Published January 26 2015
Vitamin K is a fat-soluble vitamin, and while it is known as “the clotting vitamin” due to the effects it has on prothrombin, it has other important functions as well. Some people reading this probably are aware that vitamin K is important for bone health, but it also plays a role in brain health as well. In this article I’ll discuss some of these functions, I’ll explain the difference between vitamin K1 and K2, will list some of the food sources of vitamin K, and of course I will explain how it relates to thyroid health.
There are essentially three different forms of vitamin K. Vitamin K1, also known as phylloquinone, is found in plant sources, especially leafy green vegetables. On the other hand, vitamin K2, known as menaquinones, is found in mostly animal products, although there are other sources as well such as natto, which is a fermented type of soy. Vitamin K3, also known as menadione, is found in small amounts in certain foods, but essentially is considered to be a synthetic form of vitamin K.
Vitamin K1 can be obtained through plants such as kale, spinach, beet greens, collards, broccoli, and Brussels sprouts. As mentioned before, vitamin K2 is mostly found in animal foods such as eggs, cheese, butter, beef, and is also found in natto. There are two main types of vitamin K2, which include menaquinone-4 (MK-4), and menaquinone-7 (MK-7). MK-4 is what’s mostly found in animal foods, while natto is a source of MK-7. Both of these can be excellent sources of vitamin K2, although with regards to MK-4, the quality of the animal product is important, as eating eggs from pasture-raised chickens, and meat and dairy from grass-fed animals will contain higher levels of MK-4 than grain-fed animals.
Why Is Vitamin K Important To Our Health?
As I mentioned earlier, vitamin K is primarily known as being involved in blood clotting. It is actually vitamin K1 that is involved in clotting, as vitamin K2 has a minimal effect on clotting. Because many newborns are at risk of having a vitamin K deficiency, and because there is a low concentration of vitamin K1 in the breast milk (1)  and an absence of vitamin K-synthesizing bacteria in the gut, infants born in the hospital are given vitamin K shots at birth. As one gets older a vitamin K deficiency can develop if the person doesn’t eat enough foods rich in vitamin K, or even if they do eat plenty of these foods but have a problem breaking down fats. Many people have “gallbladder” problems, and I discussed how having a bile production problem can affect the absorption of the fat soluble vitamins in a post I wrote entitled “The Importance of Bile in Thyroid Health “. So if you have a moderate to severe “gallbladder” problem, then there is a good chance you have a vitamin K deficiency, along with deficiencies in the other fat soluble vitamins (A, E, D). Having a condition such as Celiac disease or SIBO can also lead to a vitamin K deficiency, along with many other nutrient deficiencies.
With regards to bone health, vitamin K2 is mainly involved, and there are three vitamin K-dependent proteins that are synthesized in bone. These are known as osteocalcin (OC), matrix Gla protein (MGP), and protein S. Many people supplement with calcium to help increase bone density, but taking calcium supplements alone won’t increase bone density if you have a deficiency in other important nutrients. Vitamin D is one of those nutrients, as this helps with the intestinal absorption of calcium. And of course many people are deficient in vitamin D. However, vitamin K frequently gets overlooked, but a vitamin K2 deficiency can lead to vascular calcification and osteoporosis (2) . Stated another way, if someone has sufficient amounts of calcium and vitamin D, but they have low levels of vitamin K2, they can not only have osteoporosis, but they can also have deposits of calcium building up in their arteries. This vascular calcification reduces the elasticity of these blood vessels, which impairs cardiovascular hemodynamics, resulting in substantial morbidity and mortality in the form of high blood pressure, aortic stenosis, cardiac hypertrophy, myocardial and lower-limb ischemia, congestive heart failure, and compromised structural integrity (3) . In other words, a deficiency of vitamin K2 can lead to calcium buildup in the arteries, which will greatly increase the risk of cardiovascular disease.
But why is vitamin K so important for optimal bone health? And how can low levels of vitamin K2 cause calcium buildup in the arteries? Well, while vitamin D is important for the absorption of calcium, vitamin K2 is necessary to help “escort” calcium into the bones. If vitamin K2 is low, then some of the calcium will end up in the soft tissues of the body, such as the arteries. The way vitamin K2 works is by activating a protein called osteocalcin, which I briefly mentioned earlier, as this helps to guide the calcium into the bones. Vitamin K2 also activates another protein I mentioned earlier called Matrix Gla protein (MGP), and this is a potent inhibitor of arterial calcification (4) , as it helps to pull calcium out of the soft tissues. I also mentioned protein-S earlier, as this is a vitamin-K dependent protein that can play a role in bone turnover and bone mass (5) .
Vitamin K is also important for optimal brain function. There are several vitamin K-dependent hemostatic proteins which are important for the brain, and other vitamin K-dependent proteins which are not associated with blood coagulation also contribute to brain function (6) . There also is some evidence that vitamin K might play a role in cognition as we age (7) . Another study showed that using vitamin K antagonists was associated with cognitive impairment among geriatric patients (8) . Sphingolipids are a group of complex lipids present in high concentrations in cells of the central and peripheral nervous systems, and certain sphingolipids are highly correlated with MK-4 in the brain (9) . There is evidence that alterations in sphingolipid metabolism is linked to the aging process, as well as neurodegenerative conditions such as Alzheimer’s disease and Parkinson’s disease (10)  (11) . So what this essentially is saying is that vitamin K is important for sphingolipid metabolism, and a vitamin K deficiency can therefore have a negative effect on its metabolism, which can potentially increase the incidence of neurodegenerative conditions. This isn’t suggesting that a vitamin K deficiency will cause Alzheimer’s or Parkinson’s disease, but it very well might be a factor.
