Leafy green vegetables, a main source of vitamin K1

Vitamin K1 and K2 (MK-4 and MK-7): History, Warfarin, Newborn Shots, and What the Research Really Shows

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Not medical advice. This guide is for general information. Talk to your doctor, pharmacist, or other clinician before taking vitamin K, especially if you take warfarin (Coumadin, Jantoven) or another blood thinner, are pregnant, have kidney or liver disease, or have a condition that affects fat absorption. If you take warfarin, do not start, stop, or change a vitamin K supplement, or make big changes to how many leafy greens you eat, without talking to the clinician who manages your INR. Questions about your newborn’s vitamin K shot belong with your pediatrician or midwife.

Vitamin K is the vitamin most people only hear about twice: when a baby gets a shot in the delivery room, and when someone starts taking warfarin and gets told to watch the spinach. For decades, that was more or less the whole story. Vitamin K made blood clot, and that was its job.

That picture has changed. Researchers now know that vitamin K activates a whole family of proteins, some of them in bone and in the walls of blood vessels. One of those proteins keeps arteries from turning to stone in mice. Another is one of the most common proteins in bone. And the vitamin itself turned out to be not one molecule but a family: K1 from plants and a group of K2 forms, called menaquinones, from animal foods and bacteria.

That shift opened the door to big claims. Vitamin K2 is now sold as the missing partner to vitamin D, the nutrient that “steers calcium into bones and out of arteries.” Some of that is backed by real biology and a handful of decent trials. Some of it runs well ahead of the evidence.

This guide covers the history, how vitamin K works, the difference between K1, MK-4 and MK-7, how much you need, the warfarin interaction (in detail, because it matters most), newborn vitamin K shots, what the research shows for bones and arteries, lesser-known research, and safety.

The short version

  • Vitamin K is a cofactor, not a hormone. It lets one enzyme add a chemical “claw” to certain proteins so they can grab calcium. Without it, clotting factors don’t work and you can bleed.
  • There are two natural types. K1 (phylloquinone) comes mostly from green leafy vegetables and some plant oils. K2 (menaquinones) comes from fermented foods, cheese, and animal foods. MK-4 and MK-7 are the two K2 forms you’ll see on supplement labels.
  • Official intake: the US Adequate Intake is 120 mcg a day for men and 90 mcg for women. There is no upper limit, because no harm from food or supplement vitamin K has been reported in healthy people.
  • Warfarin is the big exception. Vitamin K directly counteracts warfarin and similar drugs. Even 10 to 45 mcg a day of MK-7, less than many K2 capsules, was enough to change blood clotting test results in volunteers on a warfarin-type drug.
  • Every newborn needs it. Babies are born with very little vitamin K. A single shot at birth is linked to a roughly 98% lower risk of late vitamin K deficiency bleeding, which can cause bleeding in the brain. An old cancer scare about the shot was not confirmed in larger studies.
  • Bones: mixed. Japanese trials of very high-dose MK-4 (45 mg a day) reported fewer fractures, but some of that evidence has been questioned, and Western trials of K1 and MK-4 mostly found no change in bone density. One three-year Dutch trial of 180 mcg MK-7 found slower bone loss.
  • Arteries: promising but unproven. People who eat more K2 had less artery calcification and fewer heart deaths in the Dutch Rotterdam study. Trials of supplements have been mixed. A 2026 trial found MK-7 slowed the rise in coronary calcium scores, but nobody yet knows whether that prevents heart attacks.
  • Vitamin D plus K2 makes biological sense, and a few small trials are encouraging, but the largest combined trial so far found no effect on heart valve calcification.

A short history of vitamin K

Dam’s bleeding chicks

The discovery was an accident. Between 1928 and 1930, the Danish biochemist Henrik Dam was studying cholesterol metabolism in chicks at the University of Copenhagen. He fed them a diet stripped of sterols to see whether they could make their own cholesterol. They could. But something else happened: chicks kept on the diet for more than two or three weeks developed bleeding under the skin and in their muscles, and their blood was slow to clot (Dam, 1946).

Dam ruled out the obvious suspects one by one. Adding cholesterol didn’t stop the bleeding. Neither did linseed oil, lemon juice, or injected vitamin C. Diets high in cereals and seeds did. In 1934 he announced that the disease came from the lack of an unknown dietary factor. Green leaves and hog liver turned out to be among the richest sources, and the factor dissolved in fat (Dam, 1946).

Why “K”?

In 1935, Dam described the factor as a new fat-soluble vitamin and called it vitamin K. In his 1946 Nobel lecture he explained the choice: K was the first letter of the alphabet not already claimed by another vitamin, “and it also happened to be the first letter in the word ‘koagulation’ according to the Scandinavian and German spelling” (Dam, 1946).

Dam wasn’t alone. Canadian workers had seen bleeding in chicks fed diets made with ether-extracted fish or meat meal in 1931 but didn’t follow it up. Almquist and Stokstad at the University of California, Berkeley, quickly confirmed Dam’s findings and worked on purifying the factor (Dam, 1946).

The Nobel Prize

In 1939, Dam, the chemist Paul Karrer and their co-workers reported isolating pure vitamin K from green leaves. The American biochemist Edward Doisy and his group, along with a group led by Fieser, worked out its chemical structure. Doisy also prepared a second form from putrefied fish meal, which became known as vitamin K2 (Dam, 1946).

The 1943 Nobel Prize in Physiology or Medicine was split between them: Dam “for his discovery of vitamin K” and Doisy “for his discovery of the chemical nature of vitamin K.” The prize was reserved in 1943 and awarded in 1944 (Nobel Prize, 1943). Dam gave his lecture in December 1946, after the war.

From surgery to newborns

The first human use was in surgery. Patients with obstructive jaundice, from gallstones or tumors, often bled dangerously during operations. In 1938, three groups independently showed that this bleeding came from vitamin K deficiency, because bile is needed to absorb it (Dam, 1946). Giving vitamin K before surgery greatly lowered that risk.

Doctors also learned that newborns routinely have low clotting activity in their first week. In his Nobel lecture, Dam pointed to poor transfer of the vitamin across the placenta, the sterile gut of a newborn, and the low vitamin K content of milk (Dam, 1946). That observation is the root of today’s newborn vitamin K shot.

Sweet clover disease and the birth of warfarin

At the same time, a mystery was unfolding on North American farms. In the 1920s, cattle in the northern prairies began dying of internal bleeding. Veterinarians Frank Schofield and Lee Roderick traced it to spoiled, moldy sweet clover hay; the bleeding stopped when the moldy hay was removed or the animals got fresh blood (Lim, 2017).

