Initially discovered for its role in blood clotting, this essential vitamin also prevents artery hardening, brain damage, and bone loss. Vitamin K naturally exists in two forms: K1 in leafy greens and K2 in animal-based and fermented foods. Read on to learn more about their benefits and food sources.
What is Vitamin K?
Vitamin K is an essential fat-soluble vitamin. Identified in the ‘30s as a promoter of blood clotting (the K stands for the German word ‘Koagulation’), the additional roles of this vitamin in bone, heart, and brain health weren’t revealed until the ‘90s [1+, 2+].
Vitamin K1 vs K2
- K1 or phylloquinone: abundant in green, leafy vegetables.
- K2 or menaquinones: found in animal foods or produced by gut bacteria; MK-4 and MK-7 to 11 are most important in our diet.
Bile acids help absorb vitamin K from the gut. It then travels to the liver bound to blood proteins. The liver retains most of K1 and uses it to produce blood-clotting proteins, while K2 is released into the bloodstream [5+].
Smaller molecules, such as vitamin K1, stay in the body for up to 1.5 hours. But the largest ones, such as MK-7, remain several days. That’s why you may need to take in vitamin K1 several times a day, unlike the MK-7 form of vitamin K2 .
- Prevents bone loss
- Prevents calcium buildup in blood vessels
- Promotes brain health
- Prevents bleeding
- May reduce the risk of type 2 diabetes, cancer, and death
- Interacts with blood thinners
- May cause reactions if injected
- Insufficient or contradicting evidence for some claims
Benefits of Vitamin K
It’s important to note that many of the studies regarding vitamin K are association studies, which means that a deficiency is correlated with a certain issue but doesn’t necessarily cause that issue.
How It Works
- Blood clotting (coagulation factors II, VII, IX, and X and anti-clotting proteins)
- Preventing artery clogging (MGP, GRP)
- Building bones (osteocalcin, GRP, periostin)
- Cell growth (Gas6)
Although the enzyme ultimately breaks down vitamin K, another enzyme (VKOR) regenerates it, allowing its recycling. Blood thinners such as warfarin block this enzyme and prevent vitamin K regeneration [8+].
1) Preventing Bleeding Disorders in Newborns
Vitamin K activates numerous proteins: some promote blood clotting, others block it. Vitamin K deficiency causes an imbalance, making these proteins malfunction and ultimately leading to bleeding disorders [9+].
Babies are at an increased risk of vitamin K deficiency, which may cause bleeding in the brain and bowels [9+].
Vitamin K after birth reduced bleeding in 12 studies on over 47k babies (either oral or injected). A large analysis recommended preventive vitamin K injections in newborns, given their safety and potential to prevent serious complications [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22].
However, the benefits of preventive injections in pregnant women at risk of premature delivery are unclear. The injections improved vitamin K status in newborns and reduced bleeding in trials on over 500 but not in trials on almost 300 women [23, 24, 25, 26, 27, 28, 29].
2) Improving Bone Health
Osteocalcin, a protein essential for building bones and keeping them healthy, needs vitamin K to become “active” and start working. Once activated, osteocalcin can bind calcium and integrate it into bones. High levels of inactive osteocalcin may weaken the bones [30, 31, 32, 33].
According to 5 studies on almost 9k people, those with higher vitamin K intake had increased bone density – women more so than men (excluding those on menopausal hormone therapy). However, 2 other studies on almost 3k people didn’t find a link [34, 35, 36, 37, 31, 38, 39, 40, 41].
Vitamin K reduced bone loss in 9 clinical trials on almost 2.5k people, but not in 3 trials on over 1k people. Vitamin K1 was more effective than K2, especially in combination with vitamin D and calcium. It increased overall bone mineral density [42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53].
Vitamin K reduced fractures in several trials on over 5k people but not in one on over 100 elderly women. Analyses of 14 studies concluded that supplementing with vitamin K (especially K2) reduces fractures [43, 47, 54, 50, 55, 56, 57].
Similarly, high intake or blood levels of vitamin K reduced the risk of hip fractures in 10 studies on over 220k people but not in 2 shorter studies on ~5k people. Results are still in favor of vitamin K: according to a large analysis, high vitamin K intake reduces hip fractures too [38, 58, 59, 60, 39, 61, 62, 63, 64, 65, 66, 67].
Two studies on almost 2k people found the following link: deficiency in vitamin K is associated with an increased incidence of knee osteoarthritis. In a trial on almost 400 people with hand osteoarthritis, sufficient vitamin K1 intake (both from diet and supplements) slightly improved the symptoms [68, 69, 70].
