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Vitamin K

Evidence Level
Strong
7 Clinical Trials
5 Documented Benefits
4/5 Evidence Score

Vitamin K, a fat-soluble nutrient, plays a critical role in blood clotting and bone health, existing primarily as K1 (phylloquinone) from plants and K2 (menaquinone) from fermented foods and animal products. Found abundantly in leafy greens like kale and spinach, as well as in natto and cheese, it supports proper coagulation and may reduce fracture risk.

Studied Dose 90–120 mcg/day K1 (RDA); MK-7 (K2): 90–200 mcg/day for bone/cardiovascular; MenaQ7® studies use 180–360 mcg/day MK-7
Active Compound MK-7 (Menaquinone-7) / K1 (Phylloquinone)
Deficiency information View details

Vitamin K deficiency is uncommon in healthy adults because gut bacteria produce some K2 and small amounts are stored in tissues. The main concern is VITAMIN K DEFICIENCY BLEEDING (VKDB) IN NEWBORNS — a potentially fatal condition. All US newborns receive a vitamin K injection at birth to prevent it. In adults, deficiency primarily affects people on long-term antibiotics, those with severe fat malabsorption, or warfarin users.

Common symptoms

  • Easy bruising and prolonged bleeding from minor cuts
  • Bleeding gums, frequent nosebleeds
  • Heavy menstrual bleeding
  • Blood in urine or stool (dark, tarry stools)
  • Internal bleeding — especially gastrointestinal
  • In newborns with VKDB: bleeding from the umbilical stump, mucous membranes, GI tract, or — most seriously — into the brain (intracranial hemorrhage)
  • Signs of internal bleeding in infants: extreme sleepiness, pale skin, bulging fontanelle, vomiting, seizures (MEDICAL EMERGENCY)
  • Increased risk of bone fractures (chronic deficiency affects bone metabolism via osteocalcin)

At-risk groups

  • Newborns who do not receive prophylactic vitamin K injection at birth
  • Exclusively breastfed infants (breast milk is low in vitamin K)
  • Infants with cholestatic liver disease, biliary atresia, or cystic fibrosis
  • People taking warfarin or other vitamin K antagonists (intentional, requires monitoring)
  • People on long-term broad-spectrum antibiotics (alters gut flora)
  • People with severe fat malabsorption (Crohn's, celiac, short-bowel syndrome)
  • People with chronic liver disease
  • People who've had bariatric surgery
When to see a doctor: Any unexplained bleeding in a newborn — bleeding from the umbilical stump, blood in stool, vomiting blood, or signs of brain bleeding (extreme sleepiness, seizures) — is a MEDICAL EMERGENCY requiring immediate ER care. In adults, unexplained easy bruising or bleeding warrants a coagulation panel (PT/INR). Do NOT take vitamin K supplements if you are on warfarin without your doctor's approval — it can dangerously alter your medication's effect.
Explore related deficiencies →
Sources: NIH ODS — Vitamin K Health Professional Fact Sheet · Hemorrhagic Disease of Newborn — StatPearls (NBK558994)

Benefits

Blood Clotting

Vitamin K is essential for synthesizing clotting factors (e.g., prothrombin), ensuring proper coagulation to prevent excessive bleeding from injuries.

Supported by Trial 1, Trial 2

Bone Health

It activates proteins like osteocalcin, which bind calcium to strengthen bones, potentially reducing fracture risk and supporting bone density.

Supported by Trial 2

Heart Health

Vitamin K2 may help prevent arterial calcification by directing calcium to bones instead of blood vessels, though evidence is still emerging.

Supported by Trial 7, Trial 3

Anti-Inflammatory Effects

Some studies suggest vitamin K may reduce inflammation markers, potentially benefiting conditions like arthritis.

Supported by Trial 1, Trial 2

Potential Cognitive Support

Early research indicates vitamin K may play a role in brain health, possibly reducing age-related cognitive decline.

