Vit K

Vitamin K

Vitamins & Minerals

What is Vitamin K?

Vitamin K is a fat-soluble vitamin essential for blood coagulation (clotting) and bone metabolism. It exists in two main natural forms: vitamin K1 (phylloquinone), found in green leafy vegetables and plant oils, and vitamin K2 (menaquinone), produced by intestinal bacteria and found in fermented foods, meat, and dairy. Vitamin K functions as a cofactor for the enzyme gamma-glutamyl carboxylase, which activates clotting factors II (prothrombin), VII, IX, and X, as well as anticoagulant proteins C and S.

Beyond coagulation, vitamin K activates osteocalcin—a protein essential for calcium binding in bone tissue—and matrix Gla protein, which prevents calcium deposition in blood vessel walls. This dual role in both clotting and calcium metabolism has generated significant interest in vitamin K's potential to reduce fracture risk and cardiovascular calcification. Vitamin K levels in blood are measured as plasma phylloquinone (K1), though functional assessment through prothrombin time (PT/INR) is more commonly used in clinical practice to evaluate vitamin K-dependent clotting.

Why It Matters

Vitamin K is indispensable for proper blood clotting. Without adequate vitamin K, the liver cannot produce functional clotting factors, leading to excessive bleeding that can be life-threatening. Newborns are particularly vulnerable because they are born with low vitamin K stores and limited gut bacteria, which is why vitamin K injection at birth is standard practice worldwide. Vitamin K deficiency can also impair bone mineralization, increasing fracture risk. In patients on warfarin (a vitamin K antagonist anticoagulant), vitamin K intake must be consistent to maintain stable anticoagulation.

Normal Reference Ranges

Group	Range	Unit
Adults (K1, phylloquinone)	0.2–3.2	ng/mL

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High Vit K Levels Mean

Common Causes

High dietary intake of green leafy vegetables
Vitamin K supplementation
Generally non-toxic—no established upper limit for natural forms

Possible Symptoms

No known toxicity from natural vitamin K1 or K2
Synthetic vitamin K3 (menadione) can cause hemolytic anemia and jaundice in high doses (not used clinically)

What to do: High vitamin K levels from dietary or supplemental sources are not associated with adverse effects. However, patients on warfarin or other vitamin K antagonist anticoagulants should maintain consistent vitamin K intake, as sudden increases can reduce the drug's anticoagulant effect, increasing clotting risk. Inform your doctor if you significantly change your green vegetable or supplement intake while on anticoagulation therapy.

What Low Vit K Levels Mean

Common Causes

Fat malabsorption (celiac disease, cystic fibrosis, bile duct obstruction)
Prolonged antibiotic use (reduces gut bacteria that produce K2)
Liver disease
Inadequate dietary intake
Newborns (low stores at birth)
Warfarin or other vitamin K antagonist use
Cholestyramine or orlistat medications

Possible Symptoms

Easy bruising
Excessive bleeding from cuts or surgical sites
Bleeding gums or nosebleeds
Blood in urine or stool
Heavy menstrual periods
Prolonged prothrombin time (PT/INR)
In newborns: hemorrhagic disease of the newborn (VKDB)

What to do: Mild deficiency is treated with oral vitamin K1 supplementation and dietary improvement—increase intake of kale, spinach, broccoli, Brussels sprouts, and green peas. Severe bleeding requires intravenous vitamin K and possibly fresh frozen plasma for immediate correction. Address underlying malabsorption or liver disease. Newborns receive a prophylactic vitamin K injection at birth to prevent VKDB.

When Is Vit K Testing Recommended?

When unexplained bruising or bleeding occurs
In patients with fat malabsorption syndromes
When liver disease is present
In newborns with signs of bleeding
To evaluate prolonged prothrombin time (PT/INR)
In patients on long-term antibiotic therapy

Frequently Asked Questions

Newborns are born with very low vitamin K stores because it does not cross the placenta efficiently, and their sterile intestines lack the bacteria that produce vitamin K2. Breast milk also contains relatively little vitamin K. Without supplementation, newborns are at risk for vitamin K deficiency bleeding (VKDB), which can cause intracranial hemorrhage and death. A single intramuscular injection of vitamin K at birth virtually eliminates this risk and is recommended by pediatric organizations worldwide.

Warfarin works by blocking vitamin K recycling, which reduces the production of active clotting factors. Vitamin K intake directly counteracts warfarin's effect. Patients on warfarin don't need to avoid vitamin K-rich foods, but they must keep their intake consistent from day to day so their warfarin dose remains calibrated. A sudden large intake of leafy greens can reduce INR (making blood more likely to clot), while avoiding all vitamin K-rich foods can raise INR dangerously. Newer anticoagulants like rivaroxaban and apixaban do not interact with vitamin K.

Vitamin K1 (phylloquinone) is found in green leafy vegetables and is the primary dietary form, playing a dominant role in blood clotting. Vitamin K2 (menaquinone) is produced by gut bacteria and found in fermented foods like natto, cheese, and sauerkraut. K2 has subtypes (MK-4 through MK-13) with varying half-lives and tissue distribution. Research suggests K2, particularly MK-7, may be more important for bone and cardiovascular health, as it more effectively activates osteocalcin and matrix Gla protein in those tissues.

Related Biomarkers

OCOsteocalcin Vit AVitamin A PT/INRProthrombin Time CaCalcium

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.