Unlike osteoarthritis, rheumatoid arthritis is an autoimmune disease that has little to do with bone destruction. Vitamin K1 failed to reduce joint damage and inflammation in 2 clinical trials on over 100 people with rheumatoid arthritis [71, 72].
Likely Effective for:
Preventing Heart Disease
Calcium buildup or calcification in blood vessel walls can lead to heart disease, and is a common complication of kidney disease. Vitamin K activates a protective protein called MGP, which can prevent calcium from creating deposits in blood vessels (by inhibiting BMP-2) [73, 74+].
Vitamin K slowed heart valve narrowing due to calcium buildup in a clinical trial on almost 100 people. Interestingly, people with high levels of inactive MGP levels had more severe symptoms in 3 studies on over 1k people, suggesting vitamin K deficiency might be involved [75, 76, 77, 78].
However, vitamin K2 protects from calcification better than K1.
Vitamin K supplementation might offer some protection: it slowed calcification in one trial on almost 400 elderly people. In another one on close to 400 postmenopausal women, it improved blood vessel elasticity. A study in rats revealed the same [89, 90, 91].
Possibly Effective for:
In 3 studies on almost 100k people, high vitamin K intake and blood levels reduced the risk of type 2 diabetes. In another study on 25 healthy men, those eating a diet poor in vitamin K had increased blood sugar. Their pancreas, which makes insulin, also wasn’t working well [92, 93, 94, 95].
Could vitamin K supplements help?
According to a large review, probably not. Vitamin K supplementation has no benefits in healthy people but may reduce blood sugar and insulin resistance in those with or at risk of type 2 diabetes .
In 2 trials on almost 400 elderly people, vitamin K1 reduced insulin resistance in men but not in women. In another trial on 82 women at risk of type 2 diabetes, it reduced blood sugar and insulin right after a sugary drink. It didn’t, however, improve other markers of diabetes (such as insulin resistance) [97, 98, 99].
Type 2 diabetes triggers inflammation, especially in the blood vessels. In a study on 33 people, those with type 2 diabetes had high blood CCL2 (a cytokine) and low vitamin K. Supplementing may help, since vitamin K lowered inflammation in diabetic rats and test tubes (measured by NF-kB, IL-6, and CCL2) .
Although the research so far suggests that vitamin K helps prevent diabetes, especially in those at risk of developing this condition, the evidence is still limited. Further research is needed.
Insufficient Evidence for:
Vitamin K helps produce sphingolipids, unique fats that strengthen brain cell membranes. Any change in their production can impair brain health and may lead to neurodegenerative diseases, changes in behavior, and poor cognition [101+].
Several proteins in the brain depend on vitamin K, including Gas6 and protein S: Gas6 allows brain cells to migrate, grow, live longer, and produce more insulating sheaths (myelin); protein S protects cells from the damage caused by low oxygen [110, 111, 112, 113, 114, 115, 116, 117].
In a study on over 300 elderly people, high vitamin K blood levels were associated with increased verbal memory .
A lifelong vitamin K-poor diet impaired cognition, reduced spatial learning, and damaged the hippocampus in old rats. Even slight vitamin K deficiency makes animals lethargic and less willing to explore their environment, pointing to anxiety or low mood [119, 120, 121].
People with Alzheimer’s disease eat less vitamin K-rich foods (such as leafy greens), according to a study on 62 participants. A diet lacking vitamin K may, in turn, worsen brain damage .
While promising, the evidence to support this benefit of vitamin K is limited to 2 clinical studies and animal research. Additional studies in humans are needed to confirm it.
Reducing Cancer Risk
In 2 studies on over 12k people, vitamin K2 (but not K1) intake was associated with a reduced risk of prostate cancer. However, a study on over 70k people failed to confirm this finding [123, 124, 125].
Vitamin K reduced disease progression in 3 studies on 84 people with liver cancer, some of whom survived longer. According to 2 small studies, people with liver cancer may be deficient in vitamin K (measured by PIVKA-II). Their livers also have lower vitamin K levels than the livers of healthy people [126+, 127, 128].
The anticancer drug cetuximab often causes a rash with itching and dry skin. Topical vitamin K improved and prevented this adverse effect in 2 trials on over 100 people with colorectal cancer, but not in one on over 100 people [129, 130, 131].
All in all, some studies suggest that vitamin K (especially K2) helps prevent some cancer types such as prostate and liver cancer. Because the results are mixed, the evidence cannot be considered sufficient.
Can higher vitamin K intake reduce the chance of dying in healthy people? The topic is controversial. Vitamin-K rich diets reduced the risk of dying from cancer, heart disease, or other causes in a study on over 7k people, but not in another one on over 33k [132, 133].
