Vitamin K: Benefits, How to Get It, and How Much You Need

In 1916, a chemist named Cornelia Kennedy left her job teaching “domestic science” to female students at the University of Minnesota for a research position at the University of Wisconsin, where her advisor was trying to figure out how to put together healthy animal feeds. In a paper on malnourished pigeons, Kennedy wrote of two chemicals, which she called “fat-soluble A and water-soluble B.” And just like that, we had a naming system for these newly discovered nutrients, which came to be known as vitamins.

You’re undoubtedly familiar with the first five vitamins to be identified and named: A, B, C, D, and E. But then what happened? The next (and as it turned out, final) alphabetic vitamin is K, seemingly an insignificant afterthought. What happened to F, G, H, I, and J? Did scientists not know the alphabet song? Is vitamin K really not a big deal? 

What Is Vitamin K?

Fat-soluble vitamins are generally stored in the liver and fatty tissues rather than flushed out in urine. Vitamin K is the exception, as we’ll see in a moment. What does hold true is that you absorb vitamin K best when you eat some dietary fat alongside it.

Vitamin K is primarily involved in blood clotting, bone metabolism, and building proteins. It works in your liver and other tissues, including your brain, pancreas, heart, and skeletal system. 

Unlike the other fat-soluble vitamins, only very small amounts of vitamin K circulate in your blood at any given time. In fact, if you take a one-time supplemental dose of vitamin K, your body keeps about 30–40% to use and store and gets rid of the rest. Your body has limited storage for vitamin K, so it also recycles and reuses some of it multiple times. 

Discovered in 1929, it was named vitamin K not as part of the alphabet scheme, but because Danish researcher Henrik Dam found that without it, chickens fed a fat-free diet for a few weeks developed severe hemorrhages and their blood was very slow to clot. He called the discovery the “koagulation” vitamin (using the German spelling), or vitamin K. 

And why not vitamin F, the next letter in line? It turns out that F, G, and H had been used until the chemical compounds they referred to were found not to be actual vitamins: F turned out to be the omega fatty acids; G was discovered to be part of the B vitamin group (B2, to be exact), and H was reclassified as biotin (or B7). So by the time Henrik Dam cured chicken bleed-out with hemp seeds, he was free to choose any letter he wanted. 

Types of Vitamin K

Vitamin K comes in two distinct types. The first is vitamin K1, or phylloquinone, which is primarily found in leafy green veggies like spinach, kale, and cabbage. This is the main form of vitamin K in the diet, yet some research suggests people absorb only a fraction of the vitamin K1 they get from leafy greens compared with a supplement. A small 1999 trial found absorption from raw spinach, lettuce, and broccoli was roughly one-sixth that of an equivalent dose in tablet form.

The second type is vitamin K2, or menaquinone, which actually covers a family of related forms. The longer-chain ones are made by bacteria, including bacteria in your own gut, though what your bacteria produce in your colon is poorly absorbed and probably contributes less to your K2 status than you’d think. K2 is found in certain fermented soy foods (especially natto, a Japanese dish made from soybeans), and animal products like butter, beef liver, aged cheese, and egg yolks.

Fortunately, your body isn’t simply waiting around for you to eat the right form. In 2010, researchers identified the human enzyme that converts K1 from your food into MK-4, a form of K2, right in your own tissues. It’s called UBIAD1, and it’s why MK-4 turns up concentrated in your brain, kidney, and pancreas. Studies in germ-free animals showed the conversion doesn’t even require gut bacteria. Eat a salad and your body handles part of the chemistry for you.

That conversion produces MK-4 rather than MK-7, the longer-lasting form found in natto and fermented foods, so it isn’t the whole story. And it depends on getting the K1 in the first place. Because vitamin K is fat-soluble, absorption requires bile, so conditions affecting your gut, liver, or gallbladder can limit how much you take in, and eating some fat alongside your greens helps.

Vitamin K Health Benefits

Both vitamin K1 and K2 are important for maintaining your overall health. Below are a few of the best-known health benefits of both forms of vitamin K. 

Helps with Blood Coagulation

Memorialized by its letter name, coagulation (or “koagulation”) is the best-known role of vitamin K in your body. Both vitamins K1 and K2 are involved in blood clotting, which is what keeps you from bleeding to death from a paper cut. Specifically, vitamin K is needed to manage proteins involved in blood clotting. If you don’t get enough vitamin K, you are more likely to experience hemorrhaging, or excessive bleeding, from a wound. 

Good for Your Bones

Inadequate vitamin K intake has been linked to an increased risk of osteoporosis — a disease in which bones become soft and porous — and bone fractures. Both vitamins K1 and K2 may also help with regulating calcium balance, which maintains skeletal strength. 

