Vitamin K refers to a group of related vitamins, including the naturally occurring vitamins K-1, or phylloquinone, and vitamin K-2, or menaquinone. Vitamin K was discovered in the early 1930s and for almost 50 years was thought to be only involved in blood coagulation. The 1978 discovery of osteocalcin, a protein synthesized in bone, revealed new functions of vitamin K, and led to new research illustrating the importance of vitamin K-2.
Absorption
Vitamins K-1 and K-2 are absorbed at different rates into different tissues and once there, behave differently. In animal studies, the liver absorbs most of the available vitamin K-1. On the other hand, the absorption of vitamin K-2 is more widely distributed. It is absorbed outside of the liver by the kidneys, pancreas, bone, blood vessel walls and testes. The biological availability of the two forms is also different. Unlike vitamin K-1, which only has a half-life of one hour, the half-life of vitamin K-2 is three days. Vitamin K-2 is available for longer and is more available to tissues outside of the liver.
Bone Health
Vitamin K-2 aids in the synthesis of the Gla-protein family. Specifically, vitamin K-2 is necessary to catalyze the carboxylation of these proteins, making them biologically active. The Gla proteins include osteocalcin, which contributes to bone quality, according to a 2007 study by L.J. Schurgers, which examined the positive effect of vitamin K-2 on bone strength in postmenopausal women. Schurgers also noted that vitamin K-2 increased the amount of collagen in bone cells. Vitamin K-2 apparently improves bone geometry, providing a stronger bone matrix.
Arterial Plaques
Most people over the age of 60 have plaques of calcium in their arteries, also known as vascular calcification. These plaques damage the elasticity of the arteries and obstruct arterial blood flow, leading to cardiovascular disease. Recent research has shown the formation of these plaques to be similar to bone development and metabolism, according to a June 2008 article in the journal "Circulation." In animal studies, vitamin K-2 has shown an ability to remove the arterial plaques brought on by the drug warfarin. Further research will confirm if vitamin K-2 shows the same positive effect in people with vascular calcification.
Cancer
A 2006 clinical trial suggested that menatetrenone, a subtype of vitamin K-2, might be able to reduce the recurrence rate of liver cancer after surgery, according to the American Cancer Society. The European Prospective Investigation into Cancer and Nutrition -- Heidelberg study, or EPIC, followed over 23,000 originally cancer-free people for more than 10 years. The EPIC study results, published in the May 2010 "American Journal of Clinical Nutrition," found that dietary intake of vitamin K-2, mostly in the form of cheese, was associated with a reduced risk of cancer. Dietary intake of vitamin K-1 did not show the same effect.
Dietary Sources
Vitamin K is fat-soluble and is absorbed in the intestines from food. The main source of vitamin K-1 in the diet is green vegetables, and all animals convert a small amount of that vitamin K-1 into vitamin K-2. The best dietary source of vitamin K-2 is a fermented soybean dish called natto. Other sources of vitamin K-2 are fermented cheeses, chicken egg yolks, meat, liver, butter and curd cheese. Current recommended dietary intake levels of vitamin K are based on the amount required for proper blood clotting. Research suggests, however, that other body tissues may require higher levels of vitamin K for full activation of the Gla proteins. Dietary recommendations will remain unchanged; however, until more research provides the answer for the optimum vitamin K-2 intake levels. If you are considering vitamin K-2 supplementation, consult with your doctor.
References
- American Cancer Society: Vitamin K
- "International Congress Series"; Vitamin K2 Improves Bone Strength in Postmenopausal Women; L.J. Schurgers et al; 2007
- "The American Journal of Clinical Nutrition"; Dietary Vitamin K Intake in Relation to Cancer Incidence and Mortality: Results from the Heidelberg Cohort of the European Prospective Investigation into Cancer and Nutrition; Katharina Nimptsch et al; 2010
- "Circulation"; Vascular Calcification: Pathobiology of a Multifaceted Disease; Linda L. Demer et al; 2008
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