Vitamin K is a fat-soluble vitamin that plays essential roles in blood clotting and bone metabolism. What many people don't realize is that "Vitamin K" is actually an umbrella term for a family of related compounds with distinct sources, functions, and supplemental applications. The two main forms — K1 and K2 — behave quite differently in the body, and understanding the distinction can help you make a more informed supplementation decision.
Vitamin K1 — Phylloquinone
Vitamin K1 (phylloquinone) is the primary dietary form of Vitamin K, found abundantly in leafy green vegetables such as kale, spinach, broccoli, and Brussels sprouts. It is the form most associated with the blood clotting cascade — K1 is required for the activation of clotting factors produced by the liver.
K1 is rapidly cleared from the bloodstream and preferentially taken up by the liver, where it performs its clotting-related functions. It has a relatively short half-life compared to K2 forms.
In supplement form, K1 is widely available and inexpensive. It is included in many multivitamins and is the form most commonly used in clinical settings for clotting-related applications.
Vitamin K2 — Menaquinone
Vitamin K2 (menaquinone) is found primarily in fermented foods (such as natto, aged cheeses, and fermented dairy) and certain animal products. Unlike K1, K2 is not rapidly cleared by the liver — it circulates in the bloodstream for longer and is distributed to extrahepatic tissues including bone, blood vessels, and other organs.
This broader tissue distribution is why K2 is associated with bone metabolism and cardiovascular health. K2 activates two key proteins:
- Osteocalcin — A protein produced by bone-building cells (osteoblasts) that requires K2 activation to properly bind calcium into bone matrix
- Matrix Gla Protein (MGP) — A protein found in blood vessel walls that requires K2 activation to inhibit calcium deposition in arterial tissue
In simple terms: K2 helps direct calcium to where it belongs (bones and teeth) and away from where it doesn't (arteries and soft tissues).
MK-4 vs MK-7: The Two Main K2 Forms
Vitamin K2 exists in several subtypes called menaquinones, distinguished by the length of their side chain. The two most relevant for supplementation are MK-4 and MK-7:
- MK-4 (Menatetrenone) — A short-chain menaquinone found in animal products; has a short half-life in the body (hours); requires multiple daily doses to maintain blood levels; used in some clinical research on bone metabolism at pharmacological doses
- MK-7 (Menaquinone-7) — A long-chain menaquinone derived primarily from natto (fermented soybeans); has a significantly longer half-life (days); effective at once-daily dosing at much lower doses; MK-7 is a form of vitamin K2 used in dietary supplements. For a detailed look at MK-7, see our article Understanding Vitamin K2 MK-7: What It Is and How It Works.
For general supplementation purposes, MK-7 is the preferred K2 form due to its longer half-life and effectiveness at lower doses.
Why Vitamin D3 and K2 Are Often Paired
Vitamin D3 increases intestinal calcium absorption. Vitamin K2 (MK-7) activates the proteins that direct that absorbed calcium into bones rather than soft tissues. This complementary relationship is why many practitioners recommend taking D3 and K2 together, particularly at higher Vitamin D doses. See our articles on Vitamin D2 vs D3, How to Choose a Vitamin D Supplement, and Vitamin D Supplement Forms, and browse our Vitamin D supplements for available D3+K2 combination formulas.
Explore Vitamin K Supplements
Browse our full selection of Vitamin K supplements from established supplement brands including NOW Foods, Thorne, Pure Encapsulations, Bluebonnet Nutrition, and Douglas Laboratories. We carry K1, K2 MK-7, and D3+K2 combination formulas.
Related categories worth exploring: Calcium supplements and Magnesium supplements, which work alongside Vitamin K2 in bone metabolism.