Vitamin K2 Needs an RDI Separate from Vitamin K1.

Vitamin K was first discovered almost a century ago, as was its essential role in coagulation [1]. Since then, vitamin K and vitamin K-dependent proteins have been illustrated to have a multitude of various functions beyond coagulation [2]. Vitamin K exists in various isoforms, namely, phylloquinone, commonly referred to as vitamin K1 (VK1), and menaquinones, also known as vitamin K2 (VK2). VK2 can be further classified into various subtypes, and the most well-known ones are menaquinone-4, -7, -8, and -9 (MK-4, MK-7, MK-8, MK-9) [3,4,5]. VK1 is found mainly in leafy green vegetables, such as spinach, Swiss chard, and kale, to name a few [6,7]. VK2 is primarily produced by bacteria, so it is found in high concentrations in fermented foods, such as pickled vegetables and cheeses. Relative amounts of VK2 can be found in particular meats of farmed animals. This is due to menadione supplementation in animal feed, given to prevent fractures, as well as from the naturally occurring conversion of VK1 to VK2 as MK-4.