Clear evidence. Transparent sources. Practical guidance. Our evidence policyContact
Food Health Lab
Home › Supplements › Vitamin K

Vitamin K — The Complete Supplement Guide

Essential for blood clotting and bone — and the one vitamin that directly clashes with warfarin. K1 vs K2, doses and safety.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden) Last reviewed July 2026

SupplementsVitaminsVitamin KFat-soluble

It comes in two families — K1 from leafy greens and K2 from ferments and animal foods — with growing interest in K2 for bone and arteries. For the short dietary version, see the Vitamins page.

Read first — warfarin. If you take warfarin or another vitamin K antagonist, speak with the healthcare professional managing your anticoagulant treatment before taking vitamin K supplements or making major changes to your vitamin K intake.

Key takeaways

  • K1 (leafy greens) covers clotting; K2 (MK-7, ~90–180 µg) is studied for bone and arteries.
  • Adults: about 70–120 µg/day (adequate intake).
  • No upper limit — but on warfarin, keep vitamin K intake steady and talk to your doctor.

Function

Vitamin K activates proteins that handle clotting and calcium:

  • Blood clotting: essential to activate several clotting factors — without it, blood cannot clot properly.
  • Bone: activates osteocalcin, which binds calcium into the bone matrix.
  • Artery protection: activates matrix Gla-protein (MGP), which helps keep calcium out of artery walls (the basis of interest in K2 for vascular health).
  • Cell regulation: further roles in cell growth and signalling.

Different forms — and which to choose

K1 (phylloquinone)From green leafy vegetables; the main dietary form, chiefly used for clotting.
K2 (menaquinones)From fermented foods and animal products; MK-7 (e.g. from natto) has a long half-life and is the form studied for bone and arteries.
K3 (menadione)A synthetic form not used in human supplements (toxicity concerns).

Absorption in the body

How muchFat-dependent; K1 from greens is poorly absorbed unless eaten with fat. K2 (MK-7) is well absorbed and stays in the blood longer.
NoteGut bacteria make some K2, but not enough to rely on.

Deficiency symptoms

SymptomsEasy bruising and bleeding; poor bone mineralisation over time.
NewbornsBabies are born low and are given vitamin K at birth to prevent serious bleeding.
Who's at riskFat-malabsorption, liver disease, long-term antibiotics, and newborns.

Recommended intake

Adults AI ≈ 70–120 µg/day

An adequate-intake figure:

  • Adults: ~70 µg/day (EFSA) · 90 µg ♀ / 120 µg ♂ (US)

Best sources: kale, spinach, broccoli and other greens (K1); natto, some cheeses and animal foods (K2).12

Therapeutic use

MedicalReverses dangerous over-thinning from warfarin (hospital use); routine newborn vitamin K.
Bone & vascularK2 (MK-7 ~90–180 µg/day) is studied for bone density and arterial calcification — promising, not yet definitive.

Therapeutic doses

K2 (MK-7): ~90–180 µg/day studied
EverydayGreens supply plenty of K1; K2 supplements are optional.
WarfarinIf on warfarin, do not change vitamin K intake without medical advice.

Toxicity & upper limit

No UL — low toxicity
Upper limitNo formal UL; vitamin K1/K2 have very low toxicity.1
Key interactionIt counteracts warfarin — the one situation where vitamin K matters clinically. Keep intake steady and coordinate with your doctor.

Bottom line: eat your greens for K1; a K2 (MK-7) supplement is a reasonable optional add-on for bone/artery health, often paired with vitamin D. The one hard rule: if you take warfarin, don't change your vitamin K without medical advice.

← All supplementsShort dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

  1. US NIH Office of Dietary Supplements. Vitamin K — Health Professional Fact Sheet (function, forms, intakes, food sources, deficiency, therapeutic uses, upper limit, interactions). ods.od.nih.gov
  2. EFSA. Dietary Reference Values & Tolerable Upper Intake Levels for vitamins (European reference intakes and ULs). efsa.europa.eu

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, or alongside medication.

Read next

Vitamin
Vitamin C
Vitamin
Vitamin B1
Vitamin
Vitamin B2