Years later, with the disease still common, a Wisconsin farmer named Ed Carlson drove about 200 miles to an agricultural experiment station with a dead cow and a milk can of unclotted blood. He landed in the lab of the biochemist Karl Paul Link. By 1940, after six years of work, Link’s team had isolated the culprit: dicoumarol, formed when natural coumarin in the hay oxidized (Lim, 2017).

Link later looked for a stronger version to use as rat poison. Compound number 42, out of about 150 variations, was named “warfarin” after its funder, the Wisconsin Alumni Research Foundation (WARF), and went on sale as a rodenticide in 1948. After a man survived a warfarin suicide attempt in 1951 with vitamin K treatment, researchers began studying it as a medicine. It was approved for people in 1954 as Coumadin, and President Dwight Eisenhower took it after his 1955 heart attack (Lim, 2017). It went from a cattle disease to a rat poison to a widely prescribed drug, a classic story of accidental discovery (Wardrop & Keeling, 2008).

How warfarin actually works wasn’t clear until 1978, when John Suttie’s group showed that it blocks an enzyme that recycles vitamin K (Lim, 2017). In 2004, two teams identified the gene for that enzyme, VKORC1 (Rost et al., 2004; Li et al., 2004).

Gla: the chemical secret

For decades, nobody knew what vitamin K actually did to clotting proteins. The answer came in 1974. Johan Stenflo’s group and Gary Nelsestuen’s group found an unusual amino acid, gamma-carboxyglutamic acid (Gla), in prothrombin, the clotting protein. Prothrombin made in animals given the anticoagulant dicoumarol lacked Gla and could not bind calcium (Stenflo et al., 1974; Nelsestuen et al., 1974).

That discovery opened the field. Researchers soon found Gla-containing proteins outside the blood. Osteocalcin, a small protein made by bone-building cells, was the first vitamin K-dependent protein found in bone, and new ones were still being identified in 2008 (Ferland, 2012; Hauschka, 1986). Vitamin K was no longer just “the clotting vitamin.”

Weston Price and “Activator X” (history and practitioner lore)

Any K2 discussion online eventually reaches Weston A. Price, an American dentist who studied the teeth and diets of traditional, nonindustrialized peoples. In the 1945 edition of his book Nutrition and Physical Degeneration, according to Masterjohn’s account, Price added a chapter describing a fat-soluble “activator” he found in butter from cows grazing on rapidly growing green grass, in organ meats, and in fish eggs. He believed it helped the body use minerals and protected against tooth decay. He never identified it chemically, so he called it “Activator X” (Masterjohn, 2007).

In 2007, the writer Christopher Masterjohn, in an article for the Weston A. Price Foundation’s magazine, argued that Activator X was vitamin K2. His case rested on three points: Price’s chemical test (which measured a food’s ability to release iodine) would detect K vitamins; Price’s richest sources (butterfat and organs from grass-fed animals) match foods that contain K2; and the effects Price described resemble what is now known about K2 (Masterjohn, 2007).

How to read this: it’s a plausible historical hypothesis, not an established fact. Price’s test wasn’t specific to any one compound; Masterjohn’s own article notes it would also pick up other quinones such as coenzyme Q10. Price’s reports of reversing tooth decay came from his own clinical observations, without controls. The Activator X idea is popular in traditional-diet and integrative circles, and it helped revive interest in K2, but nobody has tested it directly. Masterjohn also published a related hypothesis that vitamin D toxicity works partly by using up vitamin K; that, too, remains untested in humans (Masterjohn, 2007).

How vitamin K works

One enzyme, one job

Vitamin K’s best-understood job is to help a single enzyme, gamma-glutamyl carboxylase (GGCX). That enzyme adds a carbon dioxide group to specific glutamic acid units in certain proteins, turning them into Gla. Gla units carry a double negative charge, which lets these proteins bind calcium and do their work (Shearer & Okano, 2018; Shearer & Newman, 2014).

Each time vitamin K helps add one Gla, it gets used up, converted to a form called vitamin K epoxide. A second enzyme, vitamin K epoxide reductase (VKOR), turns it back into the active form so it can be used again. This loop is called the vitamin K cycle. It lets the body get by with tiny stores: the body holds much less vitamin K than other fat-soluble vitamins, and mice lacking VKOR die of bleeding shortly after birth (Shearer & Newman, 2014). Warfarin works by blocking VKOR, which is why it causes the body to run short of usable vitamin K (Rost et al., 2004).

The vitamin K-dependent proteins

A 2012 review counted 17 known Gla proteins (Vermeer, 2012; NIH ODS, 2026). The best known:

Protein Where What it does
Four clotting factors, including prothrombin (factor II) Made only in the liver Blood clotting
Osteocalcin Bone (made by bone-forming cells) Linked to bone strength; exact role still debated
Matrix Gla protein (MGP) Blood vessel walls, cartilage Helps stop calcium building up in soft tissue
Gas6 Many tissues Cell growth regulation

Matrix Gla protein: the artery guard

The strongest evidence that vitamin K matters outside the blood comes from MGP. In 1997, researchers bred mice without the MGP gene. They looked normal at first, but they all died within two months because their arteries filled with calcium and ruptured (Luo et al., 1997).

A year later, researchers showed that high-dose warfarin calcified the arteries and heart valves of rats, much like the MGP-deficient mice (Price et al., 1998). In other rat studies, high doses of vitamin K (especially MK-4) prevented warfarin-induced calcification (Spronk et al., 2003), and a high-vitamin K diet even cut existing artery calcium by about 50% (Schurgers et al., 2007). Human results have been far less dramatic, as we’ll see.

MGP that hasn’t been activated circulates in the blood. Researchers measure a form called dp-ucMGP (dephospho-uncarboxylated MGP). Higher levels suggest lower vitamin K status in the blood vessels, and supplements reliably bring it down.

Osteocalcin

Osteocalcin has three Gla units and is made by bone-forming cells. Its exact role in bone is still debated (Hauschka, 1986). Normally, 10% to 40% of circulating osteocalcin and MGP isn’t fully carboxylated in healthy adults, which some researchers read as a sign that typical diets leave the bones and arteries short (Theuwissen et al., 2012).

The “triage” idea

Why would bones and arteries run short while blood clots normally? Cees Vermeer and others argued for a kind of triage. When vitamin K is scarce, the liver gets first call, so clotting factors are carboxylated while proteins in other tissues are not. Official intakes were set based on clotting, so, the argument goes, they may be too low for the rest of the body (Vermeer, 2012). This is a reasonable hypothesis, but it has not been proven with hard outcomes like fractures or heart attacks.