A 2006 meta-analysis, mostly of trials of older Japanese adults, reported that a form of vitamin K2 called menaquinone-4 reduced hip fractures by 77%, spinal fractures by 60%, and non-spinal fractures by 81% in Japanese trials. Those numbers sound amazing — like everyone should be supplementing to keep their bones healthy as they age.

But 13 years later, researchers reran the numbers, stating that “the integrity of some of the previous evidence has been questioned.” This time they found a much more modest effect: roughly a 28% reduction in clinical fracture risk. Notably, when they restricted the analysis to the best-conducted trials, the effect was no longer statistically significant, and vertebral fractures showed no benefit at all.

All that said, it could be that vitamin K2 is most effective for bone health when combined with certain other nutrients (though the metaphorical jury’s still out on that, too).

As scientists love to say at the end of their papers, “more research is needed” before recommending that people with osteoporosis worries should necessarily take a K2 supplement. It may have benefits. We just can’t say for sure.

Vitamin K2-Specific Health Benefits

There are also some health benefits that seem to be tied only to vitamin K2.

May Reduce Your Risk of Heart Disease

Calcium buildup in the arteries is a serious risk factor for heart disease. Vitamin K2 may help prevent this calcification. A 2009 observational study of more than 16,000 women found that those who consumed more vitamin K2 had a lower risk of coronary heart disease. Specifically, every 10-microgram-per-day increase in K2 intake was associated with a 9% lower risk. 

For you statistics fans out there, I need to add that this finding sits at the edge of statistical significance. For non-statistics fans, this means that the data could point to a real pattern, but it could also be due to chance. For you New York Jets fans, this means nothing — keep the faith.

May Support Brain Health

Vitamin K2 participates in processes that help protect your brain as you age. For instance, it plays a role in the production of compounds called sphingolipids, which are highly concentrated in the brain (and which sound like a lot of fun!). Changes in the metabolism of sphingolipids have been linked to neurodegenerative diseases like Alzheimer’s. Recent evidence also suggests that vitamin K2 may help protect against oxidative stress and inflammation, and may influence psychomotor behavior and brain function. 

May Have Anticancer Benefits

A few studies have indicated that vitamin K2 may help reduce the recurrence of liver cancer and increase survival. And in a 2008 study published in the American Journal of Clinical Nutrition, researchers found that men with a high intake of vitamin K2 had a 63% lower risk for advanced prostate cancer.

How Much Vitamin K Do You Need?

Vitamin K is obviously doing some important work in your body, so how much should you be getting? Here the picture gets murky. Most health authorities set recommendations for K1 only and leave K2 out entirely.

The U.S.-based National Academy of Medicine puts the daily adequate intake for vitamin K1 from foods at 90 micrograms for women and 120 micrograms for men. According to survey data collected in 2011–2012, only 43% of men and 63% of women reached the adequate intake level, with older men reporting the lowest intakes.

The World Health Organization recommends significantly less K1, suggesting, on average, 55 micrograms per day for women and 65 micrograms per day for men.

There are no official recommendations for K2 specifically, and the honest reason is that we still don’t know how much it matters. K2 stays in circulation longer than K1 and reaches bone and blood vessels more readily, which is why it’s the form studied for those tissues. And there’s some evidence it performs better alongside calcium and vitamin D than on its own. But the strongest test of that combination found nothing. In a 3-year trial, 142 postmenopausal women with osteopenia all received calcium and vitamin D3, and half also took 375 micrograms of MK-7 K2 daily, which is a larger dose than you’ll find in a typical supplement. The K2 unmistakably did its biochemical job: The inactive form of a bone protein called osteocalcin dropped by 65% in the K2 group and not at all in the placebo group. However, after three years, there was no statistically significant difference in bone density between the two groups.

I’d say that’s a puzzle rather than a dead end. It’s also a reason to be careful about assuming that a K2 capsule will protect your bones. There’s reason to theorize that it might, but we don’t know.

The closest guidance comes from cohort studies linking K2 intake to cardiovascular health. Participants in those studies took in an average of 30 micrograms of K2 per day, which describes what people ate rather than a recommendation per se. A teaspoon of natto clears it comfortably. So do a number of different animal products, or a modest supplemental dose.

Vitamin K Deficiency

While deficiency of vitamin K1 is rare, it’s possible in the event of severe malnutrition. People suffering from gastrointestinal disorders, or who have trouble absorbing fat, may be at risk of K1 deficiency. Other at-risk groups include babies who are breastfed by mothers who are vitamin K deficient, and adults taking coumarin anticoagulants such as warfarin (sold under brand names including Coumadin). This is because these drugs can interfere with the synthesis of vitamin K-dependent coagulation proteins in the liver. Members of these groups may need their vitamin K status monitored and may require supplementation, but should discuss this with a qualified health care provider first.

