Vitamin K2. Vitamin K1 is produced by plants, and K2 is produced by animals and bacteria. They have very different functions in the human body, and you need them both. Supplements and fortified food are almost always K1, unless you look for K2 specifically.

Vitamin K2 is necessary for some proteins which modulate calcium in your body. Supplementing it has been found to protect both against osteoporosis and heart/artery calcification.