K1, K2, MK-4 and MK-7: what’s the difference?

All forms of vitamin K share the same ring (a naphthoquinone). They differ in the side chain attached to it (Card et al., 2014).

Vitamin K1 (phylloquinone)

K1 is made by plants, where it takes part in photosynthesis. It’s the main form in most Western diets. Leafy greens and a few plant oils (soybean, canola, cottonseed and olive) provide most of it; most other foods have less than 10 mcg per 100 grams (Booth & Suttie, 1998).

The catch is absorption. K1 is bound tightly in plant tissue. Absorption from spinach was only 4% to 17% of what people absorbed from a tablet, and eating fat with greens helps. Free K1 in supplements is absorbed at roughly 80% (NIH ODS, 2026). The body keeps only about 30% to 40% of an oral dose; the rest is broken down and excreted (NIH ODS, 2026).

Vitamin K2 (menaquinones)

Menaquinones are named MK-n, where n is the number of repeating units in the side chain, from MK-4 up to MK-13. MK-4, MK-7 and MK-9 are the most studied (NIH ODS, 2026).

MK-4 is different from all the others. It’s made in your own tissues, not by bacteria. Oral K1 is partly cleaved in the gut to menadione, which travels to tissues and is rebuilt into MK-4 (Hirota et al., 2013). The human enzyme that does this rebuilding is UBIAD1. MK-4 is concentrated in the brain, kidney and pancreas, and UBIAD1 isn’t blocked by warfarin (Nakagawa et al., 2010). MK-4 in foods comes from animal products: chicken, egg yolk, butter and cheese.

Supplement MK-4, sold as menatetrenone, is synthetic and chemically identical to the natural molecule (Iwamoto, 2014). It has a very short life in the blood. In one Japanese study, a single 420 mcg dose of MK-4 couldn’t be detected in serum at all, while MK-7 stayed detectable for 48 hours; 60 mcg of MK-4 daily for a week didn’t raise blood levels (Sato et al., 2012). That’s why MK-4 supplements are usually dosed in milligrams.

MK-7 comes mainly from bacterial fermentation. Natto, Japanese fermented soybeans made with Bacillus subtilis natto, is by far the richest source: about 850 mcg in 3 ounces (NIH ODS, 2026). Most MK-7 supplements are made by fermentation.

MK-7 stays in the blood much longer than K1. In a Dutch study, both peaked about 4 hours after a dose, but MK-7 had a very long half-life, built up to 7 to 8 times higher levels with daily use, and carboxylated osteocalcin more completely (Schurgers et al., 2007). That staying power is its selling point. It’s also why MK-7 interacts so strongly with warfarin (see below).

Longer menaquinones such as MK-8 and MK-9 come from cheese and other fermented dairy, and gut bacteria make many menaquinones too, though how much of that the body absorbs is unclear (NIH ODS, 2026).

Menadione (K3)

Menadione is a synthetic form without a side chain. It damaged liver cells in lab studies and is no longer used in dietary supplements (NIH ODS, 2026). You may still see it in animal feed.

Food sources, with amounts

Values below are from the NIH Office of Dietary Supplements and are K1 unless noted (NIH ODS, 2026).

Food Serving Vitamin K (mcg) Form
Natto 3 oz 850 MK-7
Collard greens, boiled ½ cup 530 K1
Turnip greens, boiled ½ cup 426 K1
Spinach, raw 1 cup 145 K1
Kale, raw 1 cup 113 K1
Broccoli, boiled ½ cup 110 K1
Soybeans, roasted ½ cup 43 K1
Soybean oil 1 tbsp 25 K1
Edamame ½ cup 21 K1
Iceberg lettuce 1 cup 14 K1
Chicken breast, roasted 3 oz 13 MK-4
Canola oil 1 tbsp 10 K1
Olive oil 1 tbsp 8 K1
Ground beef 3 oz 6 MK-4
Chicken liver 3 oz 6 MK-4
Cheddar cheese 1.5 oz 4 MK-4
Egg, hard boiled 1 large 4 MK-4
Milk, 2% 1 cup 1 MK-4
Salmon, sockeye 3 oz 0.3 MK-4

A few notes:

  • Natto is in a class of its own. One serving delivers several times the daily AI as MK-7. In a Dutch study, serum K2 after a natto meal was about 10 times higher than serum K1 after a spinach meal (Schurgers & Vermeer, 2000).
  • Cheese adds longer menaquinones (MK-8 and MK-9), which aren’t fully captured in the US table above. In the Dutch and German studies discussed below, cheese was a major K2 source (Walther et al., 2013; Nimptsch et al., 2010).
  • Most people get far more K1 than K2. In a Dutch cohort of 16,057 women, average intake was about 212 mcg of K1 and 29 mcg of K2 a day (Gast et al., 2009).

How much vitamin K do you need?

Adequate Intakes (AI)

In 2001, the US Food and Nutrition Board found too little data to set a Recommended Dietary Allowance, so it set Adequate Intakes instead, based on what healthy people typically eat (Shearer et al., 2012; NIH ODS, 2026):

Age Male Female Pregnancy Lactation
0–6 months 2.0 mcg 2.0 mcg
7–12 months 2.5 mcg 2.5 mcg
1–3 years 30 mcg 30 mcg
4–8 years 55 mcg 55 mcg
9–13 years 60 mcg 60 mcg
14–18 years 75 mcg 75 mcg 75 mcg 75 mcg
19+ years 120 mcg 90 mcg 90 mcg 90 mcg

The Daily Value on US labels is 120 mcg (NIH ODS, 2026). Europe’s food safety agency set a lower AI in 2017: 1 mcg per kilogram of body weight, or 70 mcg a day of K1 for adults. It didn’t set any value for K2, saying the evidence was insufficient (EFSA, 2017).

Neither body set a separate target for K2, and neither set an upper limit (see Safety below).

How much do people get?

In US national survey data, average intake from food was about 122 mcg a day for women and 138 mcg for men, rising to 164 and 182 mcg with supplements (NIH ODS, 2026). Multivitamins usually contain less than 75% of the DV (NIH ODS, 2026).

Who may run low

True vitamin K deficiency, the kind that causes bleeding, is very rare in healthy adults (NIH ODS, 2026). Groups at higher risk include (NIH ODS, 2026):

  • Newborns, especially those who don’t get vitamin K at birth (more below)
  • People with malabsorption, including cystic fibrosis, celiac disease, ulcerative colitis, short bowel syndrome, and people who’ve had bariatric surgery
  • People on certain drugs that interfere with vitamin K, such as some antibiotics, bile acid sequestrants, and orlistat

The only clinically used sign of deficiency is a longer prothrombin time (a clotting test). Normal fasting plasma K1 runs about 0.29 to 2.64 nmol/L, but no lab test for K2 status is in routine use (NIH ODS, 2026).