Similar to the case of cancer, the results are mixed. Vitamin K deficiency seems to increase the risk of death from heart failure in people with heart or kidney disease, but its effects in people who don’t suffer from these conditions are unclear.
Many skin creams contain vitamin K. Companies market them for clearing bruises, spider veins, dark eye circles, and rosacea. Is there any truth to this?
Pulsed light therapy is a beauty procedure to repair aged and damaged skin. On the downside, it may cause bruises, spider veins, and rosacea. Vitamin K creams (especially with vitamin A) sped up skin healing after such procedures in 3 small trials, but their efficacy is questionable [138, 139, 140, 141+].
This benefit is only supported by a few, small studies. Larger, more robust clinical trials are needed to validate their preliminary results.
Vitamin K Foods
The recommended adequate intake (AI) is 90 mcg/day for women and 120 mcg/day for men, which is easily achieved by healthy adults eating a varied diet. However, this only ensures the production of blood clotting factors but may be insufficient for other functions of this vitamin .
Best K1 Sources
K1 will make up to 90% of your daily vitamin K intake if you’re eating a typical Western diet. Its main sources are green leafy vegetables and their oils. Here are the best food sources and their vitamin K1 content (DV=Daily Value, which matches the AI) [144, 145+, 146+]:
817 mcg/100g (689% DV)
380 mcg/100g (317% DV)
3) Broccoli and Brussel sprouts
180 mcg/100g (150% DV)
145 mcg/100g (121% DV)
5) Bibb lettuce
122 mcg/100g (102% DV)
6) Vegetable Oils
- Soybean oil: 193 mcg/100g (160% DV)
- Canola oil: 127 mcg/100g (105% DV)
60 mcg/100g (50% DV)
Best K2 Sources
Natto is a Japanese food made from fermented soybeans. It contains 1100 mcg/100g (917% DV)
2) Foie gras
Foie gras or goose liver paste contains 369 mcg/100g (307% DV).
Butter is another good source .
3) Hard cheeses
76 mcg/100g (63% DV)
4) Soft cheeses
56 mcg/100g (47% DV)
5) Goose legs
An odd one, we’ll admit, but it contains 31 mcg/100g (26% DV)
6) Egg yolk
31 mcg/100g (26% DV)
9 mcg/100g (8% DV)
Vitamin K availability depends on its source. We absorb only 5-10% of the K1 in cooked vegetables or up to 20% if they are combined with fatty foods. In turn, K2 from animal sources is absorbed almost completely because it’s normally found together with fats that help digest it [3+].
Vitamin K Deficiency
The symptoms we list here are commonly associated with vitamin K deficiency, but are insufficient for a diagnosis. Seek medical attention if you experience several of the following symptoms, especially if you are at increased risk of vitamin K deficiency.
Work with your doctor to discover what underlying condition might be causing low vitamin K levels and to develop an appropriate plan to improve your health.
- Excessive bleeding from wounds or lining tissues
- Internal bleeding
- Bruising easily
- Heavy menstrual periods
- Blood clots under the nails
- Dark or bloody stools
People at Risk
Elderly people have a poorer vitamin K status that may be caused by a reduced food intake, a decreased uptake of this vitamin in the gut, or an increased requirement [150+].
Vitamin K deficiency may also result from gut disorders, such as:
- Cystic fibrosis 
- Inflammatory bowel disease 
- Celiac disease [153+]
- Short bowel syndrome 
- High blood galactose (galactosemia) 
- Weight-loss surgery 
Cystic fibrosis decreases the production of bile salts and pancreatic lipases, which reduces the uptake of fat-soluble vitamins such as vitamin K in the gut. A study on 81 children with cystic fibrosis revealed that 95% had vitamin K deficiency [157, 158].
Newborns and Pregnant Women
Babies are at higher risk of vitamin K deficiency and bleeding because :
- Unborn babies receive little vitamin K through the placenta.
- Premature babies start their feeding later and often require antibiotics, which delays the growth of vitamin K-producing bacteria in their gut.
- Breast milk is poor in vitamin K.
- Their immature liver fails to produce blood clotting factors.
Getting enough vitamin K during pregnancy is important. Less is known about exactly how much pregnant women need. The growing fetus takes vitamin K from the mother’s bloodstream and may cause a slight deficiency [167+].
We also know that vitamin K blockers used to reduce blood clotting, such as coumarin, are mostly unsafe in pregnancy .
In a study on over 400 women, vitamin K blockers during pregnancy caused multiple birth anomalies. A third of the fetuses had smaller brains and mental retardation; half of them died [169+].
