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
In nature, vitamin K exists in two forms [3, 4+]:
- 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
A vital enzyme called gamma-glutamate carboxylase requires vitamin K to work. This enzyme activates proteins required for [6, 3, 7]:
- 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:
3) 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.
A form of vitamin K2 (MK-7) reduced inactive MGP. But it didn’t improve other heart disease markers such as high blood pressure or cholesterol in 2 trials on 140 people [79, 80].
High K2 intake protected from calcification and heart disease in 4 trials, while K1 had no effect in 8 studies on over 85k people [81, 82, 83, 84, 85, 86, 87, 88].
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:
4) Preventing Diabetes
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:
5) Brain Support
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+].
Vitamin K deficiency reduces brain sphingolipid levels in mice and rats. Increasing vitamin K intake restored their sphingolipid production back to normal [102, 103, 104, 105].
This vitamin also helped brain cells survive and mature in dishes, protecting them from free radical damage [106, 107, 108, 109].
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.
6) 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.
7) Reducing Mortality
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].
In turn, vitamin K deficiency increased the risk of heart failure and death in 4 studies on over 5k people with chronic kidney disease, heart disease, or transplanted kidneys [134, 135, 136, 137].
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.
8) Skin Health
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+].
Vitamin K may also help soothe skin irritation after injections. It reduced pain, burning, and redness after an injection for multiple sclerosis in a clinical trial on over 100 people [142+].
This benefit is only supported by a few, small studies. Larger, more robust clinical trials are needed to validate their preliminary results.
Research 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 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
Vitamin K2 is obtained from animal and fermented foods. Here are the top K2 food sources [144, 146+]:
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+].
A nutrient-dense diet rich in vegetables, fibre, and animal products is considered to be the best and healthiest way to prevent deficiency in most people .
Making sure your diet contains enough fiber supports your microbiome, which produces most of your vitamin K2.
According to one study, eating butter or other dietary fats alog with plant-based vitamin K-rich foods (like spinach) may increase vitamin K absorption, compared to eating the same foods without added fats .
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 [151+, 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) [151+, 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 [153+].
Vitamin K exists in two forms: vitamin K1 and K2. The best sources of vitamin K1 are leafy greens like kale and spinach. Vitamin K2, on the other hand, is found only natto and animal foods like cheese and goose liver. For most people, the best way to go is to include a variety of these foods.
Vitamin K is critical for blood clotting. Sensitive populations at risk of deficiency and bleeding disorders, such as pregnant women and newborns, may be prescribed vitamin K in certain cases. Adequate vitamin K intake also supports bone and heart health and possibly helps prevent diabetes.