Supported by Trial 6, Trial 4

Mechanism of action

1

Blood Clotting (Coagulation)

Vitamin K is essential for the gamma-carboxylation of glutamate residues in clotting factors II (prothrombin), VII, IX, and X, as well as proteins C, S, and Z, in the liver. This process occurs via the enzyme gamma-glutamyl carboxylase, which uses vitamin K as a cofactor to add carboxyl groups, enabling these proteins to bind calcium ions. Calcium binding allows these factors to interact with phospholipids on cell surfaces, facilitating the clotting cascade to form a stable clot and prevent excessive bleeding.

2

Bone Health

Vitamin K activates osteocalcin, a bone protein produced by osteoblasts, through the same gamma-carboxylation process. Carboxylated osteocalcin binds calcium ions, incorporating them into bone matrix to enhance bone mineralization and strength. It also regulates bone remodeling by influencing osteoblast and osteoclast activity, potentially reducing fracture risk.

3

Vascular Health

Vitamin K2 activates matrix Gla protein (MGP), which inhibits calcium deposition in blood vessels and soft tissues, preventing arterial calcification. This helps maintain vascular elasticity and may reduce cardiovascular risk.

Clinical trials

1
Vitamin K and Cardiovascular Events/Mortality — Meta-Analysis
PubMed

2019 meta-analysis of prospective studies on vitamin K (K1, K2) intake and CV events/all-cause mortality. (Eur J Nutr 2019)

Pooled across prospective cohorts.

Higher dietary K2 intake associated with reduced CV mortality and events; K1 less consistent. CRITICAL CAVEAT: OBSERVATIONAL evidence — cannot establish causation. Vitamin K2 supplementation RCTs needed for definitive conclusions.

2
Vitamin K Intake and Hip Fracture — Meta-Analysis
PubMed

2018 meta-analysis of 23 studies (22 observational, 1 RCT) with 1,121,582 participants on vitamin K and hip fracture risk.

Pooled across 23 studies.

Higher vitamin K intake associated with modestly reduced hip fracture risk. Mostly observational evidence; OK in pooling but interventional confirmation needed. Note: bone health management primarily uses calcium, vitamin D, K2, weight-bearing exercise, and pharmacotherapy (bisphosphonates, denosumab) for established osteoporosis.

3
Vitamin K for Calcification in Kidney Transplant — RCT
PubMed

2021 RCT in 40 vitamin K-deficient kidney transplant recipients with high dp-ucMGP levels receiving vitamin K supplementation.

40 KTRs with vitamin K deficiency.

Vitamin K supplementation reduced dp-ucMGP and modestly improved arterial stiffness markers. Note: KTR vitamin K deficiency common; emerging clinical interest.

4
MK-7 for Arterial Stiffness in Postmenopausal Women — RCT
PubMed

2015 RCT (Knapen et al.) in 244 healthy postmenopausal women receiving 180 µg/day MK-7 (Vitamin K2) for 3 years. (Knapen et al. 2015, Thromb Haemost)

244 postmenopausal women. 3-year intervention.

MK-7 group showed improved arterial stiffness measures (PWV) vs placebo. Effect emerged after 12 months. Important RCT supporting K2 cardiovascular role; MK-7 form preferred for long half-life vs MK-4 (very short half-life requiring multiple daily doses).

5
Vitamin K1 for Hemodialysis Calcification — Trial Protocol
PubMed

2015 trial protocol (Holden et al.) RCT designed to assess vitamin K1 supplementation in hemodialysis patients with vascular calcification.

Hemodialysis patients (protocol).

Trial design only at this citation; subsequent results published. CKD vascular calcification is a major morbidity driver; vitamin K trials in this population emerging.

6
Vitamin K Effects on Vascular Calcification Across Ages — RCT
PubMed

Ongoing RCT (NCT04676958, registered 2020) in 80 adults (40 young ≤40 years, 40 older) examining vitamin K supplementation effects on vascular calcification.

80 adults across age ranges.

Trial ongoing. Important question — vitamin K's role in age-related vascular calcification.

7
Vitamin K and Vascular Calcification — 2023 Meta-Analysis
PubMed

2023 meta-analysis of 14 RCTs evaluating vitamin K supplementation effects on vascular calcification across populations.

Pooled across 14 RCTs.

Vitamin K (especially K2/MK-7) modestly reduced vascular calcification progression in some populations. CKD populations showed clearer benefit. Note: large outcomes-focused trials still needed; RCT evidence is emerging.

