What is vitamin K used for in your body?

December 23, 2025 •

4 min read

Over 20 years ago, it was discovered that chicks on a diet lacking a specific nutrient were prone to bleeding. That nutrient, eventually named vitamin K, is now known to be essential for blood coagulation, bone health, and more. Understanding what is vitamin K used for in your body is key to appreciating its vital role in maintaining overall health and preventing serious conditions like excessive bleeding.

Quick Summary

Vitamin K is a fat-soluble vitamin crucial for producing proteins essential for blood clotting and regulating calcium binding in bones. It also supports cardiovascular health by preventing arterial calcification.

Key Points

Blood Clotting: Vitamin K is a vital cofactor that activates liver-produced proteins, such as prothrombin, which are essential for the blood coagulation process.
Bone Health: It activates osteocalcin, a protein that binds calcium to the bone matrix, promoting strong bones and potentially reducing fracture risk.
Cardiovascular Protection: Specifically, Vitamin K2 activates Matrix Gla Protein (MGP), which helps prevent arterial calcification and supports cardiovascular health.
Two Primary Forms: The main natural forms are Vitamin K1 (phylloquinone), found in plants, and Vitamin K2 (menaquinone), found in animal and fermented foods.
Newborn Necessity: A vitamin K injection is routinely given to newborns to prevent dangerous bleeding, as they are born with very low levels of this vitamin.
Deficiency Consequences: Severe deficiency can lead to uncontrolled bleeding and bruising, while marginal deficiency may compromise bone and heart health over time.

The Core Function: Blood Clotting

The most recognized function of vitamin K is its pivotal role in blood clotting, or coagulation. This process is a cascade of events that ultimately stops bleeding and promotes healing. For this to happen, the body needs to activate specific proteins, known as clotting factors, that are produced in the liver.

Vitamin K acts as a vital cofactor for an enzyme called gamma-glutamyl carboxylase. This enzyme modifies certain proteins, such as prothrombin (Factor II), Factor VII, Factor IX, and Factor X, by adding carboxylic acid groups to specific glutamate residues. This carboxylation allows the clotting factors to bind to calcium ions, which is necessary for them to function effectively in the clotting cascade. Without sufficient vitamin K, these proteins are produced in an inactive state, which can lead to uncontrolled bleeding and hemorrhage.

This is why vitamin K is routinely given to newborns by injection shortly after birth. Babies have very low vitamin K reserves because it doesn't cross the placenta easily, and breast milk is not a rich source. This practice helps prevent a dangerous bleeding disorder known as Vitamin K Deficiency Bleeding (VKDB).

More Than Just Clotting: Bone and Heart Health

While blood clotting is its most famous use, research has uncovered that vitamin K is also critical for bone metabolism and cardiovascular health, acting beyond the liver.

Bone Health

Vitamin K is essential for activating osteocalcin, a protein produced by osteoblasts that plays a crucial role in bone mineralization. Active osteocalcin binds calcium to the bone matrix, promoting strong and healthy bones. Low vitamin K status has been associated with lower bone mineral density and an increased risk of fractures in some populations. Studies have also shown that vitamin K can work synergistically with vitamin D to improve bone health. Japanese studies, in particular, have shown that high doses of the K2 form, menaquinone-4 (MK-4), can increase bone mineral density and reduce fracture rates in people with osteoporosis.

Cardiovascular Health

Cardiovascular health is another area where vitamin K proves its worth, particularly the menaquinone (K2) form. Vitamin K2 activates Matrix Gla Protein (MGP), an inhibitor of vascular calcification. Arterial calcification, or the hardening of the arteries due to calcium buildup, is a major risk factor for heart disease. Active MGP prevents calcium from depositing in soft tissues like the arteries, helping to maintain arterial elasticity and preventing cardiovascular complications. Research suggests that higher dietary intake of vitamin K, especially K2, may be associated with a reduced risk of coronary artery disease.

