Understanding the Vitamin K Family
Vitamin K is a family of fat-soluble vitamins, primarily composed of vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). Vitamin K1, found mainly in leafy green vegetables, is used predominantly by the liver for blood clotting. Vitamin K2 encompasses several subtypes, labeled MK-4 through MK-13 based on the length of their side chain. The most well-researched forms of K2 are MK-4 and MK-7, and they differ significantly in their source, half-life, and function throughout the body.
The Key Distinction: MK-7 as a Long-Chain Vitamin K2
Put simply, MK-7 is one specific type of vitamin K2, while vitamin K2 is the broader category. MK-7 is a long-chain menaquinone, meaning it has a longer chemical side chain than other forms like MK-4. This structural difference is the root of its superior bioavailability and efficacy, especially in non-liver tissues. The source of these two main K2 subtypes also differs: MK-4 is found in animal products like eggs, meat, and dairy, while MK-7 is primarily created through bacterial fermentation and is most concentrated in the Japanese fermented soybean dish, natto.
Absorption and Bioavailability
When it comes to absorption and availability in the body, MK-7 significantly outperforms MK-4 and K1, particularly at nutritional doses. Studies show that a nutritional dose of MK-4 is not detectable in the bloodstream, while MK-7 is readily absorbed and remains active in the circulation for an extended period.
Extended Half-Life
The most crucial difference is the half-life. MK-4 has a very short half-life of only a few hours, meaning it is quickly cleared from the blood. This rapid clearance means a steady supply of MK-4 requires multiple, large doses throughout the day. In contrast, MK-7 boasts a much longer half-life of around 72 hours, allowing it to accumulate in the bloodstream and provide a consistent supply to tissues with just a once-daily dose. This sustained availability is what makes MK-7 so effective for extrahepatic (non-liver) functions.
Action in Extrahepatic Tissues
While the liver uses both K1 and K2 for blood clotting, MK-7's prolonged presence in the circulation allows it to be more effectively delivered to other tissues, including bones and blood vessel walls. Here, MK-7 activates specific vitamin K-dependent proteins:
- Osteocalcin: A protein produced by osteoblasts (bone-forming cells), which helps bind calcium to the bone matrix to strengthen bones and improve mineral density.
- Matrix Gla Protein (MGP): A protein that prevents calcium from depositing in soft tissues like arteries and blood vessels. By activating MGP, MK-7 helps protect against arterial calcification, a major risk factor for heart disease.
Comparison Table: Vitamin K2 (MK-4) vs. Vitamin K2 (MK-7)
| Feature | MK-4 (Menaquinone-4) | MK-7 (Menaquinone-7) |
|---|---|---|
| Source | Animal products (egg yolks, butter, organ meats), conversion from K1 | Fermented foods (natto, some cheeses), bacterial synthesis |
| Half-Life | Short (~6-8 hours) | Long (~72 hours) |
| Bioavailability | Low at nutritional doses; high doses needed for effect | High, effectively absorbed and utilized |
| Dosing Frequency | Needs to be taken multiple times daily for constant effect | Once-daily dosing is sufficient for constant effect |
| Tissue Distribution | Primarily used by the liver and rapidly cleared | Widely distributed to extrahepatic tissues, including bones and arteries |
| Primary Role | Liver function (clotting) and potential extrahepatic roles at high doses | Extrahepatic functions: bone mineralization and arterial calcification inhibition |
| Supplemental Form | Requires much higher dosages in supplements | Effective at lower dosages in supplements |
Synergistic Relationship with Other Nutrients
Vitamin K2, particularly MK-7, works synergistically with other key nutrients. The most notable collaboration is with vitamin D3. While vitamin D3 is responsible for increasing the absorption of calcium from the gut, vitamin K2 ensures that this absorbed calcium is directed to the bones and teeth rather than accumulating in the arteries. For optimal bone and cardiovascular health, a sufficient intake of both vitamin D3 and vitamin K2 is essential.
The Best Choice for Supplementation
Because of its superior bioavailability, longer half-life, and consistent delivery to extrahepatic tissues, MK-7 is widely considered the superior supplemental form of vitamin K2 for promoting long-term bone and cardiovascular health. While MK-4 has demonstrated benefits at extremely high, pharmacological doses used in some research, these doses are not practical for general supplementation. The lower, once-daily doses of MK-7 have been shown to be effective for activating the key proteins needed for calcium regulation throughout the body. Choosing an MK-7 supplement derived from natural fermentation, such as from natto, can ensure high isomeric purity and efficacy.
Conclusion
In summary, the core difference between vitamin K2 and MK-7 is that MK-7 is a specific, highly bioavailable form of the broader vitamin K2 family. MK-7's unique, long-chain structure gives it a significantly longer half-life than other forms like MK-4, allowing for better absorption and superior delivery to bones and arteries. This makes MK-7 more effective at supporting bone mineralization and inhibiting arterial calcification at lower, once-daily doses. For those seeking to support cardiovascular and skeletal health, especially through supplementation, MK-7 represents the most efficacious choice within the vitamin K2 family.
An excellent overview of vitamin K's biological mechanisms and differences between its forms can be found in this study: Vitamin K: Double Bonds beyond Coagulation Insights into Differences between Vitamin K1 and K2 in Health and Disease.