Side effects and drug interactions

Common Potential side effects

Excessive Intake (Rare): Hemolytic anemia (red blood cell breakdown), especially in infants or those with G6PD deficiency. Liver toxicity or jaundice in rare cases, historically linked to high-dose K3 injections. Skin reactions, such as rashes or irritation, with excessive topical or injected forms.
Allergic Reactions: Rare hypersensitivity to vitamin K supplements or injections, potentially causing: Itching, rash, or hives.
Gastrointestinal Issues: High-dose supplements may occasionally cause mild nausea, diarrhea, or stomach upset.
Injection-Specific (Medical Use): Pain or swelling at the injection site. Rare cardiovascular effects (e.g., rapid heartbeat) with rapid IV administration.

Important Drug interactions

Warfarin (Coumadin) — vitamin K directly antagonizes warfarin's anticoagulant mechanism; even small changes in K1 intake can destabilize INR; maintain consistent vitamin K intake and monitor INR closely with K2 supplementation
Antibiotics (broad-spectrum) — may reduce gut bacteria that produce vitamin K2 (menaquinones); may temporarily impair vitamin K status
Orlistat and cholestyramine — reduce absorption of fat-soluble vitamins including vitamin K
Salicylates (aspirin) — high-dose aspirin may impair vitamin K-dependent clotting factor synthesis
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Frequently asked questions about Vitamin K

What is Vitamin K?

Vitamin K, a fat-soluble nutrient, plays a critical role in blood clotting and bone health, existing primarily as K1 (phylloquinone) from plants and K2 (menaquinone) from fermented foods and animal products.

What does Vitamin K do?

Vitamin K is essential for the gamma-carboxylation of glutamate residues in clotting factors II (prothrombin), VII, IX, and X, as well as proteins C, S, and Z, in the liver. In clinical research, Vitamin K has been studied for blood clotting, bone health, heart health.

Who should take Vitamin K?

Vitamin K may be most beneficial for: Newborns who do not receive prophylactic vitamin K injection at birth; Exclusively breastfed infants (breast milk is low in vitamin K); Infants with cholestatic liver disease, biliary atresia, or cystic fibrosis; People taking warfarin or other vitamin K antagonists (intentional, requires monitoring). As with any supplement, consult your healthcare provider before starting, especially if you have medical conditions or take prescription medications.

How long does Vitamin K take to work?

Most clinical trial effects appear over weeks of consistent use; individual response varies. Acute or same-day effects (where applicable) typically appear within hours, but most cumulative benefits — particularly those affecting biomarkers, mood, sleep quality, or chronic symptoms — require 4-12 weeks of regular use to fully assess. If you don't notice benefit after 12 weeks at the appropriate dose, it may not be your responder.

When is the best time to take Vitamin K?

Vitamin K can typically be taken with breakfast or dinner — taking with food reduces GI sensitivity for most supplements. Specific timing matters less than daily consistency for cumulative effects. Always check product labeling and follow personalized guidance from your healthcare provider.

Is Vitamin K worth taking?

Vitamin K has strong clinical evidence (Evidence Level 4/5 on NutraSmarts) for its primary uses, with multiple randomized controlled trials and meta-analyses supporting its benefits. Whether it's worth taking depends on your specific goals, what you've already tried, your budget, and your overall supplement strategy. The honest framing: no supplement is essential for most people, and lifestyle factors (sleep, exercise, diet, stress management) typically produce larger effects than any single supplement. Vitamin K is most worth trying if its evidence-supported uses align with your specific goals.

What is the recommended dosage of Vitamin K?

The clinically studied dose for Vitamin K is 90–120 mcg/day K1 (RDA); MK-7 (K2): 90–200 mcg/day for bone/cardiovascular; MenaQ7® studies use 180–360 mcg/day MK-7. Always follow product labeling and consult a healthcare provider for personalized dosing recommendations.

What is Vitamin K used for?

Vitamin K is studied for blood clotting, bone health, heart health. Vitamin K is essential for synthesizing clotting factors (e.g., prothrombin), ensuring proper coagulation to prevent excessive bleeding from injuries.