Vitamin K is not a single compound but a family of fat-soluble vitamins, primarily consisting of K1 (phylloquinone) and K2 (menaquinones). While both are essential for health, they differ significantly in their dietary sources, absorption, and bioavailability within the body. For a nutritional strategy that goes beyond basic blood coagulation and aims for optimal bone and heart health, understanding these differences is key.
The Two Main Forms of Vitamin K
Vitamin K1 (Phylloquinone): This is the most common form of vitamin K found in the diet and is produced by plants. Its richest sources are dark, leafy green vegetables like kale, spinach, and broccoli. In the body, K1 is primarily used by the liver to activate proteins involved in blood clotting.
Vitamin K2 (Menaquinones): K2 is found in fermented foods, some animal products, and is also produced by bacteria in the gut. Menaquinones are a family of compounds with varying side-chain lengths, designated MK-4 through MK-13. The most studied forms are MK-4 and MK-7, which have important roles in activating proteins in extra-hepatic tissues like bones and blood vessels.
How Absorption Differs Between K1 and K2
The most significant factor affecting absorption is the chemical form of the vitamin and its source. Vitamin K1 is tightly bound within the chloroplasts of plant leaves, which severely limits its bioavailability. The body may absorb as little as 10% of the K1 from plant sources unless consumed with a healthy fat. In contrast, the menaquinone forms of K2 are more easily absorbed.
Specifically, MK-7 stands out due to its superior bioavailability and longer half-life compared to other forms of vitamin K. Studies have shown that after ingestion, postprandial serum concentrations of MK-7 can be 10 times higher than those of K1. The longer half-life of MK-7 allows it to remain in circulation for days, making it more available to extra-hepatic tissues like bone and arterial walls where it is needed to activate calcium-regulating proteins. MK-4 has a very short half-life in the bloodstream and is poorly absorbed from supplements at nutritional doses, though the body can convert K1 into MK-4 within tissues.
Comparison Table: Vitamin K Absorption & Bioavailability
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (MK-4) | Vitamin K2 (MK-7) |
|---|---|---|---|
| Primary Source | Leafy green vegetables, plant oils | Converted from K1 in tissues; animal foods | Fermented foods, some cheeses |
| Absorption Rate | Low (≤10%) from plant foods, higher from oil | Low from nutritional supplements | High, especially from fermented sources |
| Bioavailability | Poor from plants, improves with fat | Poor, short half-life | High and sustained due to long half-life |
| Half-Life in Blood | Rapidly cleared (hours); high turnover | Very short (hours), similar to K1 | Very long (days), circulates longer |
| Main Function Site | Primarily liver for blood clotting | Extra-hepatic tissues after conversion | Extra-hepatic tissues (bone, arteries) |
Maximizing Your Vitamin K Intake
To ensure you are getting adequate and well-absorbed vitamin K, consider these strategies:
- Pair K1 with Fat: Since vitamin K is fat-soluble, consuming K1-rich vegetables with a healthy fat source, like olive oil in a salad or avocado, can significantly boost absorption.
- Prioritize K2-Rich Foods: Incorporate fermented foods like natto and hard cheeses, as well as eggs and meat, into your diet for higher and more bioavailable K2 intake.
- Supplement with MK-7: For those concerned about intake or with malabsorption issues, MK-7 supplements are a reliable option due to their superior bioavailability and long half-life.
Excellent Dietary Sources for Both Types of Vitamin K
- Excellent K1 Sources: Kale, spinach, collard greens, broccoli, Brussels sprouts, and soybean oil are rich in phylloquinone.
- Excellent K2 (MK-7) Sources: Natto (fermented soybeans) is the highest source, while hard and soft cheeses (e.g., Gouda, Brie) are also valuable.
- Good K2 (MK-4) Sources: Animal products like eggs, chicken, and beef liver are decent sources of MK-4.
Factors Affecting Vitamin K Absorption
Several factors can influence how well your body absorbs vitamin K:
- Bile and Pancreatic Health: The absorption of fat-soluble vitamins, including vitamin K, depends on proper bile and pancreatic function. Disorders like celiac disease, cystic fibrosis, or blocked bile ducts can impair absorption and lead to deficiency.
- Antibiotics: Long-term use of broad-spectrum antibiotics can disrupt the gut bacteria responsible for producing K2, impacting overall levels.
- Medications: Blood-thinning medications like warfarin interfere with the vitamin K cycle. Individuals on these medications must maintain a consistent vitamin K intake under medical supervision.
- Food Matrix: As mentioned, the form in which vitamin K is consumed (e.g., raw vegetable vs. oil) significantly impacts absorption.
Conclusion
When considering which vitamin K is most absorbable, the evidence clearly points to Vitamin K2, particularly the menaquinone-7 (MK-7) subtype. While K1 is crucial for blood clotting and is abundant in green vegetables, its absorption is less efficient and its half-life shorter. MK-7's high bioavailability and sustained presence in the bloodstream allow it to benefit extra-hepatic tissues like bone and arteries, promoting long-term cardiovascular and skeletal health. To maximize your intake, focus on a varied diet that includes both K1-rich leafy greens (consumed with fat) and K2-rich fermented foods and animal products. For those with absorption concerns, MK-7 supplementation can be an effective strategy, but always consult with a healthcare professional before starting any new supplement regimen.
For more in-depth information on vitamin K and other nutrients, a trusted resource is the NIH Office of Dietary Supplements.
