Vitamin K2 (MK-4 vs. MK-7): The Calcium-Routing Vitamin
Vitamin K2 has emerged from the shadow of K1 in recent years as research has illuminated its distinct roles in calcium metabolism, bone health, and cardiovascular protection. While K1 handles blood clotting in the liver, K2 works in peripheral tissues to ensure calcium goes where it belongs — into bones and teeth — and stays out of where it doesn't — arteries and soft tissues.
How K2 Routes Calcium
K2 activates two proteins critical to calcium metabolism: osteocalcin (which incorporates calcium into bone matrix) and matrix Gla protein (MGP, which prevents calcium from depositing in arterial walls). Without adequate K2, these proteins remain in their inactive, undercarboxylated forms — meaning calcium from the diet and supplements may end up in the wrong places.
This is why the combination of vitamin D (which increases calcium absorption) and K2 (which directs where it goes) is so important. Supplementing vitamin D without K2 increases the total calcium entering the body without optimizing its distribution.
MK-4 vs. MK-7
The "MK" in K2 nomenclature refers to menaquinone, followed by the number of isoprenoid units in its side chain. MK-4 and MK-7 are the two most researched forms:
| Property | MK-4 | MK-7 |
|---|---|---|
| Source | Animal tissues (meat, eggs, butter) | Bacterial fermentation (natto, sauerkraut) |
| Half-life | Short (hours) | Long (2-3 days) |
| Effective dose | Higher (often 15-45 mg/day in studies) | Lower (45-200 mcg/day) |
| Tissue distribution | Concentrated in specific tissues | More systemic distribution |
Food Sources
Natto (fermented soybeans) is by far the richest food source of K2 (primarily MK-7). Hard and aged cheeses (Gouda, Brie, Jarlsberg), egg yolks, butter from grass-fed cows, and chicken dark meat provide MK-4. Gut bacteria also produce some menaquinones, but the contribution to K2 status is uncertain.
External resources: Linus Pauling Institute — Vitamin K