Vitamin K can be divided into two categories, one is fat-soluble vitamins, namely vitamin K1 extracted from green plants (such as alfalfa, spinach and other green leafy plants, etc.) and vitamin K2 extracted from microorganisms, human intestinal bacteria (such as Escherichia coli) can also synthesize vitamin K2. Vitamin K1 is a yellow oily substance and K2 is a light yellow crystal. Both have heat resistance, but are easily damaged by ultraviolet radiation, so they should be stored away from light. The other type is water-soluble vitamins, which are artificially synthesized, namely vitamin K3 and K4. The most important are vitamins K1 and K2.