Vitamin K: Double Bonds beyond Coagulation Insights into Differences between Vitamin K1 and K2 in Health and Disease

Abstract

Vitamin K is an essential bioactive compound required for optimal body function. Vitamin K can be present in various isoforms, distinguishable by two main structures, namely, phylloquinone (K1) and menaquinones (K2). The difference in structure between K1 and K2 is seen in different absorption rates, tissue distribution, and bioavailability. Although differing in structure, both act as cofactor for the enzyme gamma-glutamylcarboxylase, encompassing both hepatic and extrahepatic activity. Only carboxylated proteins are active and promote a health profile like hemostasis. Furthermore, vitamin K2 in the form of MK-7 has been shown to be a bioactive compound in regulating osteoporosis, atherosclerosis, cancer and inflammatory diseases without risk of negative side effects or overdosing. This review is the first to highlight differences between isoforms vitamin K1 and K2 by means of source, function, and extrahepatic activity.

Keywords: vascular calcification; vitamin K dependent proteins; vitamin K1; vitamin K2.

Figure 1

Dietary sources of vitamin K. Left side of pyramid displays K1 content gradient in dietary sources of vitamin K1. Leafy greens include spinach, kale and swiss shards. Right side visualizes K2 content gradient with natto being the most significant source. Various cheeses include hard and soft cheeses with K2 content being dependent on fermentation level.

Figure 2

Bioavailability and biodistribution of vitamin K. Vitamin K2 has a longer half-life in the circulation than vitamin K1. While vitamin K1 is retained and exerts its function in the liver, vitamin K2 is redistributed to the circulation and (extra-)hepatic tissues.

Figure 3

Functions of vitamin K2 in the body. Vitamin K2 exerts protective role and is involved in various organ systems throughout the human body (summarized in the figure).