Vitamins K1 and K2:

All You Need To Know

May 2021

Which is the Best Vitamin K Supplement?

Check out our unbiased recommendation of the best products on the market using our rigorous methodology. We screen products for the right formulation, bioavailability, safety, and efficacy to bring you only the best supplements available in 2019.

Briefly about Vitamin K (K1 & K2)

Do you bruise easily, bleed excessively, or a have a condition like celiac disease that inhibits the absorption of fat and fat-soluble nutrients? If so, you might have a vitamin K deficiency.

Vitamin K is important for proper blood clotting because it helps the body produce proteins needed for the clotting process.[1] Vitamin K also may help strengthen bones, and help the body properly process calcium and vitamin D.[2]

There are two primary types of vitamin K: K1 and K2. Their primary difference is in their source. Vitamin K1 is found in plants, particularly leafy green vegetables. As for the other one, the body creates K2 itself in the intestines.[1] Food sources of K1 include romaine lettuce, kale, broccoli, and spinach. Vegetable oils also are a good source, as are some fruits like figs and blueberries. Other foods rich in K1 include cheese, eggs, soybeans, and meat.[3]

If you’re a healthy adult in a developed country, you’re unlikely to have a vitamin K deficiency because K1 is found in many of the foods we eat, and the body makes K2 on its own. People in developing nations who have poor diets may have a deficiency in vitamin K and other nutrients. In addition, adults with certain conditions such as celiac disease, cystic fibrosis, short bowel syndrome, and ulcerative colitis are at increased risk for vitamin K deficiency because these conditions inhibit fat absorption and thus decrease the amount of many fat-soluble nutrients (including vitamin K) that can be absorbed by the body. People who have undergone bariatric surgery for weight loss also might develop a deficiency in vitamin K and other fat-soluble vitamins.[4] Finally, people who have liver disease, are taking blood thinners, are undergoing long-term hemodialysis, or have serious burns also might not be getting enough vitamin K.[5]

Infants are more likely than adults to have a deficiency. This is called vitamin K deficiency bleeding (VKDB) or hemorrhagic disease of the newborn, and although it is rare, it is potentially life-threatening and needs to be treated immediately, often with a blood transfusion. Most newborns are likely to have low reserves of vitamin K because they don’t receive this nutrient from their mothers in the womb, which is why doctors in the U.S., Canada, and many other countries give babies a shot of vitamin K right after they’re born.[6] Abnormally low vitamin K levels in infants are often signalled by unusual bleeding around where the umbilical cord was attached, the gastrointestinal tract, or other areas.[1]

You might have a vitamin K deficiency or insufficiency if you have any of the following symptoms:

  • you bruise easily
  • your stool looks black and contains blood
  • mucous membranes in your nose or other parts of your body are bleeding
  • you’re finding small blood clots under your nails[1]

If you have a condition that prevents you from absorbing enough fat (and the vitamins it contains), talk to your doctor about whether you should take a vitamin K supplement and monitor your levels. Be aware that some drugs may also cause the body to absorb and/or use vitamin K less efficiently, including Warfarin (Coumadin®), antibiotics, bile acid sequestrants, and Orlistat (Alli® and Xenical®).[3]

Vitamin K supplements might contain just vitamin K; contain additional nutrients like magnesium, vitamin D, or calcium; or be part of a multivitamin. Supplements commonly contain K1 (phylloquinone and phytonadione) and/or K2 (menaquinone-4, and menaquinone-7).[3] Aim for about 120 mcg per day if you’re male and 90 mcg per day if you’re female.[5]

A Closer Look at K1 and K2

Internal Processing of Vitamin K

Vitamin K is the general name for a group of fat-soluble compounds with a particular chemical structure. These include phylloquinone, aka vitamin K1, and various menaquinones, aka vitamin K2. Leafy green vegetables are the main dietary source of K1.[7] K2 is produced by bacteria, including bacteria found in the human gut. It is also found in small amounts in certain bacteria-rich foods, including milk, butter, cheese, eggs, and dark-meat chicken, as well as in some animal organs such as kidneys that normally aren’t consumed for food.[8]

Vitamin K serves as a coenzyme for carboxylase, another enzyme that’s required for blood clotting, building bones, and other functions of the human body.[9] The protein prothrombin is also required for blood clotting and depends on vitamin K to function properly.[10] In addition, some research shows that two proteins dependent on vitamin K — Matrix Gla-protein and osteocalcin — may play a role in keeping bones healthy by reducing calcification and helping build new bone structure.[11, 12]

Bile and pancreatic enzymes help the body’s fat cells incorporate and use fat-soluble vitamins, including vitamin K. Vitamin K is also absorbed into the small intestine, transported to the liver, and found in other parts of the body such as the heart, brain, pancreas, and bones.[13, 14, 15]

Very little vitamin K travels through the blood compared to other fat-soluble vitamins. The body also metabolizes and excretes vitamin K relatively quickly, which means less is stored in the blood and tissues compared to other fat-soluble vitamins.[15]

Although both K1 and K2 are vital for blood clotting and both health, they function somewhat differently in the body. For one thing, K1 is not absorbed well; according to one study, less than 10 percent of the K1 found in plants was metabolized by test subjects.[16] Because K2 is found in animal products that contain fat (as opposed to K1, which is found in plants), K2 might be absorbed more readily.[17] In any event, like all fat-soluble vitamins, both forms of vitamin K can be used by the body in greater quantities if they are consumed with dietary fat. Another difference is that K2 stays in the blood longer than K1 — days rather than hours.[18]

