The final chronological vitamin amongst them all, vitamin K is popularly known for its role in blood clotting. But it's important for bone and heart health as well.
Vitamin K is often referred to as the forgotten nutrient because deficiency is so rare. Blood stores are also minuscule compared to the other fat-soluble vitamins – A, D and E.
Nonetheless, vitamin K is a unique nutrient and deserves some love. Allow this to serve as a definitive guide to vitamin K benefits.
What Is Vitamin K1/K2?
Similar to vitamin E, vitamin K is a generic term for a group of compounds including phylloquinone (vitamin K1) and menaquinones (vitamin K2).
Vitamin K is a fat-soluble vitamin, so it is stored in fat tissue like vitamins A, D, and E. But unlike those, a greater amount of vitamin K is excreted via urine and stool.
Vitamin K1 is primarily found in plant foods like leafy greens while vitamin K2 is modestly present in animal and fermented foods. The bioavailability of K1 is limited, although eating these plant foods with beneficial fat sources like olive oil can enhance absorption.
Despite this, beneficial bacteria in the large intestine endogenously produce K2 – the more bioavailable and active form – which is why deficiency is quite uncommon.
Furthermore, the body stores K1 mostly in the liver, heart, and pancreas and K2 in the brain and kidneys.
Vitamin K1 Sources
• Dark leafy greens - arugula, collard greens, mustard greens, kale, spinach
• Other lettuces
Vitamin K2 Sources
• Conversion from vitamin K1 in the gut
• Fermented foods - natto, sauerkraut, kefir
• Liver and other organ meats
• Hard cheeses like parmesan
• Animal foods - beef, chicken, egg yolks, fatty fish like salmon and sardines, pork
Whereas many nutrients have an established recommended dietary allowance (RDA), vitamin K only has an adequate intake (AI) measured in micrograms (mcg). This AI amount is an estimation that ensures nutritional adequacy.
Note that babies are born with very little vitamin K, so all infants should receive a prophylactic vitamin K injection to ensure initial adequacy.
*Values based on micrograms (mcg)
Vitamin K Functions
Although vitamin K1 and K2 often work synergistically, they each exert their own functions as well.
The human body needs adequate amounts of each to produce the protein prothrombin involved in blood clotting, for bone and heart health, and energy production in the mitochondria of cells.
Vitamin K1 is mostly responsible for acting as a coenzyme to carboxylase, an enzyme required for the synthesis of proteins like prothrombin involved in blood clotting and bone metabolism. It actually helps create 4 of the 13 proteins necessary for blood clotting, which prevents wounds from continuously bleeding.
Without blood clotting, wounds, cuts, and other injuries would not heal properly!
Because of this main function and the limited storage capacity of vitamin K, it is more common to assess adequacy by measuring prothrombin time (PT) rather than blood circulation. Prothrombin time refers to how long it takes blood to clot. And whereas minor changes in vitamin K intake rarely affect PT, significant acute changes can alter PT and would warrant medical intervention.
Also important, vitamin K can dangerously interact with anticoagulant (blood-thinning) medications like warfarin (Coumadin), phenprocoumon, and more. These drugs antagonize vitamin K, therefore depleting it.
Thus, it is important for anyone taking anticoagulants to learn how to effectively maintain vitamin K status through food and supplementation in some instances.
Coronary Heart Disease
At least one vitamin K-dependent protein (MGP) plays a role in reducing the risk of heart disease and related conditions - like elevated cholesterol by reducing vascular calcification, or hardening of arteries. This is very important because this hardening can lead to weakened aortic and arterial elasticity, which can cause plaque build-up and tissue death.
Even though the complete biological function is not fully clear yet, research shows that vitamin K deficiency might increase the hardening of arteries. Medical professionals suggest this is particularly relevant for those with chronic kidney disease, as their rates of vascular calcification/arterial hardening are much higher than the general population.
Interestingly, just like vitamin C, vitamin K supplementation does not appear to reduce arterial hardening as significantly vitamin K intake from whole foods.
Still, other research showed that vitamin K2 supplementation slightly reduced the risk for coronary heart disease. On the other hand, vitamin K1 supplementation exerted no effects on the reduction of vascular calcification.
Just like vitamin K serves as a cofactor for the carboxylation of MPG discussed above, it also helps carboxylate or produces osteocalcin, a main protein in bone.
Osteocalcin helps prevent the weakening of bones, generally known as osteoporosis. In fact, some studies show that higher vitamin K intake is linked to a reduced risk for hip fractures and osteoporosis.
Conversely, insufficient or lower vitamin K status is associated with lower bone density and thus, increased risk for fractures. One study revealed that those who obtain slightly more than the recommended AI for vitamin K have a 30% less likely risk of breaking a hip.