Warfarin and vitamin K: the interaction that matters most

If you take warfarin (Coumadin, Jantoven) or a related drug, vitamin K is the single most important nutrient interaction in your life. This section is long on purpose.

Why they clash

Warfarin and its relatives (acenocoumarol, phenprocoumon and others) are called vitamin K antagonists. They block VKOR, the enzyme that recycles vitamin K, so the liver makes fewer working clotting factors (Rost et al., 2004; NIH ODS, 2026). Your dose is adjusted to keep your INR, a standardized clotting test, in a target range.

Eating or taking more vitamin K gives the liver extra raw material and can push your INR down, raising the risk of a clot or stroke. Suddenly eating much less vitamin K can push it up, raising the risk of bleeding. That’s why NIH ODS says people on these drugs should keep their vitamin K intake consistent from day to day, rather than avoid it (NIH ODS, 2026).

Consistency, not avoidance

Many people on warfarin were once told to cut out greens entirely. That isn’t the standard advice. The goal is a steady intake, so your dose is matched to your normal diet. If you usually eat a salad most days, keep doing it; just don’t suddenly start or stop.

There is even some evidence that a small, steady dose of vitamin K helps. In a UK trial of 70 people whose INR kept swinging out of range, adding 150 mcg of K1 a day increased their time in the target range by 28 percentage points, compared with 15 points on placebo (Sconce et al., 2007). That was done under medical supervision, with doses adjusted as needed. Don’t try it on your own.

MK-7 supplements are a special concern

Because MK-7 stays in the blood so long, small doses have outsized effects:

  • In a Dutch study of healthy volunteers on acenocoumarol (a warfarin-type drug), 45 mcg a day of MK-7 lowered INR by about 40%. Even 10 and 20 mcg a day caused a clinically relevant INR drop in at least 40% and 60% of participants. The authors concluded that MK-7 supplements should be avoided by people on these drugs (Theuwissen et al., 2013).
  • An earlier study warned that supplements providing 50 mcg a day or more of MK-7 may interfere with anticoagulant treatment in a clinically relevant way (Schurgers et al., 2007).

The study authors noted that 10 mcg is lower than the usual retail dose of 45 mcg, and the trials discussed below used 180 to 720 mcg a day. Vitamin D3+K2 combinations are common too. Check every supplement label for vitamin K, including multivitamins, bone formulas, and “heart health” blends. Some supplements contain as much as 4,050 mcg (NIH ODS, 2026).

What about newer blood thinners?

Apixaban (Eliquis), rivaroxaban (Xarelto), edoxaban and dabigatran (Pradaxa) work differently. They directly block specific clotting factors instead of interfering with vitamin K, and they have far fewer food interactions than warfarin (Mekaj et al., 2015; Verdecchia et al., 2016). Even so, ask your prescriber or pharmacist before adding any supplement while you’re on a blood thinner.

Warfarin, vitamin K and calcification

There’s a longer-term question too. Warfarin also blocks the activation of MGP, the artery-guarding protein, and in rats high-dose warfarin causes artery and valve calcification (Price et al., 1998). In older adults studied after death, warfarin users had 68% to 79% lower MK-4 levels in the brain than nonusers, though their odds of dementia or mild cognitive impairment weren’t different (Shea et al., 2025). Whether warfarin speeds calcification in people, and whether anything should be done about it, is a question for your cardiologist. It is not a reason to stop warfarin or to self-prescribe vitamin K.

Other medications that affect vitamin K

NIH ODS lists these (NIH ODS, 2026):

  • Antibiotics, especially cefoperazone and related cephalosporins, can lower vitamin K production by gut bacteria and may interfere with vitamin K activity.
  • Bile acid sequestrants (cholestyramine, colestipol), used to lower cholesterol, reduce absorption of fat-soluble vitamins, including K.
  • Orlistat (Xenical, Alli), a weight-loss drug, reduces fat absorption. Changes in INR have been reported in people taking orlistat with warfarin.

If you take any of these long term, ask your clinician whether your vitamin K status needs watching.

Vitamin K shots for newborns

Why babies are born low

Babies start life with very little vitamin K. Little crosses the placenta, the newborn gut has few bacteria at first, and breast milk is low in vitamin K (NIH ODS, 2026; Dam, 1946). Without extra vitamin K, some babies develop vitamin K deficiency bleeding (VKDB). The International Society on Thrombosis and Haemostasis adopted that name in 1999, replacing the older term “hemorrhagic disease of the newborn,” and described three types (Sutor et al., 1999):

  • Early VKDB: within the first 24 hours
  • Classical VKDB: days 1 to 7
  • Late VKDB: from 1 week to 6 months, most often in exclusively breastfed babies who didn’t get vitamin K at birth. NIH ODS describes it as typically occurring at 2 to 12 weeks (NIH ODS, 2026).

Late VKDB is the most dangerous kind because it often shows up as bleeding inside the skull. Vitamin K deficiency in early infancy remains a leading cause of intracranial bleeding in babies (Shearer et al., 2012).

What the shot does

The American Academy of Pediatrics recommends a single injection of 0.5 to 1 mg of K1 at birth (NIH ODS, 2026), and it has recommended intramuscular vitamin K since 1961 (Hand et al., 2022). A 2016 systematic review estimated that late VKDB occurs in about 35 of every 100,000 babies who get no prophylaxis, higher in low- and middle-income countries (80 per 100,000) than in high-income ones (8.8 per 100,000) (Sankar et al., 2016).

Against no prophylaxis, the shot was linked to a 98% lower risk of late VKDB (relative risk 0.02). Oral doses are an option in some countries, but a single oral dose protected much less well than the shot; multiple oral doses weren’t significantly different from the shot (Sankar et al., 2016). The 1999 ISTH review reached a similar conclusion: one shot and one oral dose protect equally against classical VKDB, but the shot works better against late VKDB, and repeated oral doses may protect about as well (Sutor et al., 1999).

The cancer scare

In 1992, a study from Bristol, England, reported that babies given intramuscular vitamin K had about twice the odds of childhood cancer later (odds ratio 1.97) (Golding et al., 1992). It caused real alarm, and some places switched to oral vitamin K.

Larger studies didn’t confirm it. A pooled analysis of six case-control studies found no convincing evidence that the shot raises the risk of childhood leukemia (Roman et al., 2002). The UK Childhood Cancer Study, with 2,530 children with cancer, found no link at all (Fear et al., 2003).

