An Introduction to Vitamin K2
Vitamin K is a fat-soluble vitamin crucial for blood clotting and regulating calcium distribution in the body. There are two main types: Vitamin K1 (phylloquinone), found in leafy greens, and Vitamin K2 (menaquinone), found in fermented foods and animal products. Vitamin K2 is further divided into subtypes, or menaquinones (MKs), numbered by the length of their side chain. The two most studied forms are MK-4 and MK-7, which differ significantly in absorption, half-life, and efficacy.
The Critical Difference: MK-4 vs. MK-7
The debate over the best form of vitamin K2 centers on MK-4 and MK-7. While both activate key proteins like osteocalcin (for bone health) and matrix Gla protein (MGP) (for preventing arterial calcification), their effectiveness in supplement form varies dramatically due to differences in bioavailability and half-life.
Menaquinone-4 (MK-4)
MK-4 is a short-chain menaquinone found in animal products like egg yolks and meat. It has a very short half-life in the bloodstream, often clearing out within 6-8 hours. Due to its poor bioavailability at nutritional doses, supplemental MK-4 does not effectively raise serum vitamin K2 levels. Clinical studies have shown that to see a therapeutic effect on bone health, milligram doses (as high as 45 mg daily) are required, which is significantly higher than typically found in supplements and needs to be taken multiple times per day. While the body can produce some MK-4 from other forms of vitamin K, relying on supplemental MK-4 for extra-hepatic benefits is not effective.
Menaquinone-7 (MK-7)
MK-7 is a long-chain menaquinone typically derived from fermented foods like natto. Its key advantage is a very long half-life, with studies showing it remains in the bloodstream for up to 72 hours. This prolonged presence allows MK-7 to accumulate in the blood with daily intake, ensuring a consistent and steady supply of vitamin K2 to non-hepatic tissues like the bones and arteries. Clinical trials demonstrate that even small, once-daily doses of MK-7 (around 45-180 mcg) are highly effective at activating vitamin K-dependent proteins, leading to measurable improvements in bone mineral density and cardiovascular health.
Comparison Table: MK-4 vs. MK-7 for Absorption
| Feature | Menaquinone-4 (MK-4) | Menaquinone-7 (MK-7) |
|---|---|---|
| Absorption at Nutritional Doses | Poor; not detectable in serum | Excellent; well-absorbed and elevates serum levels |
| Serum Half-Life | Very short (6-8 hours) | Very long (up to 72 hours) |
| Effect on Serum Levels | Does not effectively increase serum levels with standard daily doses | Accumulates in the bloodstream with consistent daily intake |
| Typical Supplement Dose | Requires very high doses (milligrams) multiple times daily | Effective at low microgram doses once daily |
| Target Tissues | Primarily affects the liver at nutritional levels | Effectively reaches extra-hepatic tissues like bones and arteries |
| Primary Source | Animal products, synthesized from other vitamin K forms | Fermented foods like natto; bacterial fermentation |
The Verdict: Why MK-7 is Best for Absorption
Based on the evidence, MK-7 is clearly the superior form of vitamin K2 for absorption at nutritional and supplemental levels. Its longer half-life and greater bioavailability ensure that it reaches the tissues where it is most needed, providing consistent and reliable support for bone and heart health. For most consumers, selecting a supplement containing MK-7 is the most effective and convenient way to increase vitamin K2 status.
Maximizing Vitamin K2 Absorption
Because vitamin K2 is a fat-soluble vitamin, its absorption can be optimized by following certain practices:
- Take with dietary fat: Always consume your vitamin K2 supplement with a meal containing fat, such as one with healthy oils, nuts, or avocado. This significantly enhances absorption into the bloodstream.
- Time it with a major meal: Taking your supplement with a larger meal like dinner, which often contains more fat, can be an effective strategy.
- Support gut health: As gut bacteria produce some K2, maintaining a healthy gut microbiome through diet and probiotics may also support natural production.
- Combine with Vitamin D3: Vitamin K2 works synergistically with Vitamin D3. While D3 aids calcium absorption from the gut, K2 ensures that calcium is properly directed to the bones and teeth, away from soft tissues like arteries. Many supplements combine these two vitamins for this reason.
Conclusion
For those seeking the most effective form of vitamin K2 for absorption, menaquinone-7 (MK-7) is the clear winner. Its unique chemical structure gives it a longer half-life and superior bioavailability compared to MK-4, allowing it to provide sustained benefits to extra-hepatic tissues crucial for long-term bone and cardiovascular health. While both forms have their roles, MK-7 is the more efficient choice for supplementation at nutritional dosages. By pairing MK-7 with a source of dietary fat, you can further enhance its absorption and maximize its powerful health benefits.
Foods with Vitamin K2
- Natto: A Japanese fermented soybean dish, natto is exceptionally rich in MK-7.
- Cheeses: Aged and fermented cheeses like Gouda, Edam, and Brie contain moderate amounts of MK-7.
- Animal products: Egg yolks, butter, and organ meats like liver contain MK-4.
- Sauerkraut: This fermented cabbage is another source of MK-7.
For more detailed scientific information on menaquinones, you can refer to review papers like this one published in the International Journal of Molecular Sciences: Vitamin K: Double Bonds beyond Coagulation Insights into Differences between Vitamin K1 and K2 in Health and Disease.
