Vitamin K (Phylloquinone)

vitaminphylloquinone

What is it

Phylloquinone (vitamin K1, phytonadione) is the major dietary form of vitamin K, found primarily in green leafy vegetables. It is a fat-soluble vitamin essential for blood clotting and bone metabolism through activation of specific vitamin K-dependent proteins.

How it works

Phylloquinone is absorbed from the small intestine with dietary fat and transported to the liver, where it serves as a cofactor for the enzyme gamma-glutamyl carboxylase. This enzyme adds carboxyl groups to specific glutamate residues on vitamin K-dependent proteins, including clotting factors II (prothrombin), VII, IX, and X, as well as proteins C and S that regulate clotting. In addition to coagulation, K-dependent proteins like osteocalcin (in bone) and matrix Gla protein (in vascular tissue) require carboxylation to function. The vitamin K cycle regenerates active vitamin K from its used form through the enzyme vitamin K epoxide reductase (VKOR), which is the target of warfarin and related anticoagulants. Liver storage of K1 is relatively limited, and phylloquinone is preferentially used for clotting factor synthesis. The body converts some K1 to menaquinone-4 (a K2 form) for extrahepatic tissues.

Evidence for 5 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Vitamin K deficiency bleeding (newborns and infants)

Grade A

Strong evidence

A single intramuscular K1 injection at birth (1 mg) reliably prevents vitamin K deficiency bleeding, including the late-onset form that can cause intracranial hemorrhage. Considered standard of care worldwide.

Warfarin reversal

Grade A

Strong evidence

Oral or IV phylloquinone is the standard antidote for excessive INR elevation from warfarin. Effect typically begins within 6-12 hours of administration.

Hemorrhagic disease in malabsorption

Grade A

Strong evidence

People with cystic fibrosis, cholestasis, short bowel, or chronic biliary obstruction reliably benefit from supplemental K1 (often parenteral) to prevent coagulopathy.

Bone density and fracture prevention

Grade C

Moderate evidence

Observational studies link higher K1 intake with better bone density and lower fracture risk, but RCTs of K1 supplementation alone have produced mixed results. K2 forms (especially MK-7) show more consistent bone benefits in some populations.

Cardiovascular calcification

Grade C

Moderate evidence

Observational evidence supports a role for vitamin K in reducing arterial calcification through matrix Gla protein activation. K2 (especially MK-7) shows more promising trial evidence than K1 for this purpose.

2 commercial forms

Phylloquinone (vitamin K1)

The natural plant form; absorption improves substantially with dietary fat.

Found in green leafy vegetables. The standard form used for warfarin reversal and routine supplementation.

Phytonadione (synthetic K1)

Chemically identical to natural K1; available in oral, IV, and IM forms.

The prescription form used in hospitals for anticoagulant reversal and newborn prophylaxis.

Dosage

The Adequate Intake (AI) for adults is 120 mcg/day for men and 90 mcg/day for women. Typical K1 supplements provide 50-500 mcg. There is no established UL for vitamin K from food or supplements. Most diets provide adequate K1 from green vegetables, so supplementation is often unnecessary unless absorption is impaired.

When and how to take it

Phylloquinone is fat-soluble, so take it with a meal containing some fat for best absorption. There is no preferred time of day. If supplementing alongside vitamin D, the two can be taken together with a fat-containing meal. Consistency matters more than timing for people on warfarin, where the goal is steady K intake rather than a specific schedule.

Food sources

Food	Amount	%DV
Kale (1 cup, raw)	472 mcg	—
Collard greens (1/2 cup, cooked)	530 mcg	—
Spinach (1/2 cup, cooked)	444 mcg	—
Broccoli (1/2 cup, cooked)	110 mcg	—
Brussels sprouts (1/2 cup, cooked)	150 mcg	—
Romaine lettuce (1 cup)	48 mcg	—
Soybean oil (1 tbsp)	25 mcg	—
Canola oil (1 tbsp)	10 mcg	—

Safety

Phylloquinone is considered very safe. No toxicity has been reported from oral intake, and there is no UL. Injection of synthetic vitamin K (menadione, K3) has caused hemolytic anemia in infants and is no longer used. The main safety issue with K1 is its powerful interaction with warfarin and similar anticoagulants.

Who should be cautious

Patients on warfarin or other vitamin K antagonists must coordinate any K1 supplement or major dietary change with their prescriber. Newborns receive a routine K1 injection to prevent hemorrhagic disease. People with biliary obstruction, cystic fibrosis, or other fat malabsorption may need higher intake or non-oral forms. Generally safe across other populations.

Interactions

Vitamin K1 directly antagonizes warfarin and other vitamin K antagonist anticoagulants. People on warfarin must keep K intake consistent day to day; large changes can shift INR dangerously. Antibiotics, cholestyramine, orlistat, and conditions causing fat malabsorption may lower K status. No significant interactions with DOACs (apixaban, rivaroxaban).

Frequently asked questions

What's the difference between vitamin K1 and K2?⌄

K1 (phylloquinone) is the plant form, used mainly by the liver for blood clotting. K2 (menaquinones) is bacterial and animal-derived, with greater activity in bone and vascular tissue. Both contribute to vitamin K status but have different tissue preferences.

Can I take K1 if I'm on warfarin?⌄

Only with your prescriber's coordination. K1 directly antagonizes warfarin and can dangerously lower your INR if added or stopped suddenly. The key is consistency in K intake from all sources.

Do I need a K1 supplement if I eat greens?⌄

Most likely not. A serving or two of leafy greens per day usually provides more than the AI. Supplementation matters most in malabsorption, restricted diets, or specific clinical indications.

Why do babies get a vitamin K shot at birth?⌄

Newborns are born with very low vitamin K stores and limited intestinal flora, putting them at risk of potentially fatal hemorrhagic disease of the newborn. A single K1 injection reliably prevents this.

Does K1 help bones?⌄

There is some observational evidence linking higher K1 intake with better bone density, but trial evidence is mixed. K2 (particularly MK-7) shows more consistent bone-specific effects.

References

NIH Office of Dietary Supplements - Vitamin K Fact Sheet — NIH ODS link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.