The Important Role of Vitamin K in Bone Health*
Lisa Murray RDN, LD
Many of your patients may have heard that if they are taking vitamin D, they should also be taking vitamin K – and with so many of your patients taking vitamin D, there is sure to be confusion! Here is a brief summary of what we currently know about vitamin K, and how to supplement if needed.
Your body uses 99% of its calcium to keep bones and teeth strong and to support skeletal structure and function. The other 1% of calcium is used for cell signaling, blood clotting, muscle contraction and nerve function with levels tightly controlled, within a very narrow range in the blood and tissues.
The equilibrium is maintained by an ongoing interplay of calcium absorbed from the intestines, movement of calcium into and out of the bones, and the kidney’s reclamation and excretion of calcium into the urine.
Calcium regulating hormones affect calcium transport at three primary sites: intestines, bone, and kidney. Both vitamin D and vitamin K play a role in this delicate balance.
Vitamin D has at least two ways to help maintain adequate blood levels of calcium:
- Improving calcium absorption: vitamin D enhances the absorption of calcium in the intestines.
- Taking calcium from bone: when you don’t consume enough calcium, vitamin D helps to mobilize calcium from your bones into the bloodstream.
Vitamin K also has at least two ways to help maintain proper blood calcium levels:
- It stimulates reabsorption of calcium from the kidneys back into the bloodstream.
- Builds bone by pulling calcium out of the bloodstream and holding it in the bones.
When it comes to bone building and remodeling, simplistically, vitamin D and vitamin K have opposite functions. Vitamin D mobilizes calcium out of the bones, and vitamin K pulls it back in.
The many roles of vitamin k
Vitamin K was first discovered in 1935 and dubbed the “coagulation vitamin”. It was established that vitamin K activated certain proteins that are required for normal blood clotting. Without sufficient vitamin K, blood would not clot, which results in severe bleeding.
Vitamin K activates blood-clotting proteins by making a critical chemical change in the proteins’ structure, specifically on the protein building block called glutamic acid. According to 2014 research, at least 16 different vitamin K-dependent Gla-proteins have been identified which require vitamin K to activate them in order to carry out their intended role. 
With the discovery of all these other Gla-proteins, we now know that vitamin K is vital for much more than proper blood clotting. For example, certain bone cells (osteoblasts) make the Gla-protein called osteocalcin, which is responsible for making sure calcium is deposited in bones, while the Gla-protein in arterial walls, called matrix Gla protein, prevents calcium from being deposited in arteries. This is why vitamin K is critical not only to bone health, but to cardiovascular health as well.
The function of osteocalcin is dependent on the interplay of vitamin D and K. The synthesis of osteocalcin by osteoblasts is regulated by vitamin D3, but the calcium-binding capacity of osteocalcin requires vitamin K to be activated.
Vitamin K and osteocalcin also play a role in glucose metabolism but the exact mechanism is still under investigation. Initial studies seemed to correlate low vitamin K status with better glucose metabolism, and other studies show supplementation can help restore healthier glucose metabolism by improving insulin sensitivity. The current conclusion is that adequate levels of vitamin K are supportive of proper glucose metabolism but this is still not clear. It is a good reminder, that while some supplementation can be helpful, more is not always better.
Types of vitamin k
What we refer to as vitamin K is actually a group of different fat soluble molecules with slightly differing actions. Phylloquinone, or K1, is the predominant source of vitamin K in the diet at high concentrations in green leafy vegetables and certain plant oils. Vitamin K2 includes a range of forms collectively referred to as menaquinones, which are primarily of bacterial origin and comprise only about 10% of our vitamin K consumption.
Most menaquinones are synthesized in the gut by human intestinal microbiota. MK-7 is found in fermented foods including cheese, and natto. Natto has a high concentration of MK-7 and is usually the source for MK-7 in supplements. MK-4 is unique in that it is produced by the body from vitamin K1 (phylloquinone) via a conversion process that does not involve bacterial action. Because of this, MK-4 is considered to have special qualities and functions in the body that are yet unknown. MK-4 is present in animal tissues and so meats are a dietary source.
Studies have shown that supplementation with MK-7 results in higher serum levels of vitamin K, but supplementation with MK-4 shows no rise in serum levels of MK-4. Therefore, MK-7 is considered to be more bioavailable and is the preferred choice for supplementation, especially since it is not widely available in the diet.
To supplement or not?
Insufficient blood clotting has been the primary indication of vitamin K deficiency. However, scientists have since learned that you can have enough vitamin K to promote healthy blood clotting, yet still not have enough vitamin K to activate other Gla-proteins necessary for many other critical functions. Fortunately, studies show that vitamin K supplementation can significantly increase the amount of activated Gla-proteins in tissues—without over-activating the clotting proteins.*
It is likely that both K1 and K2 are necessary for normal vitamin K function, and it appears that supplementation with both can be useful, especially in cases where the diet is low in vitamin K1.*
Vitamin k and bone health
Research studying the effects of vitamin K supplementation on bone health in humans is considered inconclusive so far in this country. However, recent human studies in Asian countries like China, Japan, Korea and Indonesia are showing good outcomes for bone health with vitamin K supplementation in postmenopausal women. *
We absolutely do know that sufficient vitamin K is required in order to activate the Gla-protein osteocalcin which is involved in bone mineralization and formation. We also know that vitamin K stimulates renal calcium reabsorption. That’s important, because when less calcium is lost in the urine, more stays in the blood stream keeping calcium levels stable and reducing the need to take it from the bone.
The bottom line
While it is clear that we don’t know everything about vitamin K, we do know that it works in tandem with vitamin D to help maintain proper calcium balance in the body, and to help maintain strong bones. Now that many people are correcting vitamin D deficiency with supplementation, it may be prudent to ensure consumption of adequate vitamin K either through diet, or through supplementation, to meet the body’s demands and balance the actions of vitamin D.
We know that MK-7 is the preferred supplement form, because it is highly bioavailable, but we don’t consume much of it in the Western diet. The American Food and Nutrition Board has set no upper limit for vitamin K because of its low potential for toxicity and the fact that no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals.
 Okano T. Gla-containing proteins. Clin Calcium. 2014 Feb; 24(2):241-8.
 Iwamoto J. Vitamin K2 Therapy for Postmenopausal Osteoporosis. Nutrients. 2014;6(5):1971-1980. doi:10.3390/nu6051971.