What is the origin of K2 vitamin?

December 24, 2025 •

4 min read

While most people associate vitamin K with leafy greens, which provide vitamin K1, the origin of K2 vitamin is significantly different, stemming primarily from bacterial synthesis. This crucial fat-soluble vitamin is largely produced by microbes and found in specific fermented and animal products.

Quick Summary

Vitamin K2 is a collection of compounds called menaquinones that are synthesized by bacteria in both fermented foods and the animal gut, leading to distinct subtypes like MK-4 and MK-7 found in different dietary sources.

Key Points

Bacterial Synthesis: Vitamin K2 is primarily synthesized by bacteria, a key distinction from vitamin K1, which comes from plants.
Fermented Food Production: Fermented foods like natto are major sources of long-chain menaquinones (like MK-7) due to bacterial fermentation.
Animal Tissue Conversion: MK-4, another important subtype of K2, is produced in animal tissues through the conversion of vitamin K1, not by bacteria.
Varied Dietary Sources: K2 is found in different foods depending on the subtype, including fermented products and animal fats like egg yolks and grass-fed butter.
Distinct Functions: The unique origin and structure of K2 mean it has distinct benefits for bone and cardiovascular health compared to K1's primary role in blood clotting.
Complex Family of Vitamins: Vitamin K is not a single compound but a family, with K1 and K2 representing different classes with varied origins and functions.

The Initial Discovery of the 'Koagulationsvitamin'

The story of vitamin K begins in 1929 with Danish scientist Henrik Dam. Investigating cholesterol metabolism in chickens, Dam observed that a fat-free diet caused the birds to develop spontaneous hemorrhages. Supplementing their diet with cholesterol didn't stop the bleeding, leading him to conclude that a different fat-soluble compound was missing. He named this anti-hemorrhagic factor "Koagulationsvitamin" in a German scientific journal, from which the letter 'K' was derived. However, it wasn't until later that the distinct nature of vitamin K1 and vitamin K2 was fully understood.

The Tale of Two Vitamins: K1 vs. K2

It is now known that the term "vitamin K" refers to a family of related fat-soluble compounds. The two most important forms are:

Vitamin K1 (Phylloquinone): This is the form most commonly found in the human diet, making up about 90% of total vitamin K intake. It is synthesized by plants and algae and is abundant in green leafy vegetables like spinach, kale, and broccoli. Its primary function is in hepatic (liver) carboxylation, which is crucial for blood clotting.
Vitamin K2 (Menaquinone): This form is produced by bacteria and found primarily in fermented foods and animal products. Vitamin K2 is further divided into subtypes, known as menaquinones (MK-n), which differ based on the length of their side chain. The most relevant subtypes are MK-4 and MK-7.

The Bacterial and Conversion Origins of K2

The origin of vitamin K2 is a story of microbial activity and animal biology. The two primary pathways are bacterial synthesis and tissue-specific conversion.

Bacterial Synthesis via Fermentation

Many types of bacteria can synthesize menaquinones. This process is most famously exploited in the creation of fermented foods, where specific bacterial strains produce large quantities of long-chain menaquinones. The most notable example is natto, a traditional Japanese food made from fermented soybeans, which is exceptionally rich in MK-7. The bacteria responsible, Bacillus subtilis natto, ferments the soybeans, yielding high levels of MK-7 not present in the unfermented version. Other fermented products, like certain cheeses and sauerkraut, also contain menaquinones synthesized by bacteria during their maturation.

Tissue-Specific Conversion from K1 to MK-4

Unlike the long-chain menaquinones produced by bacteria, MK-4 is primarily created within animal tissues. It is not produced by gut bacteria but rather by a specific process where dietary vitamin K1 is converted into MK-4. This conversion occurs in various animal tissues, including the testes, pancreas, and arterial walls. This explains why MK-4 is found in animal products, with its concentration often influenced by the animal's diet, such as whether chickens are pasture-raised. Consequently, foods like egg yolks, liver, and butter from grass-fed cows are notable sources of MK-4.

Dietary Sources of Vitamin K2

Because of its unique origins, vitamin K2 is found in different types of food compared to vitamin K1. Here is a breakdown of where to find the various menaquinones:

Natto: The richest source of MK-7, with up to 1000 mcg per 100 grams.
Cheeses: Certain hard and soft cheeses, like Gouda, Edam, and Brie, contain moderate amounts of menaquinones like MK-8 and MK-9.
Organ Meats: Beef and chicken liver are excellent sources of MK-4.
Egg Yolks: The yolk, especially from pasture-raised hens, contains MK-4.
Grass-Fed Butter: Butter from grass-fed cows is a good source of MK-4.
Sauerkraut: This fermented cabbage product provides a lesser, but still beneficial, amount of menaquinones.
Kefir: As a fermented dairy drink, kefir offers probiotics and can contain vitamin K2.

Comparison Table: Vitamin K1 vs. K2

To better understand their differences, here is a comparison of vitamin K1 and vitamin K2:


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinone)
Primary Source	Green leafy vegetables	Fermented foods, animal products
Origin	Plant photosynthesis	Bacterial synthesis, tissue conversion
Subtypes	One form	Multiple menaquinone subtypes (MK-4 to MK-13)
Bioavailability	Lower absorption rate	Higher absorption, longer half-life
Key Functions	Primarily blood clotting	Bone and heart health, calcium metabolism

Conclusion: The Unique Origin of K2

Vitamin K2's origins are a fascinating story of microbiology and nutritional science. Unlike the single-source K1, K2 is derived from multiple pathways, either through bacterial fermentation or internal animal conversion. This dual origin leads to different menaquinone subtypes found in distinct food categories. Understanding where K2 comes from is vital for appreciating its specific health benefits, particularly for bone mineralization and cardiovascular health. As research continues to explore the nuances between K1 and K2, the unique journey of menaquinones from microbial synthesis to our dinner plate remains a testament to the complex interplay of diet and physiology.

Learn more about the differences between Vitamin K1 and K2 from Healthline.

Frequently Asked Questions

The primary difference lies in their origin and function. Vitamin K1 comes from plants and is mainly for blood clotting, while vitamin K2 is made by bacteria and primarily benefits bone and cardiovascular health.

Specific bacteria, such as Bacillus subtilis natto used in fermenting soybeans, produce menaquinones like MK-7. Bacteria in the gut microbiome can also produce vitamin K2.

MK-4 is produced in animal tissues (like the pancreas and testes) by converting vitamin K1. It is found in animal products such as egg yolks, liver, and dairy from grass-fed animals.

Natto is a traditional Japanese food made from fermented soybeans. It is an exceptionally rich source of MK-7 because the fermentation process utilizes specific bacteria that synthesize this form of vitamin K2.

While gut bacteria do produce menaquinones, the amount is minimal and not well absorbed. Relying solely on gut production is insufficient for maintaining optimal vitamin K2 status.

Yes, vitamin K1 can be partly converted to MK-4 in animal and human tissues, but this conversion process is considered inefficient for ensuring sufficient K2 levels.

The 'K' comes from the German word for coagulation, 'Koagulation.' The vitamin was named by Danish scientist Henrik Dam in the 1930s after his discovery of its role in blood clotting.