommendations are for total vitamin K, and most people meet their needs largely through K1 from vegetables [1]. K2 supplements are popular and generally well tolerated, but the case that healthy people *need* supplemental K2 specifically is not settled. For a form-by-form K2 comparison, see vitamin K2: MK-4 vs. MK-7.
The warfarin warning
This is the most important safety point. Vitamin K counteracts the blood thinner warfarin: NIH notes that 'sudden changes in vitamin K intakes can increase or decrease the anticoagulant effect' [1]. People on warfarin should keep their vitamin K intake consistent (not necessarily low) and tell the clinic managing their dose before starting a K1 or K2 supplement — see supplements and blood thinners.
Practical guidance
Most people get enough vitamin K from a normal diet with some greens. If you take a K2 supplement (often paired with vitamin D), it's generally safe, but it's not a substitute for eating vegetables — and it's not for people on warfarin without medical guidance.