Refusals are rising, and babies are being hurt

In 2013, doctors in Tennessee reported seven babies with confirmed vitamin K deficiency over eight months, five of whom developed bleeding. None had received vitamin K at birth. Four had bleeding in the brain, and two needed urgent brain surgery (Schulte et al., 2014; Centers for Disease Control and Prevention, 2013). A 2022 AAP statement noted that VKDB appears to be rising, driven by parents declining the shot and by less effective alternatives (Hand et al., 2022).

If you have concerns about the shot, talk them through with your pediatrician or midwife before the birth. If you choose an oral regimen, make sure you know the full schedule, because a single oral dose is much less protective than the shot.

What the research says, use by use

For each use below, we give an evidence grade:

  • Strong: consistent results from good trials, accepted by major guidelines
  • Moderate: several good trials or meta-analyses, with some uncertainty
  • Limited: a few trials, mixed results, or benefits only in subgroups
  • Weak or negative: good trials mostly found no benefit

Preventing bleeding from deficiency — Evidence: Strong

This is the one use nobody argues about. Vitamin K reverses bleeding caused by deficiency, whether in newborns, in people with malabsorption, or in people with bile duct blockage (Dam, 1946; NIH ODS, 2026). Doctors also give vitamin K to reverse warfarin’s effect when needed. That’s medical care, not a supplement decision.

The Rotterdam study and dietary K2 — Evidence: Limited (observational; consistent but not proof)

The study that launched modern K2 interest came from the Netherlands. In the Rotterdam Study, researchers followed 4,807 older adults from the early 1990s to 2000 and estimated their K1 and K2 intakes from food questionnaires. People in the top third of menaquinone (K2) intake, above about 32.7 mcg a day, compared with those in the bottom third, below about 21.6 mcg, had (Geleijnse et al., 2004; NIH ODS, 2026):

  • 57% lower risk of dying from coronary heart disease (relative risk 0.43)
  • 26% lower risk of dying from any cause (relative risk 0.74)
  • 52% lower odds of severe aortic calcification (odds ratio 0.48)

K1 intake wasn’t linked to any of these outcomes (Geleijnse et al., 2004).

Other cohorts pointed the same way:

  • In 16,057 Dutch women (Prospect-EPIC), each extra 10 mcg a day of K2 was linked to 9% lower coronary heart disease risk, driven by MK-7, MK-8 and MK-9. K1 wasn’t linked (Gast et al., 2009).
  • In 564 postmenopausal Dutch women, higher menaquinone intake was linked to less coronary calcification; K1 wasn’t (Beulens et al., 2009).
  • In 53,372 Danes followed for about 21 years, the highest K1 intake was linked to 21% fewer hospital admissions for atherosclerotic heart disease, and the highest K2 intake to 14% fewer (Bellinge et al., 2021).

Why it isn’t proof: in Dutch diets, K2 comes largely from cheese and other dairy (Walther et al., 2013). People who eat more of those foods may differ in many ways, and food questionnaires are imprecise. These studies point to a question that trials have to answer.

Arterial calcification and stiffness (supplement trials) — Evidence: Limited (mixed; benefit on scans, no proof of fewer heart attacks)

This is where most of the excitement is, and the trial results are genuinely mixed.

Trials with a positive signal:

  • Knapen MK-7 (2015): 244 healthy postmenopausal Dutch women took 180 mcg of MK-7 or placebo for three years. Arterial stiffness (carotid-femoral pulse wave velocity) improved, especially in women who started with stiffer arteries, and inactive MGP fell by 50% (Knapen et al., 2015).
  • Shea K1 (2009): 388 older adults took 500 mcg of K1 or placebo for three years. Overall, there was no difference in coronary calcium progression. Among people who took at least 85% of their pills, progression was slower on K1, and among people who already had some calcification, those on K1 had 6% less progression. The authors called these findings hypothesis-generating (Shea et al., 2009).
  • VitaK-CAC (2026): 180 Dutch patients with symptomatic coronary artery disease and moderate coronary calcium scores (50 to 400) took 360 mcg of MK-7 or placebo for two years. Calcium scores rose from 145 to 214 on placebo and from 135 to 184 on MK-7, a significant difference. No significant side effects were seen. The authors said the clinical meaning, for plaque stability and heart attacks, remains to be determined (Vossen et al., 2026).

Trials that found little or no effect:

  • Type 2 diabetes: 68 people with type 2 diabetes and heart disease took 360 mcg of MK-7 or placebo for six months. MK-7 didn’t reduce calcification on CT, and an imaging marker of active calcification tended to increase (not statistically significant) (Zwakenberg et al., 2019).
  • AVADEC (2022): 365 men with aortic valve calcification took 720 mcg of MK-7 plus 25 mcg (1,000 IU) of vitamin D or placebo for two years. There was no effect on valve calcification, coronary calcification, or aortic calcification (Diederichsen et al., 2022).
  • Kidney dialysis: a one-year trial of 200 mcg MK-7 and a pilot trial of 10 mg K1 three times a week both lowered inactive MGP sharply but didn’t slow calcification (Oikonomaki et al., 2019; Holden et al., 2023).

The meta-analyses:

  • A 2019 review of 13 trials found vitamin K supplements were linked to about 9% less progression of vascular calcification, but not to a significant change in stiffness. The authors noted the conclusions were limited by the small number of studies and their differences (Lees et al., 2019).
  • A 2020 review of nine trials concluded vitamin K doesn’t consistently prevent calcification or stiffening, though it might help people who already have calcification (Vlasschaert et al., 2020).
  • A 2023 review of 14 trials (1,533 patients) found a modest slowing of coronary calcium progression, no increase in adverse events, and a call for better trials (Li et al., 2023).

Bottom line: vitamin K supplements reliably activate MGP. They may slightly slow calcification in some groups, such as people with existing coronary disease. No trial has yet shown fewer heart attacks, strokes or deaths. NIH ODS calls the clinical trial results “inconclusive” (NIH ODS, 2026).

Bone density and fractures — Evidence: Limited (mixed, with integrity concerns in older trials)

Vitamin K is needed to activate osteocalcin, and Europe’s food safety agency accepted a health claim that vitamin K contributes to “the maintenance of normal bone.” The US FDA has not authorized any health claim for vitamin K (NIH ODS, 2026). Whether supplements prevent fractures is another matter.

