Vitamin K is an essential vitamin that plays an important role in blood coagulation, bone metabolism, and vascular health, and it is one of the four fat-soluble vitamins (along with vitamin A, vitamin D, and vitamin E).[1][2][3] Vitamin K is actually the collective term for several fat-soluble molecules called 2-methyl-1,4-naphthoquinones.[1][2][3]
There are two naturally occurring forms of vitamin K: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). K1 is the major dietary form and is found in several plant-based foods including spinach, broccoli, kale, Brussels sprouts, collards, and soybeans.[4][1][2][3] Vitamin K2 is synthesized by bacteria — including gut bacteria in our microbiome — and is found in meat and fermented foods (e.g., nattō; fermented soybeans).[5][6][1][2][3]
Phylloquinone (vitamin K1) is the predominant form used in vitamin K supplements, but menaquinones (vitamin K2) are also used.[1][7] Another form of vitamin K called menadione (or vitamin K3) is an intermediate molecule in vitamin K metabolism.[1][3][8] It is not typically used in human supplements, but it is used in animal feed.[9]