Which form of vitamin K is most bioavailable?

December 17, 2025 •

4 min read

According to scientific studies, the bioavailability of vitamin K varies significantly between its forms, impacting its effectiveness throughout the body. The critical question for many is: which form of vitamin K is most bioavailable? Emerging research indicates that Vitamin K2, specifically the menaquinone-7 (MK-7) subtype, is absorbed more efficiently and remains active longer than other forms.

Quick Summary

This article explains the differences in bioavailability between Vitamin K1, Vitamin K2 (MK-4), and Vitamin K2 (MK-7). It details how MK-7's longer half-life and better absorption make it the superior choice for delivering benefits to extrahepatic tissues like bones and arteries, crucial for optimal health.

Key Points

MK-7 is the most bioavailable form: Vitamin K2 as menaquinone-7 (MK-7) has the highest bioavailability and the longest half-life in the bloodstream compared to Vitamin K1 and other K2 forms like MK-4.
K1 is poorly absorbed from plants: Phylloquinone (K1) found in leafy greens is often poorly absorbed by the body, although eating it with fat can increase absorption.
MK-7 reaches extrahepatic tissues: The long half-life of MK-7 allows it to be more effectively distributed to tissues outside the liver, such as bones and arteries.
MK-4 is poorly absorbed at nutritional doses: Nutritional doses of supplemental MK-4 are not well-absorbed and do not effectively raise serum levels, unlike MK-7.
Source and supplement quality matter: Bioavailability depends on the source, with supplemental MK-7 generally being better absorbed than dietary K1. High-quality supplements use highly bioavailable, all-trans MK-7 isomers.
Both K1 and K2 are important: K1 is critical for liver function and blood clotting, while K2 (specifically MK-7) offers broader systemic benefits for bone and cardiovascular health.

Understanding Vitamin K and Bioavailability

Vitamin K is a fat-soluble nutrient essential for activating proteins involved in blood clotting, bone metabolism, and cardiovascular health. It is not a single compound but a family of similar molecules, with the two most prominent forms being Vitamin K1 (phylloquinone) and Vitamin K2 (menaquinones). Bioavailability refers to the proportion of a nutrient that is absorbed and utilized by the body after consumption. The bioavailability of vitamin K varies widely depending on its specific form and dietary source, a crucial distinction for anyone looking to optimize their intake through food or supplements.

The Major Forms of Vitamin K: K1 and K2

Vitamin K1 (Phylloquinone): Primarily found in leafy green vegetables like kale, spinach, and broccoli, Vitamin K1 is the most common form consumed in a typical Western diet. However, the phylloquinone in plants is tightly bound within the plant's chloroplasts, leading to poor absorption. Studies have shown that less than 10% of K1 from plant sources may be absorbed, although absorption can improve when consumed with dietary fats. Once absorbed, K1 is rapidly cleared from the bloodstream and preferentially used by the liver for blood clotting factors.

Vitamin K2 (Menaquinones): The menaquinones (MKs) comprise a series of compounds produced by bacteria, found in fermented foods, animal products, and through conversion from K1 in the body. The two most researched subtypes for supplementation are MK-4 and MK-7, which exhibit different bioavailability profiles.

MK-4 (Menaquinone-4): Found in small amounts in animal products like meat and dairy, MK-4 has a very short half-life in the bloodstream, similar to K1. Research has shown that nutritional doses of supplemental MK-4 are poorly absorbed and do not significantly increase circulating serum levels. While the body can convert other forms of Vitamin K to MK-4 in tissues, directly consuming supplemental MK-4 is not an efficient way to boost systemic K2 status.
MK-7 (Menaquinone-7): Found in fermented foods like the Japanese dish natto, MK-7 stands out for its superior bioavailability and stability. It is well-absorbed and has a significantly longer half-life, lasting for days in the bloodstream compared to just hours for K1 and MK-4. This prolonged circulation allows MK-7 to be effectively distributed to extrahepatic tissues like bones and arteries, where it activates important proteins for bone mineralization and the prevention of vascular calcification.