The Japanese MK-4 story

In Japan and other parts of Asia, high-dose MK-4 (menatetrenone, 45 mg a day, roughly 375 times the US AI for men) is used as an osteoporosis medicine (NIH ODS, 2026; Iwamoto, 2014). Early results looked spectacular. A 2006 meta-analysis found that all seven trials reporting fractures were Japanese MK-4 trials, and pooled together they showed 60% lower odds of vertebral fractures, 77% lower odds of hip fractures, and 81% lower odds of other nonvertebral fractures (Cockayne et al., 2006).

Later evidence was more sobering:

  • The largest trial didn’t confirm it. The Japanese OF Study randomized 4,378 postmenopausal women with osteoporosis, open-label, to 45 mg of menatetrenone plus calcium or calcium alone. It found no overall reduction in vertebral fractures. There was a possible benefit only in women with five or more fractures at the start, and more adverse events in the MK-4 group (Inoue et al., 2009).
  • Integrity concerns. A 2019 update covering 36 trials noted that “the integrity of some of the previous evidence has been questioned.” In postmenopausal or osteoporotic patients, vitamin K was linked to 28% lower odds of clinical fractures (OR 0.72), but that shrank to a nonsignificant 0.76 when only low-risk-of-bias trials were counted. There was no effect on vertebral fractures or bone density (Mott et al., 2019).
  • Earlier smaller trials were modest: one open-label Japanese trial of 241 patients found menatetrenone maintained spinal bone density rather than increasing it, with fewer clinical fractures (Shiraki et al., 2000). A review of eight small trials described “modest” spinal density gains (Iwamoto, 2014).

Western trials

  • Knapen MK-4 (2007): 325 Dutch postmenopausal women took 45 mg of MK-4 or placebo for three years. Bone density by DXA didn’t change, but hip bone strength was maintained while it declined in the placebo group (Knapen et al., 2007).
  • Knapen MK-7 (2013): 244 Dutch postmenopausal women took 180 mcg of MK-7 or placebo for three years. MK-7 slowed the age-related decline in bone density at the lumbar spine and femoral neck (not the total hip), helped bone strength, and reduced loss of vertebral height in the lower thoracic spine (Knapen et al., 2013). This is the strongest positive trial for a nutritional dose of K2.
  • Emaus MK-7 (2010): 334 Norwegian women who were one to five years past menopause took 360 mcg of MK-7 (as natto capsules) or placebo for one year. No effect on bone loss (Emaus et al., 2010).
  • Rønn MK-7 (2016): 148 postmenopausal women took 375 mcg of MK-7 or placebo for a year. The inner structure of the shin bone was better preserved, but there was no difference in bone density (Rønn et al., 2016).
  • Binkley (2009): 381 North American postmenopausal women took 1 mg of K1, 45 mg of MK-4, or placebo for a year. Neither form changed bone density or bone turnover (Binkley et al., 2009).
  • Booth K1 (2008): 452 adults aged 60 to 80 took 500 mcg of K1 for three years, with calcium and vitamin D. No difference in bone density (Booth et al., 2008).
  • ECKO (2008): 440 postmenopausal women with low bone mass took 5 mg of K1 or placebo for two years, and some continued for up to four. Bone density didn’t improve. There were fewer clinical fractures (9 vs 20) and fewer cancers (3 vs 12), but the trial wasn’t designed or powered to test either (Cheung et al., 2008).

Observational studies

Low dietary K1 was linked to higher hip fracture risk in the Nurses’ Health Study, where women in the lowest fifth of intake (under 109 mcg a day) had higher risk (Feskanich et al., 1999), and in the Framingham Heart Study, where the top quarter of intake had 65% lower hip fracture risk than the bottom quarter (Booth et al., 2000). In both, the intake link was likely partly about overall diet, and Framingham found no link with bone density (Booth et al., 2000).

Bottom line: vitamin K is clearly needed for bone, but supplements haven’t reliably increased bone density in Western trials, and the strongest fracture evidence comes from older Japanese trials that have been questioned. Low-dose MK-7 has one good three-year trial behind it. If you have osteoporosis, vitamin K isn’t a substitute for proven osteoporosis drugs.

Vitamin D plus K2 — Evidence: Limited (good rationale, small trials, one large negative trial)

This pairing is everywhere in supplement aisles. The logic: vitamin D increases calcium absorption and the production of some vitamin K-dependent proteins, and vitamin K is needed to activate them. A 2017 review concluded that combined supplementation might work better than either alone for bone and heart health, though most trials were small and in postmenopausal women (van Ballegooijen et al., 2017).

What the trials show:

  • Braam (2003, 2004): 181 Dutch postmenopausal women took a supplement with minerals and vitamin D, the same plus 1 mg of K1, or placebo for three years. The group with K1 had less bone loss at the femoral neck (Braam et al., 2003) and kept their carotid arteries more elastic (Braam et al., 2004). This used K1, not K2.
  • Kurnatowska (2015): 42 people with chronic kidney disease (stages 3 to 5) took 90 mcg of MK-7 plus 10 mcg (400 IU) of vitamin D, or vitamin D alone, for about nine months. Thickening of the carotid artery wall slowed on the combination; coronary calcium didn’t differ significantly (Kurnatowska et al., 2015).
  • AVADEC (2022): the largest trial of D plus K2, 365 men with valve calcification, found no benefit on calcification (Diederichsen et al., 2022).

Bottom line: the biology is sound, and the combination looks safe for most people, but proof that adding K2 to vitamin D improves fractures or heart outcomes is missing. The idea that K2 is required to take vitamin D safely is a hypothesis, not established fact (Masterjohn, 2007).

Living longer — Evidence: Weak (observational only)

  • In three US cohorts with 3,891 adults, people with low blood K1 (0.5 nmol/L or less) had 19% higher all-cause mortality, but not more heart disease (Shea et al., 2020).
  • In the PREDIMED trial population of 7,216 Spaniards at high heart risk, higher K1 intake was linked to lower cancer mortality and 36% lower all-cause mortality, and people who increased their K1 or K2 intake during follow-up had lower mortality than those who didn’t (Juanola-Falgarona et al., 2014).

These are associations from diet and blood tests, not trials of supplements.

Cancer — Evidence: Weak (observational and mixed)

  • In 24,340 Germans (EPIC-Heidelberg), higher menaquinone intake, mostly from cheese, was linked to 28% lower cancer mortality (Nimptsch et al., 2010).
  • In the ECKO trial, fewer women on 5 mg K1 developed cancer (3 vs 12), but cancer wasn’t a planned outcome (Cheung et al., 2008).
  • A Japanese trial of 548 people, given 45 or 90 mg of MK-4 a day after treatment for liver cancer, found no reduction in recurrence and was stopped early (Yoshida et al., 2011).