Vitamin K2 deficiency is harder to pin down because it doesn’t produce obvious short-term symptoms. Its effects are slow and show up in bone and vascular health rather than in bleeding, which makes it easy to miss.

Vitamin K-Rich Foods

Some of the best vitamin K1-containing foods are: 

• Leafy greens, such as spinach, mustard greens, and collard greens
• Cruciferous vegetables, such as Brussels sprouts, kale, and broccoli
• Asparagus
• Prunes
• Peas
• Parsley
• Scallions

Plant foods that contribute vitamin K2:

• Natto, which is by far the highest plant-based source of vitamin K2
• Tempeh
• Sauerkraut
• Kimchi
• Fermented pickles

Animal products, like butter, egg yolks, organ meats, and high-fat dairy (more so if they are grass-fed or pasture-raised), are all high in vitamin K2. If you eat animal foods, these are potential K2 sources. If you’d rather get your K2 from plants, natto is by far the most concentrated option, and a supplement is a reasonable fallback.

Natto is a dish traditionally served for breakfast in Japan, consisting of soybeans fermented with natto bacillus. According to many Westerners, it’s an acquired taste. Some natto fans claim it tastes a bit like ripe brie cheese, and may even have overtones of bacon. You can find natto at specialty food stores and online, or you can be adventurous and make your own. A single teaspoon provides 50 micrograms of vitamin K2. There is also some vitamin K2 in sauerkraut, tempeh, and other fermented foods, though amounts vary, so it’s hard to know how much you’re getting.  

3 Vitamin K-Rich Recipes

You’ll see in the recipes below that getting vitamin K1 and K2 on a plant-based diet can be delicious. Easy-to-prepare, tasty dishes like Natto Black Rice Bowl with Veggies, Spinach Artichoke Dip, and Tempeh Italiano Farro and Spinach provide nutrients your body needs and loves to support heart, bone, and cognitive health. 

1. Natto Black Rice Bowl with Veggies 

Ready to open your culinary mind (and kitchen!) to something new, adventurous, and worldly? Meet natto! This Japanese staple hasn’t (yet) made its way to mainstream Western cuisine, but it deserves to. While you may not be super familiar with natto, what you probably do know well are soybeans. Natto is simply fermented soybeans. But during that fermentation process, something magical happens — vitamin K2 is created. The Natto Black Rice Bowl is your chance to experiment with this nutrient-packed food. We hope you love it!

2. Spinach Artichoke Dip

You can get an abundance of vitamin K1 in this tasty appetizer or snack. If you’re looking for a creamy, savory, and tasty treat that delivers a bundle of nutrients — then try out this mouth-watering Spinach Artichoke Dip!

3. Tempeh Italiano Farro and Spinach

You’ll find both vitamins K1 and K2 in this nutrient-dense grain bowl. Also, farro is packed with fiber (five grams in just ¼ cup of the dry grain!), which helps to cultivate diverse, healthy bacteria in your gut. Not to mention, this delicious dish is sure to please the entire family!

Vitamin K Testing

Testing for vitamin K1 deficiency is not typically done routinely, except in the case of severe bleeding or hemorrhaging, which can be a symptom of severe deficiency. The primary lab test used to evaluate vitamin K1 deficiency is prothrombin time (PT). It measures how long it takes your body to form blood clots and stop active bleeding. If the result of the PT test is prolonged — meaning that blood clotting takes longer than it should — and is suspected to be a result of low vitamin K1, then vitamin K1 supplementation is typically prescribed. 

While vitamin K1 tests focus on blood clotting and coagulation, vitamin K2 tests focus on bone health markers. Testing for vitamin K2 is more difficult because it’s hard to measure directly. But labs can measure osteocalcin, a protein found in bones and teeth that vitamin K2 activates. If you can measure the level of activation of this vitamin K2-dependent protein, you get a rough idea of your vitamin K2 status.

In the U.S., both LabCorp and Quest Diagnostics offer Osteocalcin tests. 

Should You Take a Vitamin K Supplement?

Vitamin K1 is found widely in foods. If you eat a decent amount of leafy vegetables, you’re almost certainly getting more than you need, and as we’ve seen, your body converts some of it to MK-4 K2 on its own. So for many people, the answer here is probably no.

