Vitamin K

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Vitamin K is the name given to a group of structurally similar compounds. Natural phylloquinone is K1, which is best known for its role in blood coagulation. The synthetic form of K1 is phytonadione. Vitamin K2 includes at least 14 different menaquinones, identified as MK plus a number.
When supplementing with vitamin D, it is appropriate to also supplement vitamin K2. Vitamin D aids in the absorption of calcium from diet or supplements. Adequate Vitamin K2 helps to ensure this additional calcium goes into bone rather than calcifying soft tissue.
Vitamins A and D play a role in the secretion of osteocalcin, a protein in bone. When osteocalcin is activated by vitamin K2, it can bind to calcium and move it into bone. A second protein called matrix GLA protein, which is responsible for moving calcium out of areas where it is harmful, is also dependent for its actions on vitamin K2.
Research on vitamin K is still quite limited and the subject has only recently been of particular interest. Studies have been done with both K2 MK4, which has a very short half life and requires dosing every eight hours, and K2 MK7, which can be dosed once a day. There is evidence that the body can convert K1 and K2 MK7 to K2 MK4, which appears to be the form that is needed to get calcium into bone.
Most supplements contain very low doses of vitamin K2 MK7 measured in mcg (frequently 45 to 200), and this may be all that is needed. However, in Japan prescription vitamin K2 MK4 is used as a treatment for osteoporosis in very high doses of 45 mg a day split into three 15 mg doses taken 8 hours apart.
Studies of vitamin K2 dietary intake and supplementation have fairly consistently shown better vitamin K2 status results in a decrease in fracture risk for post menopausal women, and that improved vitamin K2 reduces arterial calcification. Children of vitamin K2 replete mothers are more likely to have strong, straight teeth. Research has shown vitamin K2 status has implications for cancer, gum disease, cardiovascular disease, bone strength and fracture risk, varicose veins, insulin resistance and skin wrinkling.
The changes in our food supply have lowered the vitamin K2 most people consume. Everyone, and most particularly those supplementing vitamin D, should be eating foods high in vitamin K2 or supplementing.
The exception to supplementing would be anyone who is taking prescription blood thinners like warfarin (Coumadin®), who should take any vitamin K only under supervision of a physician. However, there have been studies that suggest supplementing with vitamin K2 may actually have a stabilizing effect for someone on warfarin.
There is no way to directly measure vitamin K2 status at this time, however, carboxylation of osteocalcin requires vitamin K2 and a blood test measuring uncarboxylated osteocalcin (ucOC) can be used to evaluate vitamin K2.
An excellent book on the subject of vitamin K2 is Vitamin K2 and the Calcium Paradox by Dr. Kate Rheaume-Bleue, published at the end of 2011. It is available as an e-book and in paperback.