There’s no good evidence that vitamin K supplements prevent cancer.

Beyond the mainstream: what lesser-known research suggests

Vitamin K has an active fringe, much of it built around K2. Some of it comes from careful researchers, some from traditional-diet writers, and some from one intriguing trial that hasn’t been repeated. Each topic below gets a label for how much weight it can carry:

  • Early: lab or animal research, reviews of how vitamin K might work, or open-label studies with no comparison group
  • Observational: studies that follow people or compare populations. They can show links but can’t prove cause.
  • Small trial(s): one or a few controlled trials, usually with fewer than a few hundred people
  • Trial substudy: a secondary analysis from a larger randomized trial
  • Reanalysis: a new look at existing data, often contested

A promising small study is a reason to keep watching, not proof. None of this means vitamin K treats or prevents any condition, and none of it replaces care from your doctor.

Nighttime leg cramps — Evidence: Small trial(s)

This is one of the most surprising recent results. In a 2024 Chinese trial published in JAMA Internal Medicine, 199 adults aged 65 and older with frequent night leg cramps took 180 mcg of MK-7 or placebo for eight weeks. Cramps fell from about 2.6 to 0.96 a week on MK-7, while they rose from 2.7 to 3.6 a week on placebo. Cramps were also shorter and less severe, and no side effects were linked to MK-7 (Tan et al., 2024).

Where it falls short: it’s one trial, eight weeks long, in one country. The mechanism isn’t clear, and cramps in the placebo group got worse rather than holding steady, which makes the gap look larger. It needs repeating before anyone can call it a reliable effect.

Is the official intake too low? The triage argument — Evidence: Early

Vermeer and colleagues argue that since 10% to 40% of osteocalcin and MGP in healthy adults isn’t fully activated, current intakes, set to support clotting, are too low for bones and arteries (Vermeer, 2012; Theuwissen et al., 2012). In a dose-finding trial of 42 healthy Dutch adults aged 18 to 45, MK-7 doses from 10 to 360 mcg improved activation of these proteins without affecting clotting markers (Theuwissen et al., 2012).

Where it falls short: better activation of a protein in a blood test isn’t the same as fewer fractures or heart attacks. Neither the US nor European expert panels found enough evidence to set higher intakes or a K2-specific target (Shearer et al., 2012; EFSA, 2017).

Natto and hip fractures in Japan — Evidence: Observational (ecological)

Natto is eaten far more in eastern Japan than in the west. In one study, women in Tokyo had average blood MK-7 levels of 5.26 ng/mL, compared with 1.22 in Hiroshima and 0.37 in British women. Across Japanese prefectures, higher natto consumption tracked with lower hip fracture rates (Kaneki et al., 2001).

Where it falls short: this compares regions, not individuals, so many other differences could explain it.

Osteocalcin as a hormone — Evidence: Early (mostly animal studies)

In 2007, researchers working with mice reported that osteocalcin acts like a hormone, influencing insulin secretion, insulin sensitivity and fat mass (Lee et al., 2007). That suggested bone might help regulate blood sugar. In a related human trial (a secondary analysis of a bone study), 500 mcg of K1 for three years lowered insulin resistance in older men but not in women (Yoshida et al., 2008).

Where it falls short: the hormone findings come from genetically altered mice, and the human effect was seen only in men, in a secondary analysis of a bone trial.

Vitamin K in the brain — Evidence: Observational

MK-4 is the main form of vitamin K in the human brain. In 325 older adults from the Rush Memory and Aging Project studied after death, higher brain MK-4 was linked to 17% to 20% lower odds of dementia or mild cognitive impairment, and to less Alzheimer’s-type brain pathology (Booth et al., 2022). A follow-up in 381 participants found warfarin users had 68% to 79% lower brain MK-4, but their odds of dementia weren’t higher (Shea et al., 2025).

Where it falls short: these are associations in brain tissue after death. No trial has tested whether vitamin K supplements protect memory.

COVID-19 and MGP — Evidence: Observational, plus one small trial

Early in the pandemic, Dutch researchers found that patients hospitalized with COVID-19 had high levels of inactive MGP, a sign of low vitamin K status, and that higher levels were linked to worse outcomes (Dofferhoff et al., 2021). In a later trial in 40 hospitalized patients, 999 mcg a day of MK-7 lowered inactive MGP but didn’t change a marker of elastic fiber breakdown (Visser et al., 2024).

Where it falls short: low vitamin K status may be a result of severe illness rather than a cause, and the small trial was negative on its main marker.

Calcium paradox, arterial stiffness and kidney transplant — Evidence: Small trial(s)

In a single-arm study of 60 kidney transplant recipients, eight weeks of 360 mcg a day of MK-7 was linked to a 14% drop in arterial stiffness and a 55% drop in inactive MGP (Mansour et al., 2017).

Where it falls short: there was no placebo group, and stiffness can change for many reasons over eight weeks.

Activator X and traditional diets — Evidence: Early (historical hypothesis)

As covered in the history section, the idea that Weston Price’s “Activator X” was vitamin K2 is a hypothesis built on matching Price’s descriptions to modern knowledge (Masterjohn, 2007). It’s popular among people who follow traditional or ancestral diets, and many recommend grass-fed butter, egg yolks, organ meats, aged cheese and natto as K2 sources. Those foods do contain K2 (NIH ODS, 2026; Schurgers & Vermeer, 2000). The claims that K2 alone explains Price’s observations about teeth and disease haven’t been tested.

Vitamin D toxicity as vitamin K depletion — Evidence: Early (hypothesis)

Masterjohn proposed in 2007 that vitamin D toxicity works partly by using up vitamin K, with vitamin A helping to protect against it (Masterjohn, 2007). It’s an interesting idea that would explain why some people pair high-dose vitamin D with K2. It hasn’t been tested in human trials.

Supplement forms compared

Form What it is Typical amounts in studies Notes
K1 (phylloquinone, phytonadione) Plant form; also the drug form used for newborns and to reverse warfarin 150 mcg to 5 mg Well absorbed from supplements; mostly used by the liver (NIH ODS, 2026)
MK-4 (menatetrenone) Synthetic copy of the K2 form made in your tissues 45 mg a day in Japanese osteoporosis use Very short time in the blood; low doses barely raise blood levels (Sato et al., 2012)
MK-7 Long-chain K2, usually from fermentation (natto bacteria) 90 to 720 mcg a day in trials Long half-life; builds up with daily use; strongest warfarin interaction per microgram (Schurgers et al., 2007; Theuwissen et al., 2013)
K2 “complex” blends MK-4 plus MK-7, sometimes with K1 Varies Check the amount of each form
D3 + K2 combinations Vitamin D3 with MK-7 or MK-4 Varies Count both vitamins toward your totals
Multivitamins Usually K1, sometimes K2 Usually under 75% of the DV (NIH ODS, 2026) Count this before adding a standalone product
Menadione (K3) Synthetic, no side chain Not used No longer used in supplements because of safety concerns (NIH ODS, 2026)

Vitamin K is fat-soluble, so take it with a meal that contains some fat.