Also, those who eat leafy greens daily cut their hip fracture risk in half compared to those who only obtain one serving of greens per week!
Even so, research once again shows that vitamin K supplementation may not be as effective at maintaining healthy bones as getting enough from food.
Vitamin K Supplementation
Speaking of supplementation, taking an isolated vitamin K supplement is pretty rare. Vitamin K is present in many multivitamins, but these often contain less than 75% of the recommended daily value.
Vitamin K is available in isolated form. But more often than not, it is combined with other specific nutrients like vitamins A, D, and E, calcium, and magnesium, or a combination of these to be helpful.
Both K1 and K2 are used in dietary supplements as phylloquinone or phytonadione and MK-4 or MK-7 respectively. Although not much data surrounds the different bioavailability of the specific types, some research shows K2 circulates in the blood longer than K1, therefore may be better utilized by tissues.
Another limited study suggested phytonadione and MK-7 are better absorbed than their counterparts, but more research is needed.
Despite the limited research concerning the bioavailability of different forms, most health professionals consider K2 as the gold standard. If taking vitamin K supplements, it is likely better to choose one with a K2 form.
Medication Interactions of Vitamin K
Note again that anyone taking anticoagulants needs to closely monitor vitamin K intake as these medications can reduce vitamin K absorption and activity.
In addition, long-term antibiotic use can reduce and even destroy vitamin K-producing bacteria in the gut. Supplementation is not generally needed with the short-term use of antibiotics unless intake of this nutrient is very low.
Bile acid sequestrants or cholesterol-lowering medications can reduce the absorption of vitamin K and other fat-soluble vitamins, although the clinical significance is still unclear. Still, those on these medications should ensure adequacy by eating a serving of leafy greens per day.
Finally, the over-the-counter and prescription versions of Orlistat, a weight loss drug, can decrease the absorption of vitamin K because it reduces dietary fat absorption in general. It is recommended to take a multivitamin with K2 if taking this medication and necessary if taking this medication along with warfarin.
Vitamin K Deficiency
Vitamin K deficiency is assessed based on prothrombin time. If PT is significantly increased, that means the vitamin K deficiency is clinically important. If not, then the deficiency is interestingly rarely considered significant or important in terms of general health.
As mentioned, vitamin K deficiency is pretty rare because the body can endogenously produce it. In fact, for the average healthy person, it is nearly impossible to have a low enough vitamin K status to alter PT.
However, deficiency is slightly more common at birth through the first couple weeks of infancy due to the low placental transfer of K2, low clotting factor levels, and low vitamin K content of breast milk.
Moreover, those with nutrient absorption issues like cystic fibrosis, Celiac disease, ulcerative colitis, and short bowel syndrome are also at higher risk of deficiency.
Signs and Symptoms of Deficiency
• Increased prothrombin time (PT)
• Excessive bleeding
• Weakened bones or osteoporosis
• Uncontrollable hemorrhaging
Vitamin K Toxicity
Finally, vitamin K toxicity is so rare that it is hardly discussed in the literature. Because the body regulates it endogenously and it’s partially excreted via urine and stool, it’s not likely to accumulate in toxic amounts within the body.
For some time in the '80s and '90s, supplements contained the form of vitamin K, menadione, which did increase the risk of toxicity and resulted in symptoms of jaundice, hyperbilirubinemia, anemia, and brain damage in infants.
After this was discovered, menadione was ruled unsafe for human consumption and has since been banned from supplements to prevent toxicity concerns.
The Takeaway on Vitamin K
From clotting blood to supporting bone health, vitamin K is one powerful vitamin wearing many health hats. Overall vitamin K deficiencies and toxicities are rare, though the vitamin is known to interact with many medications - especially blood thinners.
The biggest takeaway from vitamin K is that a cup of leafy greens a day keeps the doctor away – just be sure to add a healthy fat source to increase absorption!
Eske J. Vitamin K-2: Functions, Sources, Benefits, and Deficiency Symptoms. Medical News Today, MediLexicon International. Written April 29, 2019. https://www.medicalnewstoday.com/articles/325059#_noHeaderPrefixedContent.
Vitamin K. NIH Office of Dietary Supplements. Updated March 29, 2021. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/.
Pearson K. Vitamin K1 vs K2: What's the Difference? Healthline. Updated January 22, 2022. https://www.healthline.com/nutrition/vitamin-k1-vs-k2#TOC_TITLE_HDR_5.
Vitamin K. The Nutrition Source. https://www.hsph.harvard.edu/nutritionsource/vitamin-k/.