Comparison of Vitamin K Bioavailability and Efficacy


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (MK-4)	Vitamin K2 (MK-7)
Primary Sources	Leafy greens, vegetable oils	Animal products (small amounts)	Fermented foods, natto
Dietary Absorption	Poor from plant sources; improves with fat	Poor at nutritional doses	Excellent, especially from supplements
Half-Life	Short (1-2 hours)	Short (approx. 2 hours)	Long (up to 72 hours)
Distribution	Primarily to the liver for blood clotting	Short-lived in circulation; converted from other forms in tissues	Wide distribution to extrahepatic tissues (bones, arteries)
Effective Dosage	Effective for clotting at lower doses	Requires very high pharmacological doses (mg range) for effects	Effective at lower nutritional doses (mcg range)
Health Benefits	Supports blood clotting	Supports tissue-specific functions (often supplied via conversion)	Supports bone density and cardiovascular health

Why MK-7 is Considered Most Bioavailable

Several factors contribute to MK-7's superior bioavailability and effectiveness. The prolonged half-life is a key advantage, allowing for a more consistent and stable presence in the blood. This long circulation time ensures that MK-7 is available to activate proteins in extrahepatic tissues, which K1 and nutritional doses of MK-4 cannot effectively reach. In fact, some studies suggest that MK-7 is a better supplier of MK-4 to these tissues than supplementing with MK-4 itself.

Furthermore, the form of vitamin K in dietary supplements matters. While MK-7 from natto is highly bioavailable, supplements containing high-quality, all-trans MK-7 isomers mimic this effectiveness. In contrast, dietary K1 is bound to plant matter, which significantly hinders its absorption. While K1 effectively supports liver function, its poor absorption makes it less reliable for other systemic benefits compared to MK-7, which has shown more potent effects on extrahepatic markers like osteocalcin carboxylation.

How to Choose the Right Vitamin K Form

For optimal systemic benefits, particularly for bone and heart health, supplemental MK-7 is often the most effective choice due to its high bioavailability and long half-life. However, this does not diminish the importance of Vitamin K1. A balanced approach involves consuming a variety of vitamin K sources. This ensures the body receives both the liver-focused benefits of K1 from green vegetables and the broader systemic effects of K2 from fermented foods or a quality supplement.

Conclusion

While Vitamin K1 is vital for blood clotting and readily available in vegetables, its poor absorption and short half-life limit its systemic reach. In contrast, Vitamin K2 as menaquinone-7 (MK-7) offers the highest bioavailability due to its efficient absorption and significantly longer presence in the bloodstream. This prolonged circulation allows MK-7 to effectively activate proteins in extrahepatic tissues, supporting bone density and cardiovascular health more robustly than other forms. Ultimately, a combination of dietary sources and, if needed, a high-quality MK-7 supplement represents the best strategy for ensuring optimal vitamin K status for whole-body health.

Disclaimer: The information in this article is for informational purposes only and does not constitute medical advice. Consult with a healthcare professional before starting any new supplement regimen.

For further reading on the different types of vitamin K and their roles, see this comprehensive review from the National Institutes of Health: Vitamin K: Double Bonds beyond Coagulation Insights into Differences between Vitamin K1 and K2 in Health and Disease.

Frequently Asked Questions

Vitamin K1 (phylloquinone) is found primarily in leafy green vegetables and is crucial for blood clotting in the liver. Vitamin K2 (menaquinone) is a group of compounds found in fermented foods and animal products, with subtypes like MK-4 and MK-7 known for supporting bone and cardiovascular health.

Vitamin K2, particularly the MK-7 subtype, is significantly more bioavailable than Vitamin K1. MK-7 is absorbed more efficiently and remains in the bloodstream for days, while K1 is poorly absorbed from plant sources and has a very short half-life.

MK-7 is often recommended for bone health due to its long half-life and superior distribution to extrahepatic tissues like bones. It efficiently activates osteocalcin, a protein that helps bind calcium into the bone matrix.

While it is possible to get adequate Vitamin K from food, especially if you consume a diet rich in both leafy greens (K1) and fermented foods or animal products (K2), many Western diets are lower in K2. The poor bioavailability of K1 from plants also makes it less reliable for systemic effects, making supplements a consideration.

The long half-life of MK-7 (up to 72 hours) allows it to maintain consistent and stable blood levels. This sustained presence ensures a steady supply of vitamin K for the activation of proteins in extrahepatic tissues, such as those crucial for bone and cardiovascular health.

The best food source of highly bioavailable vitamin K is natto, a Japanese fermented soybean dish, which contains high levels of MK-7. Other sources of K2 include hard cheeses and sauerkraut, though they contain varying amounts.

Yes, many dietary supplements on the market combine Vitamin K1 and Vitamin K2 (often as MK-7 or MK-4). Combining both forms may offer complementary benefits, leveraging K1 for liver function and K2 for broader systemic health.