Vitamin D and vitamin K are both fat-soluble vitamins.
Vitamin D is commonly referred to as the sunshine vitamin because sunlight is required for the production of vitamin D in the skin. However, while the inactive form of vitamin D is produced in the body during sun exposure, a healthy liver and kidneys are also essential for the conversion of inactive vitamin D into its active form (calcitriol).
Biological functions of vitamin D go beyond maintaining healthy bones and can benefit other organs and systems in the body especially the immune, cardiovascular, respiratory and nervous systems.
Vitamin K2 encompasses a group of vitamers called menaquinones (MKs). They all share the common structure (menadione) but differ in the length of the side chain which determines their bioavailability and tissue distribution. In the human diet, meat and dairy products are the main source of the short-chain menaquinones such as MK-4, while the longer-chain menaquinones, especially MK-7, are predominantly of microbial origin and can be found in fermented food such as natto and cheese. After absorption, MK-7 can linger in the blood for 72 hours which makes it available for tissues outside the liver (e.g., bones, arteries, etc.) while the lifespan of MK-4 is much shorter (1.5 hours).
In the body vitamin K2 acts as a cofactor for an enzyme that activates proteins involved in regulating calcium deposition. Proper functioning of these proteins promotes calcification of bones and prevents calcification of soft tissues like the blood vessels. Therefore, vitamin K2 may help reduce calcium accumulation in arteries and prevent arterial stiffening.