There are some real exceptions, though. A supplement is worth at least discussing with your health care provider if:

• You have a condition that interferes with fat absorption, such as celiac disease or inflammatory bowel disease. Vitamin K is fat-soluble, so absorption problems affect it directly.
• You take a bile acid sequestrant or orlistat long-term, or you’ve been on broad-spectrum antibiotics for an extended stretch. See the section below.
• You eat few leafy greens. There’s no K1 foundation for your body to build on.
• You eat plant-based, and natto isn’t part of your life. You’re likely getting very little MK-7 from any source. That’s a potential gap, even though we can’t yet say how much it costs you.
• You’re postmenopausal or have been diagnosed with osteoporosis. This is the unresolved category, and the reason to raise it with a provider rather than either supplement reflexively or dismiss the idea.

And if you take an anticoagulant, don’t add or drop vitamin K without talking to whoever manages your prescription. Consistency matters more than quantity.

What Else to Know Before You Supplement

Vitamin K1 from food and from supplements has no established upper limit, because no toxic dose has been identified even at large intakes. There’s no evidence of toxicity from K2 either. This is unusual among the fat-soluble vitamins.

The one form with a documented safety problem is menadione, sometimes labeled vitamin K3. It’s a synthetic, water-soluble precursor that the body converts to usable vitamin K, and in infants it has caused red blood cell destruction, jaundice, and, in severe cases, brain damage. It isn’t sold in human supplements in the U.S., but it does appear in some animal feeds and in supplements sold elsewhere. If a label says menadione or vitamin K3, avoid it. Look for K1 (phylloquinone) or K2 (as MK-7 or MK-4) instead.

A handful of medications can also interfere with your vitamin K status:

• Anticoagulants like warfarin, as described above. This is the interaction that matters most.
• Broad-spectrum antibiotics, taken for extended periods, can reduce the gut bacteria that produce some of your vitamin K. Short courses are unlikely to matter much.
• Bile acid sequestrants, prescribed to lower cholesterol, reduce absorption of fat-soluble vitamins including K.
• Orlistat, a weight-loss drug that works by blocking fat absorption, can reduce vitamin K absorption for the same reason.

If you’re taking any of these long-term, it’s worth asking your health care provider whether your vitamin K status is something to keep an eye on. And as with any supplement, it’s a good conversation to have before you start, not after.

Vitamin K2 and Vitamin D

Vitamin D, another fat-soluble vitamin, works closely with vitamin K to perform various bodily functions, like regulating calcium metabolism for bone and heart health. 

Vitamin K2 may also boost the absorption of vitamin D. While having high circulating levels of vitamin D can result in excessive calcium buildup in your blood, vitamin K2 appears to help make sure the calcium is used for your bones instead of ending up in your arteries and causing potential heart health issues. 

How does this happen? After vitamin D helps transport calcium into your blood, vitamin K2 activates osteocalcin, a protein that moves the calcium from your blood to your bones and teeth. 

This is a well-established biochemical partnership, which is why vitamin D and K2 are often sold together.

Get Your Coagulation Vitamin 

Vitamin K comes in two forms, and both are involved in blood clotting and in the proteins that manage calcium. What separates them is where they go. K2 stays in circulation longer and reaches bone and blood vessels more readily, which is why it’s the form researchers study for those tissues. That research is both promising and unsettled.

For most people, though, vitamin K may be the most forgiving nutrient in the whole alphabet. No established upper limit, no identified toxic dose, and K1 comes packaged in food that’s good for you for dozens of other reasons. Your own enzymes handle part of the conversion work. If you’re eating leafy greens and cruciferous vegetables with any regularity, and none of the exceptions above apply to you, this is one nutrient you can mostly stop worrying about.

So eat your greens, try the natto if you’re feeling adventurous, and if you’re plant-based or in one of those exception groups, make your K2 status a question you actually ask rather than one you guess at.

Get Your Coagulation Vitamin

Vitamin K is an essential fat-soluble vitamin that comes in two forms, vitamin K1 and K2. K1 primarily has benefits for blood clotting, while K2 has additional benefits for bone, heart, and brain health, as well as potential cancer prevention. Although the recommended daily intake for vitamin K is only based on vitamin K1, K2 supplementation may be recommended to avoid deficiency, especially for plant-based eaters, since its plant-based sources are limited.

Tell us in the comments:

• What food sources of vitamin K1 and/or K2 do you currently eat? What are some sources of vitamin K2 you could add?
• Have you ever had your vitamin K or osteocalcin levels checked?
• Do you take any supplements containing vitamin K2?

Feature image: iStock.com/Gargonia

Read Next:

• Vitamin D Guidelines: What Is It, How Much Do You Need, & How to Get Enough
• 5 Key Supplements for Vegans and Vegetarians to Thrive on A Plant-Based Diet