Last updated: 11/23/2022
One of the lesser-known and lesser-studied vitamins, Vitamin K refers to a group of fat-soluble vitamins that share a similar molecular structure. Vitamin K includes Vitamin K1, or phylloquinone, and a handful of menaquinones, also called Vitamin K2. Vitamin K facilitates the production of several proteins that are involved in blood clotting and bone mineralization - particularly important during pregnancy when the baby's bones are developing and mom can lose calcium in her bones and teeth.
Types and sources of Vitamin K
Phylloquinone (Vitamin K1) is the form of Vitamin K found in green leafy vegetables like collard greens, spinach, kale and broccoli. This is the form of Vitamin K most abundant in the American diet. While the data is limited, the absorption rate of phylloquinone form of Vitamin K from leafy vegetables appears quite low, likely because phylloquinone is tightly bound to chloroplast (the structure in the cells where photosynthesis occurs) and difficult to break down.
The other form of Vitamin K, menaquinone (Vitamin K2), is produced by bacteria. As such, menaquinone is found in fermented foods like cheese and fermented soybean products. Natto – the fermented soybean product from Japan famous for its “acquired taste” smell and slimy texture – is known to contain an unusually large amount of menaquinone in the MK-7 form.
Poultry and pork products sometimes contain menaquinone in the MK-4 form, because animals can synthesize MK-4 from a synthetic Vitamin K that some animal feed contains.
Interestingly, the bacteria in our intestines – gut microbiota – also synthesizes menaquinone. Some studies suggest that our gut microbiota is involved in the remodeling of Vitamin K from diet and that Vitamin K intake in turn influences the composition of the gut microbiota. However, this is still an emerging area of research and scientists don’t know a lot about how much Vitamin K is produced in our intestines, how it’s absorbed into the bloodstream and how it functions in the body.
Limited research suggests that menaquinones – especially Vitamin K2 with longer chains – may be more bioavailable than Vitamin K1.
Roles of Vitamin K in the body
All forms of Vitamin K act as a coenzyme in various chemical reactions in the body. These physiological functions are driven by enzymes, but they also require the presence of Vitamin K as a catalyst. The three major processes dependent on Vitamin K are hemostasis (blood clotting), bone metabolism and coronary elasticity.
Blood clotting:
- Vitamin K is necessary in the synthesis of a protein called prothrombin. Also called a clotting factor II, prothrombin circulates in the blood and facilitates coagulation (blood clotting) by converting another clotting factor, fibrinogen, into fibrin and fibrin-based blood clot when there is an injury.
- Vitamin K’s role in blood clotting is why people who are on blood thinners (anticoagulants) like Warfarin need to watch their Vitamin K intake.
Bone metabolism:
- Synthesis of another protein, called osteocalcin, is dependent on the presence of Vitamin K. One of the most abundant proteins in the bone, osteocalcin regulates the mineralization of the bone. Through osteocalcin, Vitamin K helps maintain bone health by facilitating the bonding of calcium to the bone.
- Vitamin K works synergistically with Vitamin D in maintaining bone health. This is why high-quality Vitamin D supplements usually also contain some Vitamin K.
Coronary elasticity:
- Similar to Vitamin K’s role in regulating calcium metabolism in the bone, Vitamin K also plays a part in how much calcium accumulates on the blood vessels. Vitamin K is indispensable in the production of a protein called matrix Gla-protein (MGP), which inhibits calcifications in the arteries that supply oxygen to the heart. Studies suggest that Vitamin K supports heart health by helping keep the elasticity of these important blood vessels.
Population studies suggest a few other potential areas in which Vitamin K may play an important role in human health, including inflammation, brain function and endocrine function. However, we don’t have as much insight into these areas, compared to Vitamin K’s roles in blood clotting and bone metabolism – more research is needed.
How much Vitamin K do you need daily?
For adult women, adequate intake of Vitamin K is considered 90 mcg/day. This amount doesn’t change for pregnant or nursing women. For adult men, AI is 120 mcg/day.
Interesting fact: Most vitamins and minerals have RDAs (recommended daily allowance) set by the American National Academies. Vitamin K, however, has only AI (adequate intake) set as of 1998, because the scientific evidence was not enough to set a firm guideline like RDAs.
How common is Vitamin K deficiency?
Severe Vitamin K deficiency can lead to poor blood clotting and osteoporosis, but the good news is that for healthy people who eat a balanced diet, Vitamin K deficiency is rare. The endogenous production of Vitamin K in the gut constitutes a safety net as well - we have our gut microbiome to thank for.
Interestingly, American Academy of Pediatrics recommends a prophylactic dose of Vitamin K at birth. This is because not much Vitamin K passes through the placenta. Even when moms had more than adequate amount of Vitamin K in her diet while pregnant, newborns can still be low enough on Vitamin K to have problems with blood coagulation.
Takeaway on Vitamin K
Vitamin K helps facilitate blood clotting, binds calcium to the bone and maintains elasticity of the blood vessels. Eat a balanced diet that includes leafy green vegetables and fermented foods to ensure that your body is getting enough Vitamin K. (Bonus tip: Eat your green leaves with some fat to make it easier for the body to absorb this fat-soluble vitamin.)
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