Vitamin K
Also known as: Phylloquinone, Menaquinone
What Does Vitamin K Measure?
Vitamin K is a fat-soluble vitamin that exists in two primary natural forms: Vitamin K1 (phylloquinone), found mainly in leafy green vegetables, and Vitamin K2 (menaquinone), produced by bacteria and found in fermented foods and animal products. A blood test for Vitamin K measures the circulating level of these compounds in the bloodstream, typically reported in nanograms per milliliter (ng/mL) or micrograms per liter (mcg/L). The test reflects recent dietary intake and the body's ability to absorb and utilize fat-soluble vitamins.
Why Does Vitamin K Matter?
Vitamin K plays an essential role in two critical physiological processes: blood coagulation and bone metabolism. It acts as a cofactor for enzymes that activate clotting factors (II, VII, IX, and X), meaning that without adequate Vitamin K, the blood cannot clot properly, leading to excessive bleeding. Beyond coagulation, Vitamin K activates osteocalcin and matrix Gla-protein (MGP), proteins responsible for directing calcium into bones and preventing its deposition in arteries. Low Vitamin K levels are associated with increased fracture risk, arterial calcification, and cardiovascular disease, making it a clinically important marker beyond just bleeding disorders.
Normal Ranges
Males
0.2 - 3.2 ng/mL (Vitamin K1/Phylloquinone)
Females
0.2 - 3.2 ng/mL (Vitamin K1/Phylloquinone)
Children
Varies by age; generally 0.1 - 2.2 ng/mL; newborns are at highest risk for deficiency
Causes of High Levels
- Excessive supplementation with high-dose Vitamin K supplements
- High dietary intake of leafy green vegetables (kale, spinach, collard greens) in large quantities
- Use of Vitamin K2 supplements (MK-4 or MK-7 forms) without medical oversight
- Liver disease in certain phases, altering Vitamin K metabolism and storage
- Recent consumption of Vitamin K-rich foods prior to blood draw affecting serum levels
Causes of Low Levels
- Inadequate dietary intake of leafy greens and fermented foods
- Fat malabsorption disorders such as Crohn's disease, celiac disease, or cystic fibrosis
- Long-term use of antibiotics, which destroy gut bacteria that produce Vitamin K2
- Anticoagulant medications such as warfarin, which antagonize Vitamin K activity
How to Improve Your Vitamin K
Diet
- Increase consumption of leafy green vegetables such as kale, spinach, Swiss chard, and collard greens (1 cup cooked provides 400-1000 mcg of K1)
- Include fermented foods rich in Vitamin K2 such as natto (a Japanese fermented soybean dish, highest known food source at ~900 mcg per 100g), sauerkraut, and aged cheeses
- Consume grass-fed animal products including egg yolks, liver, and butter, which contain meaningful amounts of Vitamin K2 (MK-4 form)
- Pair Vitamin K-rich foods with healthy fats (olive oil, avocado) to enhance absorption, as it is a fat-soluble vitamin
- Reduce ultra-processed foods and alcohol consumption, which can impair fat-soluble vitamin absorption and gut health
Supplements
- Vitamin K2 as MK-7 (menaquinone-7): 90-200 mcg/day is the most bioavailable and longer-acting form, often combined with Vitamin D3 for synergistic bone and cardiovascular benefits
- Vitamin K1 (phylloquinone): 1000 mcg/day may be used therapeutically under medical supervision for deficiency correction
- Combined Vitamin D3 + K2 supplements (e.g., 5000 IU D3 with 100-200 mcg MK-7) are widely recommended for bone health and arterial protection
Related Biomarkers
Frequently Asked Questions
What is the difference between Vitamin K1 and Vitamin K2?
Vitamin K1 (phylloquinone) is found primarily in leafy green vegetables and is the main dietary form in Western diets. It is predominantly used by the liver for blood clotting factor production. Vitamin K2 (menaquinone) is produced by bacteria and found in fermented foods, grass-fed animal products, and aged cheeses. K2 has a longer half-life in the body and is more effective at directing calcium into bones and out of arteries. K2 exists in several subtypes, with MK-4 and MK-7 being the most studied. Many experts consider K2 more important for long-term cardiovascular and bone health, while K1 is critical for acute clotting functions.
Can I take Vitamin K if I am on blood thinners like warfarin?
This requires careful medical supervision. Warfarin works by blocking Vitamin K activity, so changes in your Vitamin K intake — whether through diet or supplements — can significantly alter your INR (clotting time) and make your anticoagulation therapy unpredictable. Dramatically increasing or decreasing Vitamin K while on warfarin can either raise your risk of clotting or bleeding. However, some physicians now recommend keeping Vitamin K intake consistently stable rather than avoiding it, as a steady intake leads to more predictable INR levels. Always consult your prescribing physician before making any changes to Vitamin K intake if you are on anticoagulant medications.
What are the symptoms of Vitamin K deficiency?
Vitamin K deficiency primarily manifests as impaired blood clotting. Symptoms include easy bruising, excessive bleeding from minor cuts or wounds, heavy menstrual periods, blood in urine or stool, and prolonged bleeding after injuries or surgery. In severe cases, spontaneous internal bleeding can occur. In newborns, deficiency can cause a life-threatening condition called Vitamin K Deficiency Bleeding (VKDB), which is why newborns routinely receive a Vitamin K injection at birth. Subclinical deficiency — not severe enough to cause obvious bleeding — may still increase risks of osteoporosis and arterial calcification over time.