How to choose and dose a supplement

  • Start with food. One cup of raw spinach or kale, or half a cup of cooked broccoli, gets most adults close to or past their AI (NIH ODS, 2026). If you eat natto, you’re getting far more K2 than any typical capsule.
  • If you take warfarin or a similar drug, stop here. Talk to your anticoagulation clinic first. Don’t add K2 on your own.
  • For most healthy adults, vitamin K from a varied diet meets official needs. If you choose a K2 supplement, doses used in positive trials were 180 mcg of MK-7 a day (bone and stiffness) and 360 mcg (coronary calcium in heart patients) (Knapen et al., 2013; Knapen et al., 2015; Vossen et al., 2026).
  • MK-4 at 45 mg a day is a drug dose. In Japan it’s used under medical care for osteoporosis. Don’t start it on your own.
  • Choose third-party tested products with a USP Verified, NSF Certified, or similar seal. Supplements aren’t approved by the FDA before sale.
  • Check which form you’re buying. “Vitamin K2” on a label could mean MK-4, MK-7, or a mix, and the doses aren’t interchangeable.
  • Tell every clinician you see. Vitamin K matters before surgery and when any new medicine is started.

Brands we may link to

We may link to vitamin K products from Thorne and Life Extension, and to multivitamins from Ritual that may include vitamin K. We haven’t tested these products ourselves, and we’re not ranking them over other brands or over the store brand at your pharmacy. Product formulas and strengths change, so check the current label rather than relying on any number you’ve seen before.

Whatever you look at, check:

  • the form (K1, MK-4, MK-7, or a blend)
  • the amount of each form per serving, in mcg
  • whether it’s combined with vitamin D or calcium, and how much
  • whether there’s third-party certification on the label or product page

Prices, strengths and formulas change. Check the current label and price on the brand’s site before you buy.

Safety: can you take too much?

No upper limit, for a reason

The Food and Nutrition Board didn’t set an upper limit for vitamin K, because “no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals” (NIH ODS, 2026). Trials of up to 5 mg of K1 and 45 mg of MK-4 a day for years haven’t shown serious harm from the vitamin (Cheung et al., 2008; Knapen et al., 2007), though the large Japanese OF study did report more adverse events with menatetrenone (Inoue et al., 2009).

The real risks

  • Blood thinners. This is the big one. Vitamin K, especially MK-7, can counteract warfarin and similar drugs (Theuwissen et al., 2013; NIH ODS, 2026).
  • Menadione (K3). It damaged liver cells in lab studies and is no longer used in supplements (NIH ODS, 2026).
  • Interactions with other drugs that affect absorption, listed above.

Pregnancy and breastfeeding

The AI during pregnancy and breastfeeding is the same as for other women of the same age (NIH ODS, 2026). There’s little research on high-dose K2 supplements in pregnancy, so talk to your obstetric provider before taking one.

Frequently asked questions

Do I need K2 if I eat lots of greens?

Probably not to meet official needs. Greens provide plenty of K1, and your body turns some of it into MK-4 (Hirota et al., 2013). Whether extra K2 adds benefits for bones or arteries beyond that is still being studied. The best K2 evidence comes from dietary studies in the Netherlands and a handful of MK-7 trials.

Is MK-7 better than MK-4?

They behave differently. MK-7 stays in the blood much longer, so small doses raise levels and activate proteins (Schurgers et al., 2007; Sato et al., 2012). MK-4 is the form your own tissues make, but as a supplement it’s cleared quickly, so it’s used in milligram doses. The two have rarely been compared head to head on real outcomes.

Can I take vitamin K with warfarin?

Only with your prescriber’s direct involvement. Keep your intake steady, and tell your anticoagulation clinic before starting, stopping or changing any supplement that contains vitamin K (NIH ODS, 2026; Theuwissen et al., 2013).

Does vitamin K2 remove calcium from arteries?

There’s no good evidence that it does in people. In rats, high vitamin K intake reduced warfarin-induced artery calcium (Schurgers et al., 2007), but human trials have shown, at best, a slower rise in calcium scores, not a reversal (Vossen et al., 2026; Li et al., 2023).

Should I take vitamin D with K2?

It’s reasonable and generally safe if you’re not on a blood thinner, and the biology supports it. Proof of better outcomes is still missing, and the largest trial of the combination was negative (van Ballegooijen et al., 2017; Diederichsen et al., 2022).

Is the newborn vitamin K shot safe?

Large studies didn’t confirm the early link to childhood cancer, and the shot is linked to a 98% lower risk of late vitamin K deficiency bleeding (Fear et al., 2003; Roman et al., 2002; Sankar et al., 2016). Discuss any concerns with your pediatrician.

What does natto taste like, and how much K2 is in it?

Natto has a strong smell and a sticky, stringy texture that many people find takes getting used to. Three ounces provide about 850 mcg of MK-7 (NIH ODS, 2026). If you’re on warfarin, natto is one food to discuss with your clinic before eating.

The bottom line

Vitamin K began as a factor that stopped chicks from bleeding, gave the world warfarin almost by accident, and now saves newborns from brain bleeds with a single shot. Here’s what the evidence supports today:

  • Getting enough vitamin K matters, and most people do from leafy greens and plant oils.
  • Every newborn should get vitamin K at birth. It’s one of the best-supported uses of any vitamin.
  • If you take warfarin, consistency is everything. Even small MK-7 doses can change your INR.
  • K2 looks promising for arteries and bones, with strong observational data and some positive trials, but no trial has yet shown fewer heart attacks or fractures in Western populations.
  • The Japanese high-dose MK-4 fracture data are less certain than they once looked.
  • Vitamin D plus K2 is a reasonable pairing, but its benefits aren’t proven.
  • Vitamin K has no known toxicity from food or supplements in healthy people, but that doesn’t make high doses useful.

For most people, a sensible plan is a daily serving of leafy greens, some fermented foods or cheese if you enjoy them, and a K2 supplement only if you’ve weighed the evidence and aren’t on a blood thinner. Talk to your doctor first if you have heart or kidney disease, osteoporosis, a bleeding or clotting disorder, or take any prescription medicine.

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