You need vitamin K for your blood to clot properly, but deficiency is quite rare among adults.
To begin, we have to distinguish between the different K vitamins. Vitamin K1 (phylloquinone) is found mostly in plants, particularly green vegetables like spinach and broccoli. Vitamin K2 (menaquinone) is the form found in animals. However, animal products are a negligible source of vitamin K in our diet; almost all the vitamin K we consume is phylloquinone from plant sources. But the distinction is academic. Vitamin K1 is converted into vitamin K2 in our bodies, in part by bacteria in our intestines.
Vitamin K’s main biological role is the activation of several blood coagulation factors. You need vitamin K for your blood to clot properly, and without it you would have severe hemorrhages. Medically, it was used to reverse the effects of a blood thinner like Coumadin, though Coumadin is used less frequently than it once was. There is one other situation in which vitamin K is used routinely: Infants are uniformly vitamin K deficient and need a single dose at birth to prevent bleeding.
For adults, vitamin K deficiency is quite rare and would occur only if they had liver failure or some disease that prevented its absorption from the intestine, like Crohn’s disease.
Given its central role in bleeding and coagulation, one may wonder why we are discussing it in the context of osteoporosis. But vitamin K also activates proteins like osteocalcin that have a role in bone mineralization, and this has led some people to champion its use as an osteoporosis treatment.
Vitamin K, in whatever variety you take it, has one clear medical purpose: It prevents major hemorrhage and can reverse severe bleeding if you are an infant with no vitamin K in your system or an adult taking too high a dose of Coumadin. It doesn’t delay osteoporosis or coronary calcifications to any meaningful degree. Come find me when the next vitamin fad hits.