Vitamin K1 vs. Vitamin K2

Vitamin K is a family of fat-soluble compounds with two primary natural forms: K1 (phylloquinone) and K2 (menaquinone). They differ in their sources, absorption, and primary functions.

Comparison Table: Vitamin K1 vs. Vitamin K2


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinone)
Primary Sources	Green leafy vegetables like kale, spinach, and broccoli.	Animal products (meat, eggs, cheese) and fermented foods like nattō.
Produced By	Plants (photosynthesis).	Gut bacteria, or converted in animal tissues from K1.
Main Function	Primarily involved in activating blood-clotting factors in the liver.	Acts in extrahepatic tissues, influencing bone and vascular health.
Storage	Less efficiently stored and has a shorter half-life in the body.	Longer half-life and more bioavailable to extrahepatic tissues, particularly MK-7.

Deficiency and Dietary Intake

Clinical vitamin K deficiency is rare in healthy adults but can occur in individuals with fat malabsorption disorders, liver disease, or those taking medications that interfere with vitamin K metabolism. Long-term antibiotic use can also deplete the beneficial gut bacteria that produce K2. Symptoms include easy bruising, excessive bleeding from wounds, and bleeding from the nose or gums. In severe cases, it can be life-threatening.

Ensuring an adequate daily intake of vitamin K is crucial. For healthy adults, the recommended Adequate Intake (AI) is 120 micrograms per day for men and 90 micrograms per day for women. A balanced diet rich in green leafy vegetables, healthy oils, and fermented foods is usually sufficient for most individuals to meet their needs. However, those on anticoagulant medications like warfarin must maintain a consistent intake to avoid drug interactions.

Conclusion

In summary, vitamin K is a multifunctional nutrient whose uses extend far beyond its well-known role in blood clotting. By acting as a key cofactor, it enables the activation of proteins crucial for coagulation, bone mineralization, and the prevention of vascular calcification. From ensuring a newborn’s safety to contributing to long-term cardiovascular and skeletal health, what is vitamin K used for in your body is a testament to its broad and vital biological importance. Maintaining an adequate intake through a varied diet is a cornerstone of overall health and wellness.

Visit the NIH Office of Dietary Supplements for more detailed information on vitamin K.

Frequently Asked Questions

A vitamin K deficiency impairs the body's ability to produce necessary clotting factors, which can lead to excessive bleeding and easy bruising. In newborns, it can cause a life-threatening condition known as Vitamin K Deficiency Bleeding (VKDB). In adults, it is also linked to lower bone density and increased fracture risk.

Yes, the two main natural forms are Vitamin K1 (phylloquinone), primarily from green leafy vegetables, and Vitamin K2 (menaquinone), found in animal products and fermented foods. These forms are absorbed and used by the body differently, with K2 playing a more significant role in extrahepatic tissues like bones and arteries.

Foods rich in Vitamin K1 include leafy greens such as kale, spinach, and broccoli. Good sources of Vitamin K2 include fermented foods like nattō, as well as meat, eggs, and cheese.

Infants are at high risk due to poor placental transfer and low levels in breast milk. Adults with fat malabsorption disorders (like celiac disease or cystic fibrosis), liver disease, severe malnutrition, or those on long-term antibiotic therapy are also at increased risk.

Vitamin K is essential for activating osteocalcin, a protein that directs calcium to bind with the bone matrix, thereby promoting bone mineralization and strength. Without enough vitamin K, this process is compromised, which can lead to weakened bones.

Yes, particularly Vitamin K2, which activates Matrix Gla Protein (MGP). MGP is a potent inhibitor of arterial calcification, the process where calcium deposits harden the arteries. This helps maintain arterial elasticity and reduces the risk of cardiovascular disease.

Excessive intake of natural vitamin K1 and K2 is generally not associated with toxicity and no upper limit has been established. However, the synthetic form, Vitamin K3, can be toxic and is not used in supplements for humans.