What is the Classification of Vitamin K?

November 19, 2025 •

4 min read

First discovered in 1929, vitamin K was identified as an essential nutrient for blood coagulation. However, the term actually refers to a group of fat-soluble compounds, leading to a specific classification. Understanding what is the classification of vitamin K is key to appreciating its diverse functions throughout the body.

Quick Summary

Vitamin K is a group of fat-soluble compounds classified into natural forms, K1 (phylloquinone) and K2 (menaquinones), and synthetic versions. K1 primarily supports liver functions like blood clotting, while K2 is crucial for bone and cardiovascular health outside the liver.

Key Points

Two Primary Natural Forms: Vitamin K is classified into two main natural forms: phylloquinone (K1) from plants and menaquinones (K2) primarily from bacterial synthesis and fermented foods.
Distinct Functions: Vitamin K1 is crucial for activating blood-clotting proteins in the liver, while vitamin K2 supports extrahepatic tissues, promoting bone mineralization and preventing arterial calcification.
Bioavailability and Half-Life: K2, particularly the MK-7 subtype, has a longer half-life and is more bioavailable than K1, allowing it to have a more systemic effect on extrahepatic tissues.
Synthetic Forms: Menadione (K3) is a synthetic vitamin K form no longer used for human supplementation due to toxicity, although a synthetic K1 (phytonadione) is used in medicine.
Dietary Importance: A diverse diet including leafy greens (K1) and fermented foods or animal products (K2) is recommended to ensure adequate intake of both forms for comprehensive health benefits.
Deficiency Risks: While rare in healthy adults, vitamin K deficiency can cause uncontrolled bleeding and poor bone health. It is a major concern for newborns, who routinely receive a prophylactic dose.

Vitamin K is not a single entity but a family of fat-soluble compounds with a shared chemical structure, 2-methyl-1,4-naphthoquinone. This family is primarily classified into three types: vitamin K1 (phylloquinone), vitamin K2 (menaquinones), and synthetic vitamin K (menadione). These different forms arise from distinct sources and play varied yet critical roles in human physiology.

Vitamin K1: Phylloquinone

Vitamin K1, or phylloquinone, is the most abundant dietary form of vitamin K and is synthesized by plants. Its name is derived from its association with chlorophyll and photosynthesis. This form is found predominantly in green leafy vegetables, where it is tightly bound to plant cells, leading to a relatively lower absorption rate compared to K2.

The primary physiological role of K1 is to act as a cofactor for the enzyme gamma-glutamyl carboxylase in the liver, which is essential for activating blood-clotting proteins. Without sufficient vitamin K1, the liver cannot produce these clotting factors properly, which can lead to excessive bleeding or hemorrhage. Due to its short half-life, K1 is primarily used by the liver for coagulation, with less availability for other tissues. For example, a study showed that less than 10% of K1 from plants is typically absorbed, and it stays in the blood for only a few hours.

Food Sources of Vitamin K1

Good sources of K1 include:

Spinach
Kale
Broccoli
Collard greens
Brussels sprouts
Turnip greens
Vegetable oils

Vitamin K2: Menaquinones

Vitamin K2 represents a class of compounds known as menaquinones (MKs). Unlike K1, menaquinones are primarily synthesized by bacteria. They are found in certain animal products and fermented foods, and are also produced by bacteria in the human gut. K2 has a longer half-life than K1, allowing it to circulate in the blood for a longer period and benefit extrahepatic tissues, such as bones and blood vessels.

Subtypes of Menaquinones

There are several subtypes of K2, designated MK-4 through MK-13 based on the length of their chemical side chain. The two most studied subtypes are MK-4 and MK-7, which have different sources and bioavailabilities. MK-4 is found in animal products like egg yolks and meat and can also be produced from K1 in certain body tissues. MK-7 is primarily found in fermented foods like the Japanese dish natto, where it is produced by Bacillus subtilis bacteria. MK-7 is known for its higher bioavailability and longer half-life, making it particularly effective at reaching extrahepatic tissues.

Key Functions of Vitamin K2

K2's main functions are related to calcium regulation outside of the liver. It activates proteins like osteocalcin, which helps integrate calcium into bone mineral matrix, strengthening bones. It also activates Matrix Gla Protein (MGP), which helps prevent calcium from depositing in soft tissues like arteries, thereby protecting cardiovascular health. Several studies have shown that a higher intake of K2 is linked to a reduced risk of coronary heart disease and osteoporosis.

