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Fat-soluble vitamin

Vitamin K

K1 phylloquinone / K2 menaquinones

Blood clotting factors (II, VII, IX, X), bone protein (osteocalcin) activation

Essential nutrient

Reference intakes (RDA or AI) are set by the National Academies of Sciences, Engineering, and Medicine. Deficiency thresholds come from clinical and WHO guidance.

At a glance

Women, 19-50
90 mcg (AI)
Men, 19-50
120 mcg (AI)
Upper limit (adults)
Not established
How common is deficiency?
Uncommon in healthy adults; COMMON in newborns without a vitamin K injection

Food sources

Animal sources
K2: natto is plant-based; animal K2 in egg yolk, hard cheeses, chicken, beef liver, butter
Plant sources
K1: kale, spinach, collards, broccoli, Brussels sprouts, lettuce, parsley; K2: natto (fermented soybeans)

Deficiency and testing

Signs of deficiency
Easy bruising, prolonged bleeding, nosebleeds, heavy periods, blood in urine/stool; bleeding in newborns (VKDB)
Who is at higher risk
Newborns, long-term antibiotic use, fat malabsorption, bariatric surgery, anticoagulant (warfarin) users, severe liver disease
How it is tested
Prothrombin time (PT) / INR; PIVKA-II (des-gamma-carboxy prothrombin); undercarboxylated osteocalcin (research)
Cut-offs
Prolonged PT/INR not explained by other causes; PIVKA-II elevated

Symptoms are not specific. Only a clinician with the right tests can tell whether you are deficient.

Supplement forms

Common forms
K1 (phylloquinone), K2 as MK-4, K2 as MK-7 (from natto/fermentation), combined K1+K2 products.
More effective
MK-7 (long half-life, steady blood levels at low daily doses, good evidence for osteocalcin activation); K1 for clotting needs or deficiency; always with a fat-containing meal.
Less effective
MK-4 at low supplement doses (very short half-life; studies of bone use pharmacologic doses of about 45 mg); fasting use. People on warfarin must keep vitamin K intake steady and talk to a clinician first.

Safety, interactions and halal notes

Safety and interactions
Upper limit for adults: Not established. Going above the upper limit over time can cause harm, so check labels and talk to a clinician before high-dose supplements.
Halal note
The nutrient itself is not a halal question. For supplements, check the delivery form: gelatin (pork or non-halal bovine), capsule shells, glycerin, and alcohol-based carriers need halal certification. Ask for the halal certificate, not just a label claim. This is general guidance, not a certification.

Frequently asked questions

How much Vitamin K do adults need per day?

Women 19-50: 90 mcg (AI). Men 19-50: 120 mcg (AI). Upper limit: Not established. Needs change with age, pregnancy and health conditions.

What are the signs of Vitamin K deficiency?

Easy bruising, prolonged bleeding, nosebleeds, heavy periods, blood in urine/stool; bleeding in newborns (VKDB). These signs are not specific, so only a clinician with the right tests can confirm a deficiency.

What are the best food sources of Vitamin K?

Animal sources: K2: natto is plant-based; animal K2 in egg yolk, hard cheeses, chicken, beef liver, butter. Plant sources: K1: kale, spinach, collards, broccoli, Brussels sprouts, lettuce, parsley; K2: natto (fermented soybeans).

Which form of Vitamin K supplement is best?

MK-7 (long half-life, steady blood levels at low daily doses, good evidence for osteocalcin activation); K1 for clotting needs or deficiency; always with a fat-containing meal.

Sources

  1. NIH Office of Dietary Supplements fact sheet

← Back to the full nutrient guide

This page is for education only and is not medical advice. These statements have not been evaluated by the Food and Drug Administration. Supplements are not intended to diagnose, treat, cure or prevent any disease. Talk to a qualified clinician before starting a supplement, especially if you are pregnant, breastfeeding, take medicine or have a medical condition. Daily needs are US reference intakes for adults 19-50.

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