Certain Drugs Can Interact With Vitamin K
There are numerous prescription drugs which can interact with vitamin K. For example, since intestinal bacteria synthesize vitamin K, if someone takes antibiotics then this can potentially cause a vitamin K deficiency. Warfarin is one of the main drugs that can interact with vitamin K. Warfarin inhibits certain clotting factors which are vitamin K-dependent. If someone is taking Warfarin and also takes a supplement consisting of vitamin K1, or eats large amounts of foods rich in vitamin K1, then this can inhibit the action of warfarin. This is why many medical doctors will tell their patients to minimize their consumption of green leafy vegetables when taking warfarin. Eating foods rich in vitamin K2 shouldn’t cause a problem when taking warfarin, since as mentioned earlier, vitamin K2 has a minimal influence on blood clotting mechanisms.
What Role Does Vitamin K Play In Thyroid Health?
As for the relationship between vitamin K and thyroid health, thyroid hormone does seem to affect the vitamin K-dependent plasma factors of blood coagulation, including factors II, VII, IX, and X (12) . I’m not going to get into great detail about blood clotting, but this involves platelets, as well as proteins called clotting factors, which create a network of fibrin, and this in turn acts like glue to hold blood clots together. So both sufficient numbers of platelets and clotting factors are important.
In any case, thyroid hormone, along with other hormones of the hypothalamic-pituitary-thyroid axis (TSH and TRH) help to regulate the synthesis and secretion of vitamin K-dependent plasma coagulation factors (12)  (13) . Having low thyroid hormone levels seems to lead to a decrease in blood clotting, and therefore patients with hypothyroidism have an increased risk of bleeding (R3). On the other hand, those with hyperthyroidism have increased coagulation factor levels and therefore have an increased risk of thrombosis, which is the formation of blood clots inside a blood vessel (14)  (15) .
How Can One Measure a Vitamin K Deficiency?
Doing a serum test for phylloquinone is one method of determining vitamin K1 nutritional status. Using prothrombin time is yet another method. Proteins Induced by Vitamin K Absence (PIVKA-II) is a more sensitive method of testing for a vitamin K1 deficiency (16) . However, most people aren’t deficient in vitamin K1, as a vitamin K2 deficiency is much more common. And unfortunately, there currently isn’t a great method of testing for deficient vitamin K2 levels, as most vitamin K2 doesn’t remain in the circulation, which is why the blood isn’t a great source of measuring the vitamin K2 levels. However, there are blood tests available that measure the levels of active versus inactive MGP (17) , and this type of test can predict how much calcium buildup in the arteries one has, which is common with a vitamin K2 deficiency.
Food Sources of Vitamin K
I mentioned some of the food sources earlier, as certain vegetables are the main source of vitamin K1, including leafy green vegetables such as kale and spinach, as well as broccoli and Brussels sprouts. Vitamin K2 can be found in animal products such as egg yolks, meats, cheese, butter, as well as natto.
Supplementing With Vitamin K
Since most people aren’t deficient in vitamin K1, supplementation with this usually isn’t necessary. The exception might be if someone has frequent bleeding, which might indicate a deficiency in vitamin K since this is involved in the clotting mechanisms. Of course other factors could be involved as well. With regards to vitamin K2 supplements, you can usually get this in the MK-4 form, or the MK-7 form. Both seem to raise vitamin K2 levels in an experimental setting, but lower doses of MK-7 typically are required when compared to MK-4. However, if someone is trying to avoid soy products then they will want to avoid MK-7 supplements, since this is from natto. When taking these supplements you want to take them with food, and ideally with a meal that contains fat.
Can Vitamin K Be Toxic?
Unlike some of the other fat soluble vitamins, it doesn’t appear that vitamin K1 or K2 are toxic in high amounts. I wasn’t able to find any studies which showed that either vitamin K1 or K2 were toxic, even when high doses were taken. I did come across one study which demonstrated that synthetic MK-7 is of low acute oral toxicity (18) . With that being said, since vitamin K is a fat soluble vitamin I’d still be cautious about consuming large doses of this nutrient in supplement form.
So hopefully you have a better understanding about the different functions of vitamin K, as well as the main differences between vitamin K1 and vitamin K2. Vitamin K1 is mostly involved in clotting, while vitamin K2 has numerous functions, as it is involved primarily in bone health, but also has other important roles. The two main sources of vitamin K2 are MK-4 and MK-7. Vitamin K1 can be obtained through green leafy vegetables and some other vegetables. MK-4 can be obtained through animal sources such as eggs, cheese, beef, and butter, while the best source of MK-7 is natto. Thyroid hormone has an effect on the vitamin K-dependent plasma factors of blood coagulation. Those with hypothyroidism typically have an increased risk of bleeding, whereas those with hyperthyroidism typically have an increased risk of thrombosis.