Food Sources of Vitamin K2

Foods rich in K2 include:

Natto (fermented soybeans)
Hard and soft cheeses
Egg yolks
Organ meats (e.g., chicken liver)
Grass-fed dairy products
Sauerkraut

Synthetic Vitamin K (Menadione)

A synthetic form of vitamin K, called menadione (or vitamin K3), is no longer used in human supplements or fortified foods due to its potential toxicity. In high doses, it was shown to damage hepatic cells. However, a synthetic version of K1 called phytonadione is still used therapeutically in medical settings to treat severe bleeding issues.

Comparison of Vitamin K1 and Vitamin K2


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinones)
Primary Source	Green leafy vegetables and some plant oils	Fermented foods, animal products, and gut bacteria
Bioavailability	Relatively low absorption from plants; short half-life	Higher bioavailability, especially long-chain forms like MK-7; longer half-life
Primary Function	Cofactor for liver-produced blood clotting proteins (e.g., prothrombin)	Activates proteins in extrahepatic tissues for bone mineralization and prevention of arterial calcification
Target Tissues	Primarily the liver	Bones, blood vessels, brain, and pancreas

The Critical Role of Vitamin K in the Body

Beyond its classification, vitamin K's overarching function is to act as a crucial co-factor in the gamma-carboxylation of specific glutamic acid residues in various proteins. This modification allows these proteins to bind calcium ions, a process vital for their function. In addition to blood clotting and bone mineralization, research suggests vitamin K may play a role in cardiovascular health by preventing arterial calcification, cognitive function, and cellular growth regulation. A deficiency in vitamin K, while rare in healthy adults, can lead to serious health problems. It is most concerning in newborns, who are born with very low levels and are given a prophylactic injection to prevent vitamin K deficiency bleeding (VKDB). Malabsorption issues or interactions with blood-thinning medications like warfarin can also cause deficiency in adults. It is important to maintain a consistent dietary intake of vitamin K, especially K1 from leafy greens, which can be converted by the body into some forms of K2. A varied and balanced diet that includes both plant-based K1 and fermented or animal-based K2 is the best approach to ensuring adequate intake. For more in-depth information on vitamin K for health professionals, the Office of Dietary Supplements provides excellent resources on its website.

Conclusion

In summary, the classification of vitamin K reveals a complex family of compounds with specialized functions. Vitamin K1 (phylloquinone), from plants, is essential for blood clotting in the liver, while vitamin K2 (menaquinones), from bacteria and fermented foods, plays a more systemic role in promoting bone and cardiovascular health. The synthetic K3 form is no longer used in human supplements due to toxicity. While both forms are crucial, their differing sources, half-lives, and primary functions emphasize the importance of a varied diet to support overall health and prevent deficiency-related complications like poor bone development and excessive bleeding.

Frequently Asked Questions

The main difference is their source and primary function. Vitamin K1, found in leafy green vegetables, primarily functions in the liver for blood clotting. Vitamin K2, found in fermented and animal foods, acts systemically to support bone and cardiovascular health.

Vitamin K2, particularly the long-chain menaquinones like MK-7, is considered more effective for bone health. It helps activate osteocalcin, a protein that binds calcium to the bone matrix, promoting stronger bones.

Newborns are given a vitamin K shot, typically a synthetic K1 (phytonadione), shortly after birth to prevent vitamin K deficiency bleeding (VKDB). This is necessary because infants have low vitamin K stores at birth and low amounts in breast milk.

Yes. A synthetic form called menadione (K3) is toxic and no longer used in human supplements. However, a synthetic version of K1, phytonadione, is used in medical treatments.

Yes, the body can convert some dietary vitamin K1 into vitamin K2, specifically the MK-4 subtype. However, this conversion process is generally considered inefficient, so direct dietary intake of K2 is still important.

The best food sources for vitamin K2 include fermented foods like natto and sauerkraut, as well as animal products such as egg yolks, hard and soft cheeses, and liver.

For individuals taking blood-thinning medications like warfarin, maintaining a consistent daily intake of vitamin K is crucial. Sudden changes in vitamin K consumption can interfere with the medication's effectiveness, potentially leading to dangerous bleeding or blood clots.

Vitamin K deficiency is rare in healthy adults who consume a balanced diet. It is more common in newborns, individuals with malabsorption disorders, or those on certain medications like long-term antibiotics.