Understanding the Connection: Is Vitamin K Synthesized by the GI Flora?

December 4, 2025 •

3 min read

The human gastrointestinal tract is colonized by a vast array of microorganisms, with up to $10^{11}$ bacteria per gram of intestinal content. This complex ecosystem, known as the gut microbiome, is not only crucial for digestion but also plays a role in whether and how is vitamin K synthesized by the GI flora.

Quick Summary

The gut microbiome contributes to the body's vitamin K supply by synthesizing menaquinone (K2), though dietary phylloquinone (K1) is the primary source. Several factors, including antibiotics, diet, and malabsorption, can influence this synthesis and overall vitamin K status.

Key Points

Gut bacteria produce Vitamin K2: Intestinal microbes, such as Bacteroides and E. coli, synthesize menaquinone (K2) in the large intestine.
K2 synthesis is a secondary source: The body relies more heavily on dietary intake of Vitamin K1 from leafy greens, as absorption of bacterially produced K2 is less efficient.
Antibiotics can cause deficiency: Prolonged use of broad-spectrum antibiotics can wipe out vitamin K-producing gut bacteria, leading to potential deficiency and impaired blood clotting.
Newborns are at risk: Due to an underdeveloped gut flora, infants have low vitamin K levels and receive a prophylactic injection to prevent dangerous bleeding.
Dietary K1 and K2 have different roles: K1 is critical for blood clotting in the liver, while K2 supports bone and cardiovascular health more effectively due to higher bioavailability.
Gut health is critical for absorption: Conditions causing malabsorption or bile salt deficiencies can significantly impair the uptake of all fat-soluble vitamins, including both dietary and gut-synthesized vitamin K.

The Different Forms of Vitamin K

Vitamin K is a fat-soluble vitamin crucial for several physiological processes, including blood clotting and bone health. It exists in two primary forms found naturally:

Vitamin K1 (Phylloquinone): This form is primarily obtained from plant-based foods, especially dark green leafy vegetables like kale, spinach, and broccoli.
Vitamin K2 (Menaquinone): This form has several subtypes (MK-4 to MK-13) and is mainly produced by bacteria, both in fermented foods like natto and within the human gut.

The Gut's Role in Vitamin K Synthesis

Yes, certain bacteria within the GI flora are capable of synthesizing vitamin K. Specifically, they produce menaquinones (vitamin K2) through specific enzymatic pathways. While this production is a notable function of the gut microbiome, its overall contribution to the body's vitamin K status is often debated and considered a secondary source to dietary intake. This is because the majority of menaquinone synthesis occurs in the colon, while the primary site of absorption is the small intestine. Additionally, factors like the composition of a person's gut microbiota and bile acid availability can influence how much of this bacterially produced vitamin K is actually absorbed and utilized by the body.

Bacteria Responsible for K2 Production

Several bacterial species within the human gut are known to be involved in the synthesis of menaquinones, including:

Bacteroides: Major producers of longer-chain menaquinones like MK-10 and MK-11. Studies have shown that a reduced abundance of Bacteroides is associated with poor vitamin K status.
Eubacterium lentum: Synthesizes MK-6.
Veillonella: Primarily produces MK-7.
Escherichia coli: A facultative anaerobe that can produce MK-8 during anaerobic respiration.

Factors Influencing Vitamin K Production in the Gut

Several factors can affect the delicate balance of gut flora and its ability to synthesize vitamin K:

Antibiotics: Prolonged use of broad-spectrum antibiotics can deplete the normal gut flora, including the bacteria responsible for producing menaquinones, potentially leading to vitamin K deficiency. This is a well-established cause of impaired blood clotting in susceptible individuals.
Diet: Dietary fiber, which gut bacteria ferment, can impact the overall health and composition of the microbiome, indirectly influencing menaquinone synthesis. Fermented foods containing specific strains of bacteria, like natto and some cheeses, also provide a direct dietary source of K2.
Malabsorption: Conditions that impair fat absorption, such as celiac disease, cystic fibrosis, or liver and pancreatic diseases affecting bile salts, can significantly reduce the absorption of all fat-soluble vitamins, including both K1 and K2.
Infant Immaturity: Newborn infants have an underdeveloped gut flora and low baseline levels of vitamin K, putting them at risk for vitamin K deficiency bleeding (VKDB). This is why all newborns are given a prophylactic vitamin K injection at birth.

Comparison of Vitamin K1 and K2

While both K1 and K2 are essential forms of vitamin K, they differ in their origin, function, and bioavailability. The following table outlines some key distinctions:


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinone)
Primary Source	Green leafy vegetables	Gut bacteria, fermented foods (e.g., natto), and some animal products
Subtypes	One primary form	Multiple subtypes (MK-4 to MK-13) with varying chain lengths
Production	Synthesized by plants for photosynthesis	Synthesized by bacteria, as well as tissue-specific conversion from K1 to MK-4 in animals
Absorption Site	Primarily absorbed in the small intestine	Primarily produced in the colon, but absorption is less efficient and variable
Bioavailability	Lower bioavailability from plant sources due to strong binding in chloroplasts	Higher bioavailability, especially MK-7, which has a longer half-life and accumulates in the blood longer than MK-4
Key Functions	Essential for blood clotting factor activation in the liver	Important for bone health (activating osteocalcin) and cardiovascular health (inhibiting vascular calcification)

Conclusion: The Collaborative Vitamin K Supply

The synthesis of vitamin K by the GI flora, primarily menaquinones (K2), is a valuable and natural contribution to the body's overall vitamin K status. However, it is not a sufficient source on its own and should be viewed as a complement to dietary intake, especially vitamin K1 from leafy greens. Factors that disrupt the microbiome, such as antibiotic use or malabsorptive diseases, can compromise this internal production, potentially increasing the risk of deficiency. Therefore, a diverse diet rich in both plant-based K1 and fermented or animal-sourced K2 is crucial for maintaining adequate vitamin K levels, supported by a healthy and balanced gut ecosystem. For further health information, consult the National Institutes of Health (NIH) fact sheet on Vitamin K.

Frequently Asked Questions

Gut bacteria primarily synthesize menaquinones, which are also known as Vitamin K2.

No, the vitamin K produced by your gut is generally not enough to meet all your needs. It acts as a complement to the vitamin K you get from your diet, especially from leafy greens and fermented foods.

Prolonged use of broad-spectrum antibiotics can disrupt the natural balance of gut bacteria, destroying the flora that produce vitamin K2 and potentially causing a deficiency.

Newborns are born with very low levels of vitamin K and have an immature gut flora, putting them at high risk for vitamin K deficiency bleeding. The shot provides immediate protection.

Vitamin K1 (phylloquinone) comes from plants, particularly green leafy vegetables. Vitamin K2 (menaquinone) is produced by gut bacteria and found in fermented foods like natto, as well as some animal products.

Yes, vitamin K is an antagonist to blood thinners like warfarin. Changes in gut flora from antibiotics or diet can affect vitamin K levels, which in turn can alter the effectiveness of the medication.

Several types of bacteria produce menaquinone, with Bacteroides and E. coli being notable examples. Different species produce menaquinones with different chain lengths.