Understanding the Source and Function of Vitamin K

December 27, 2025 •

3 min read

Vitamin K is a vital fat-soluble vitamin discovered in 1929 and named for its role in blood coagulation. This essential nutrient is naturally found in two main forms, K1 and K2, each with distinct dietary sources and functions for overall health.

Quick Summary

This article details the primary dietary sources of vitamin K1 and K2, explains its crucial role in blood clotting and bone mineralization, and distinguishes between its different forms.

Key Points

Two Main Forms: Vitamin K exists as K1 (phylloquinone) from plants and K2 (menaquinone) from animal and fermented foods.
Blood Coagulation: Its most critical function is activating proteins in the liver that are essential for blood coagulation, preventing excessive bleeding.
Bone Health: Vitamin K is necessary for the mineralization of bones by activating osteocalcin, a protein that helps bind calcium.
Heart Protection: Vitamin K, especially K2, helps prevent arterial calcification by activating Matrix Gla Protein (MGP).
Dietary Diversity is Key: Different sources provide different forms of the vitamin, so a balanced diet including both leafy greens and animal products is important.
Medication Interactions: Individuals on blood-thinning medication, such as warfarin, must maintain a consistent intake of vitamin K, as sudden changes can interfere with the drug's efficacy.

What is Vitamin K?

Vitamin K is a group of fat-soluble vitamins essential for producing proteins crucial for several physiological processes, notably blood clotting. The body rapidly metabolizes vitamin K, storing only small amounts, primarily in the liver. This highlights the need for consistent dietary intake. The vitamin exists in two natural forms: K1 (phylloquinone), mainly from plants, and K2 (menaquinone), found in animal products and produced by bacteria.

The Diverse Sources of Vitamin K

Obtaining sufficient amounts of both K1 and K2 requires a varied diet due to their different sources.

Dietary Sources of Vitamin K1 (Phylloquinone)

K1 is the most common form in diets and is synthesized by plants. Key sources include:

Dark green leafy vegetables: Excellent sources include kale, spinach, collard, and turnip greens.
Other vegetables: Broccoli, Brussels sprouts, cabbage, and asparagus also provide K1.
Vegetable Oils: Soybean and canola oils contain some K1.

Dietary Sources of Vitamin K2 (Menaquinone)

K2 primarily comes from animal foods, fermented products, and gut bacteria. Sources include:

Fermented foods: Natto is a very rich source of K2.
Dairy: Cheeses and butter provide varying levels of menaquinones.
Meats: Liver, beef, pork, and chicken are good sources.
Eggs: Egg yolks contain K2.
Gut Bacteria: Bacteria in the large intestine synthesize and provide some K2.

The Critical Functions of Vitamin K

Vitamin K is a crucial cofactor for proteins (Gla-proteins) that require carboxylation to bind calcium, enabling their biological functions.

Blood Coagulation

Vitamin K is essential for the liver to produce blood clotting factors like prothrombin and factors VII, IX, and X. Deficiency leads to inactive factors and increased bleeding risk.

Bone Health

Vital for bone health, vitamin K is needed to activate osteocalcin, a protein that incorporates calcium into bone matrix, promoting strength. Low vitamin K is linked to lower bone density and higher fracture risk.

Cardiovascular Health

Research suggests vitamin K helps prevent arterial calcification by activating Matrix Gla Protein (MGP). Insufficient K leads to undercarboxylated MGP, potentially hardening arteries and increasing heart disease risk.

Vitamin K1 vs. Vitamin K2: A comparison

Though both are vital, K1 and K2 differ in sources and how the body utilizes them.


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinone)
Primary Sources	Green leafy vegetables, vegetable oils	Animal products, fermented foods, gut bacteria
Absorption	Tightly bound in plant cell membranes; requires dietary fat	Better absorbed, especially long-chain MK-7
Distribution	Primarily taken up by the liver	Distributes more widely to bones, kidneys, and blood vessels
Function Focus	Mostly supports liver blood clotting factors	Significant role in bone and cardiovascular health outside the liver

Deficiency and Risk Factors

Deficiency is uncommon in healthy adults with balanced diets. Risks increase with malabsorption disorders (like Crohn's), cystic fibrosis, celiac disease, or liver disease. Long-term antibiotic use may reduce K2 production by gut bacteria. Those on warfarin must maintain consistent vitamin K intake to avoid interfering with the medication.

Conclusion

Vitamin K, including K1 and K2, is a crucial fat-soluble vitamin necessary for blood clotting and bone mineralization. Sources vary, with K1 in leafy greens and K2 in animal and fermented foods. A balanced diet is key for adequate intake to support proper healing, bone strength, and heart health. The NIH Fact Sheet on Vitamin K offers more details.

Frequently Asked Questions

The main function of vitamin K is to activate proteins responsible for blood clotting and to promote proper bone mineralization.

The best sources of vitamin K1 are dark green leafy vegetables, such as kale, spinach, and collard greens, as well as vegetables like broccoli and Brussels sprouts.

Vitamin K2 is found in animal products like meat, eggs, and cheese, and in fermented foods like the Japanese dish natto. Gut bacteria also produce K2.

Vitamin K is important for bone health because it activates osteocalcin, a protein that helps incorporate calcium into your bones to maintain their strength and density.

While gut bacteria do produce some vitamin K2, the amount is generally considered insufficient to meet all the body's needs, making a dietary intake essential.

Newborns and individuals with fat malabsorption conditions (like celiac or Crohn's disease) or liver disease are most at risk for vitamin K deficiency.

For individuals taking warfarin, a blood thinner, inconsistent vitamin K intake can be dangerous. It is important to maintain a stable daily intake to avoid interfering with the medication's effectiveness.