Understanding Vitamin K2: The Foundation
Vitamin K is a group of fat-soluble vitamins essential for a variety of bodily functions. The two primary forms are K1 (phylloquinone), found in leafy greens, and K2 (menaquinone), which includes several subtypes, most notably MK-4 and MK-7. While K1 is crucial for blood clotting, K2 plays a more significant role in regulating calcium metabolism, directing it to bones and away from arteries.
The Chemical Difference: Short-Chain vs. Long-Chain
The fundamental difference between MK-4 and MK-7 lies in their chemical structure. They are both menaquinones, but the number in their name refers to the length of their side chain, or "tail," made of isoprene units. MK-4 has a short side chain (4 units), while MK-7 has a longer side chain (7 units). This structural distinction dramatically impacts how the body absorbs and utilizes each form.
Natural Sources: Where to Find MK-4 and MK-7
The natural dietary sources for these two vitamin K2 forms differ significantly. MK-4 is found predominantly in animal products, including eggs, meat, and high-fat dairy from grass-fed animals. In the body, MK-4 is also endogenously produced through the conversion of other vitamin K forms. On the other hand, MK-7 is produced by bacteria during fermentation. The most abundant and well-known source is natto, a traditional Japanese food made from fermented soybeans. Cheeses and other fermented foods also contain some MK-7, though in lower concentrations.
Bioavailability, Half-Life, and Efficacy
This is where the debate over which K2 is best, MK-4 or MK-7, really heats up. The longer side chain of MK-7 results in dramatically different pharmacokinetic properties compared to MK-4.
- Absorption and Transport: MK-4 is typically absorbed into triglycerides and has a very short half-life of just a few hours. Because of this, it's cleared from the bloodstream quickly and requires multiple daily doses to maintain sufficient levels. In contrast, MK-7 is packaged into cholesterol and circulates in the blood for much longer, with a half-life of approximately 72 hours.
- Extrahepatic Availability: The longer half-life of MK-7 allows it to reach tissues outside the liver, such as bones and blood vessels, more effectively. This makes it particularly important for extrahepatic functions like preventing arterial calcification and supporting bone health.
- Dosing: Due to its superior bioavailability and stability, MK-7 is effective at much smaller, daily nutritional doses, typically in the microgram range (e.g., 90-180 µg/day). MK-4 requires much higher, often pharmacological, doses (milligrams) to achieve therapeutic effects, and even then, may be cleared too rapidly.
Functions Beyond Calcium Regulation
While both forms activate key proteins involved in calcium metabolism, research points to other specific functions. MK-4 acts as a ligand for the SXR/PXR receptor, a different mechanism from the gamma-carboxylation pathway. This may indicate unique physiological roles for MK-4, but because the body can convert MK-7 into MK-4 in extrahepatic tissues, MK-7 can be considered a better overall supplier.
Comparison Table: MK-4 vs. MK-7
| Feature | MK-4 | MK-7 |
|---|---|---|
| Natural Source | Animal products (meat, dairy) | Fermented foods (natto, some cheeses) |
| Half-Life in Blood | Very short (approx. 2-6 hours) | Very long (approx. 72 hours) |
| Bioavailability | Poor at nutritional doses, high doses needed for effect | High, well-absorbed at nutritional doses |
| Typical Dose | Milligram range (1.5-45 mg) | Microgram range (90-180 µg) |
| Frequency | Needs multiple daily doses for stable levels | Once-daily dosing is effective |
| Main Role | Activates liver proteins (clotting factors) and converted in tissues; requires high doses for extrahepatic activity | Provides long-term, systemic support for extrahepatic tissues (bones, arteries) |
| Overall Efficacy | Can be effective at high doses, but bioavailability is poor at typical nutritional levels | Superior bioavailability and accumulation lead to higher efficacy for bone and cardiovascular health |
Conclusion: The Verdict on Which K2 is Best, MK-4 or MK-7
Based on decades of research, MK-7 stands out as the superior supplemental form for most people seeking vitamin K2's benefits. Its high bioavailability, long half-life, and ability to accumulate effectively in non-liver tissues—such as bones and arteries—make it the more efficient choice for promoting long-term bone density and cardiovascular health. While MK-4 is the biologically active form found in the body, it is poorly absorbed from dietary supplements at nutritional doses. The body can and does produce MK-4 from other forms, including MK-7, in the tissues where it is needed. For reliable, once-daily support for extrahepatic health, supplemental MK-7 is the clear winner. Individuals taking blood thinners like warfarin should consult a doctor before supplementing with any form of vitamin K2, as it can interfere with medication.
Understanding the Differences Between Vitamin K1 and Vitamin K2
