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 [3].

Snapshot

Pros:

  • 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

Cons:

  • Interacts with blood thinners
  • May cause reactions if injected
  • Insufficient or contradicting evidence for some claims

Benefits of Vitamin K

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) Improves 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 [9, 10, 11, 12].

Bone Strength

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 [13, 14, 15, 16, 10, 17, 18, 19, 20].

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 [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32].

Fractures

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 [22, 26, 33, 29, 34, 35, 36].

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 [17, 37, 38, 39, 18, 40, 41, 42, 43, 44, 45, 46].

Osteoarthritis

Two studies on almost 2k people found the following link: people deficient in vitamin K are more likely to develop 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 [47, 48, 49].

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 [50, 51].

Getting enough vitamin K from food or supplements keeps your bones strong, reduces fracture risk, and may ease the symptoms of osteoarthritis.

2) May Prevent 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) [52, 53+].

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 [54, 55, 56, 57].

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 [58, 59].

High K2 intake protected from calcification and heart disease in 4 trials, while K1 had no effect in 8 studies on over 85k people [60, 61, 62, 63, 64, 65, 66, 67].

Vitamin K 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 [68, 69, 70].

Both vitamin K1 and 2 may prevent the calcification of blood vessels, but K2 is probably more effective.

3) Keeps Your Brain Healthy

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 [71+].

Vitamin K deficiency reduces brain sphingolipid levels in mice and rats. Increasing vitamin K intake restored their sphingolipid production back to normal [72, 73, 74, 75].

This vitamin also helped brain cells survive and mature in dishes, protecting them from free radical damage [76, 77, 78, 79].

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 [80, 81, 82, 83, 84, 85, 86, 87].

Cognition

In a study on over 300 elderly people, high vitamin K blood levels were associated with increased verbal memory [88].

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 [89, 90, 91].

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 [92].

Vitamin K may keep your brain cells strong and healthy, while deficiency can impair your cognition.

4) Prevents 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 [93+].

Babies are at an increased risk of vitamin K deficiency, which may cause bleeding in the brain and bowels [93+].

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 [94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106].

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 [107, 108, 109, 110, 111, 112, 113].

We all need vitamin K to maintain normal blood clotting. Babies and pregnant women may need more, but this should be determined by a doctor.

5) May Prevent 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 [114, 115, 116, 117].

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 [118].

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) [119, 120, 121].

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) [122].

Vitamin K helps lower inflammation and prevent health complications in people with type 2 diabetes.

6) May Reduce Cancer Risk

In 2 studies on over 12k people, vitamin K2 (but not K1) intake was associated with a reduced risk of prostate cancer. In turn, 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].

In test tubes, vitamin K1 reduced the growth, development, survival, and spreading of the following cancer types:

In some cases, it enhanced the activity of an anticancer drug (sorafenib) [129, 130, 131, 136, 137].

In turn, K1 promoted breast cancer cell growth (while K2 had the opposite effect) [138].

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 [139, 140, 141].

Vitamin K2 seems to be more important for cancer prevention than K1, but more studies are needed to determine their relevance.

7) May Reduce 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 [142, 143].

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 [144, 145, 146, 147].

All in all, getting more vitamin K might help if you have chronic heart or kidney disease, but it probably won’t do much if you’re healthy.

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) may speed up skin healing after such procedures, although its efficacy is questionable [148, 149, 150, 151+].

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 [152+].

Vitamin K Foods

Recommended Intake

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 [153].

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) [154, 155+, 156+]:

1) Kale

817 mcg/100g (689% DV)

2) Spinach

380 mcg/100g (317% DV)

3) Broccoli and Brussel sprouts

180 mcg/100g (150% DV)

4) Cabbage

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)

7) Asparagus

60 mcg/100g (50% DV)

Best K2 Sources

Vitamin K2 is obtained from animal and fermented foods. Here are the top K2 food sources [154, 156+]:

1) Natto

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 [157].

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)

7) Chicken

9 mcg/100g (8% DV)

Tips

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

Symptoms

The main symptom of vitamin K deficiency is poor blood clotting, resulting in [106, 158, 159+]:

  • 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 [160+].

Vitamin K deficiency may also result from gut disorders, such as:

  • Cystic fibrosis [161]
  • Inflammatory bowel disease [162]
  • Celiac disease [163+]
  • Short bowel syndrome [164]
  • High blood galactose (galactosemia) [165]
  • Weight-loss surgery [166]

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 [167, 168].

Vitamin K is routinely given to people with cystic fibrosis to prevent deficiency and the resulting bleeding disorders [169, 170, 171, 172, 173, 174, 175].

Newborns and Pregnant Women

Babies are at higher risk of vitamin K deficiency and bleeding because [176]:

  • 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 [177+].

We also know that vitamin K blockers used to reduce blood clotting, such as coumarin, are mostly unsafe in pregnancy [178].

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 [179+].

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

Oral Supplements

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 [180+].

Injections

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 [181, 182+, 183+, 184+, 185+, 186+].

2 studies on over 800 children associated vitamin K injection during labor with leukemia, but further studies couldn’t confirm this finding [187, 188, 189].

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 [190+, 191].

Drug Interactions

Blood Thinners

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 [192, 193].

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 [194].

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 [195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206]

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 [207, 208, 209, 210, 211].

Other Drugs

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 [212].

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
  • Lifestyle
  • 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

Formulations

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 [215]
  • Injectable vitamin K1 [106, 106]
  • Topical creams and gels with vitamin K1 [139, 140]

Common Dosage

The most common K1 doses in clinical trials were:

  • Bone loss: 0.2-10 mg/day [216+, 46]
  • Heart disease: 0.5-2 mg/day [68, 54+]
  • Diabetes: 0.5-1 mg/day [118+]
  • Cystic fibrosis 0.25-5 mg/day (injected) or 5-10 mg/week (by mouth) [175]
  • Skin reactions: 0.1-5% cream 2x-3x/day [139, 140]

Some studies used K2 at the following doses:

  • Bone loss: 45 mg/day (MK-4) or 0.1-1 mg/day (MK-7) [216+, 36+]
  • Heart disease: 0.1-0.36 mg/day MK-7 [58, 59]

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 [217+].

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Takeaway

Vitamin K is an essential vitamin needed for proper blood clotting, brain and heart health. It exists in two forms: K1 and K2. Adequate intake might also prevent diabetes and reduce cancer risk.

Most people get enough vitamin K through a typical diet. If you have an intestinal disorder or are pregnant, you may need more. Aim for a balanced intake of both leafy greens and vegetable oils (K1 sources) and animal fats (K2 sources).

About the Author

Carlos Tello, PhD (Molecular Biology)

PhD (Molecular Biology)

Carlos received his PhD and MS from the Universidad de Sevilla.

Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.

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