Talk to your doctor if you’re pregnant and worried about your vitamin K intake or the medication you’re taking.
Vitamin K Side Effects & Toxicity
The normal doses of dietary vitamin K and oral supplements are generally safe. Doses as high as 10 mg K1 and 45 mg K2 (MK-4) per day have been tested in clinical trials without any toxic effects [170+].
Skin reactions with itching and redness from vitamin K injections have been reported on rare occasions. They normally resolved within a few weeks. Severe reactions to injected vitamin K (anaphylaxis) are even less frequent [171, 172+, 173+, 174+, 175+, 176+].
Synthetic Vitamin K
A synthetic vitamin K form (menadione or vitamin K3) caused high blood bilirubin in premature babies in an old study, but only at very high injected doses (over 10 mg/day). K3 is no longer used because it was toxic to liver cells in test tubes [180+, 181].
Some drugs may cause vitamin K deficiency by reducing its production, regeneration, and uptake. Alternatively, vitamin K may interfere with the effects of drugs that prevent blood clotting (blood thinners). To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking and discuss how they may affect your vitamin K status.
Blood thinners such as warfarin prevent the formation of blood clots by blocking vitamin K regeneration. People on these drugs should carefully monitor their vitamin K status and maintain a consistent intake of this vitamin, since high doses (above 100 mcg/day) may interfere with the therapy [182, 183].
Bacterial overgrowth in the bowel may cause a leaky gut. This increases vitamin K1 uptake and may require the use of a higher blood thinner dose .
However, it may be desirable to reverse the effects of blood thinners with vitamin K in certain situations.
When taken for long periods or at excessive doses, blood thinners may cause bleeding disorders. Supplementation with vitamin K reversed them in 12 clinical trials on almost 2.5k people [185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196]
People about to undergo surgery should discontinue blood thinners to prevent life-threatening bleeding during the procedure. Vitamin K helped restore normal blood clotting after stopping the drugs in trials on over 300 people, but was unnecessary in some. Talk to your doctor to understand how you should prepare for surgery [197, 198, 199, 200, 201].
Antibiotics may cause vitamin K deficiency by killing the gut bacteria that produce it. Cephalosporin antibiotics may additionally block vitamin K action in the body. However, vitamin K supplementation is only needed in case of long-term antibiotic use .
Drugs that block fat and cholesterol uptake in the gut may also cause vitamin K deficiency. They include:
Limitations and Caveats
Although the studies overall support the role of vitamin K in preserving bone strength, heart health, and blood clotting, some showed no effects. These discrepancies may be due to differences in the:
- Populations (gender, age, nationality)
- Vitamin K status and measurement
- Vitamin K type, dosage, and form of supplementation
- Follow-up period
- Supplement combinations
Some clinical trials had design flaws such as small populations, lack of blinding or randomization, and measurement of objective parameters. A limitation of observational studies is the difficulty to prove a cause-effect relationship between vitamin K intake or status and the condition studied.
The role of vitamin K in other conditions is insufficiently investigated. There were fewer studies, some of them had the same design flaws previously described, and the results were sometimes contradictory.
To sum up, larger, longer, and better-designed studies are needed to confirm the results.
Vitamin K Supplements & Dosage
Vitamin K supplements are available in several forms:
- Oral vitamin K: tablets, capsules, or drops of vitamin K1, K2 (MK-4 or MK-7), or complexes with other vitamins and minerals 
- Injectable vitamin K1 
- Topical creams and gels with vitamin K1 [129, 130]
The most common K1 doses in clinical trials were:
- Bone loss: 0.2-10 mg/day [206+, 67]
- Heart disease: 0.5-2 mg/day [89, 75+]
- Diabetes: 0.5-1 mg/day [96+]
- Cystic fibrosis 0.25-5 mg/day (injected) or 5-10 mg/week (by mouth) 
- Skin reactions: 0.1-5% cream 2x-3x/day [129, 130]
Some studies used K2 at the following doses:
- Bone loss: 45 mg/day (MK-4) or 0.1-1 mg/day (MK-7) [206+, 57+]
- Heart disease: 0.1-0.36 mg/day MK-7 [79, 80]
Vitamin K Therapy for Dogs
Rat poison contains blood thinners such as warfarin. They are also toxic to dogs if accidentally eaten and cause symptoms such as bleeding, weakness, and difficulty breathing. Because blood thinner poisoning can be deadly if not diagnosed on time, take your dog immediately to the vet if you notice these symptoms.
The vet will administer an initial vitamin K injection (5 mg/kg), often combined with activated charcoal to absorb the toxins. Once stable, the vet will prescribe vitamin K pills or oral drops for 4-6 weeks [207+].
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