Health Benefits of Vitamin K

Bleeding Problems

It is well-established that K1 can address several types of bleeding issues. In particular, it is given to newborns orally or by muscle injection to treat hemorrhagic disease. In addition, people with low prothrombin levels as a side effect of certain medications can take K1 orally or through an intravenous injection to help with bleeding problems. Oral or intravenous K1 injections also are prescribed for people with vitamin K-dependent clotting factors deficiency (VKCFD), an inherited bleeding disorder.[19]

Bone Health

Vitamin K is believed to aid in bone health because it is required to produce a key protein found in bones. However, studies of the actual effects of vitamin K on bone health have been inconsistent or inconclusive. According to the National Institutes of Health, epidemiologic studies have found that a diet high in vitamin K1 and K2 is associated with a lower risk of hip fractures in older people, but randomized controlled trials showed that vitamin K supplements did not increase bone mineral density.[20]

One review of controlled studies examined 13 trials that included data on bone loss; seven reported fracture data, and all except one showed that K2 reduced the incidence of fractures.[21] However, according to other research, most clinical trials don’t support the idea that vitamin K supplements reduce bone loss or fracture risk in the general population.[22] In summary, more high-quality studies are needed to determine this question.[20]

Heart Disease

Some studies have shown that vitamin K can help prevent arterial calcium deposits that lead to the development of plaque, which in turn can lead to heart disease. K2 has been shown to be better at this than K1.[23, 24, 25, 26] However, more studies are needed in this area to determine the mechanism for how this works and the optimal dose of K2 for slowing the formation of calcium deposits.[23, 25]

Other Conditions

Although people take vitamin K to treat a variety of other conditions, there is little or no evidence that the nutrient has any beneficial effect. These conditions include various forms of cancer, cystic fibrosis, high cholesterol, diabetes, stroke, spider veins, bruises, scars, stretch marks, burns, and swelling.[19]

Getting Enough Vitamin K

Recommended dosages have been established for K1, but not for K2.[27] Men generally should consume about 120 mcg of K1 per day, and women should consume about 90 mcg. Pregnant and breastfeeding women 14 – 18 years of age should aim for 75 mcg daily, and 90 mcg if they’re 19 or older. Recommended dosages for children are: 1 – 3 years: 30 mcg; 4 – 8 years: 55 mcg; 9 – 13 years: 60 mcg; adolescents 14 – 18 years: 75 mcg.[5]

The U.S. diet is relatively rich in vitamin K, and thus most Americans get sufficient quantities in the food they eat and do not need to take supplements. The most common sources of vitamin K in foods consumed by Americans include iceberg lettuce, spinach, broccoli, and fats and oils like soybean and canola oil. Although lots of foods are fortified with various vitamins, K isn’t usually one of them, with the exception of some meal replacement shakes and nutrition bars.[28] According to the 2011–2012 National Health and Nutrition Examination Survey, children and teens aged 2–19 years average 66 mcg of vitamin K from foods every day. Adults 20 and older average 122 mcg per day for women and 138 mcg for men. The average daily vitamin K intake from both food and supplements is 164 mcg for women and 182 mcg for men.[29]

Because most people in the U.S. get enough vitamin K in their diets and reported deficiencies of the nutrient are rare, levels of most patients aren’t checked routinely by U.S. health care professionals. There are exceptions, including people who are taking anticoagulants or have bleeding disorders. Fasting concentrations of phylloquinone in the blood of healthy people have been reported to be between 0.29 to 2.64 nmol/L, but people with phylloquinone concentrations slightly below this range have shown no clinical indications that they are deficient in vitamin K.[10]

Your medical history, including whether you take anticoagulants, take antibiotics, or have a condition that inhibits fats absorption, will help your doctor decide whether you’re at risk of a vitamin K deficiency. The main indication of vitamin K status that has been found to be clinically significant is the time it takes for blood to clot.[12, 30] Thus, if you might have a deficiency based on your medical history, the doctor will perform a coagulation test to see if your blood clots at a normal speed (about 11-13 seconds). If it takes longer than that, your doctor might ask you to take a vitamin K supplement or prescribe phytonadione (vitamin K1). Both K1 and K2 are probably safe when taken as recommended and don’t cause side effects in most people.[19]

Drug Interactions

Vitamin K has few side effects if you’re taking the recommended dosage. However, talk to your doctor because of possible interactions with other drugs. For example:

Warfarin (Coumadin): Warfarin is prescribed to help prevent blood clots. If you’re taking this drug, be sure to keep your vitamin K intake consistent. If you suddenly increase your vitamin K consumption, this might decrease the drug’s effectiveness. Conversely, consuming less vitamin K might make Warfarin more effective. Either of those events could affect your health in a negative way.[36] On the other hand, your doctor might prescribe vitamin K to counteract the effects of taking too much Warfarin or to stabilize clotting time while taking the drug.[19]

Fish Oil: Fish oil (Omega-3 fatty acids) might provide additional anticoagulation effects along with warfarin by lowering blood clotting factors that depend on vitamin K. However, the research is inconclusive.[37]

Antibiotics: Some antibiotics, especially cephalosporins like Cefamandole (Mandol), Cefoperazone (Cefobid), Cefmetazole (Zefazone), and Cefotetan (Cefotan),kill the “good” bacteria in your gut that produce vitamin K. As a result, your doctor might tell you to take a vitamin K supplement if you’re talking these antibiotics, especially for more than 10 days.[5]

Drugs that lower cholesterol: Bile acid sequestrants like Cholestyramine (Questran), Colestipol (Colestid), and Colsevelam (Welchol), lower cholesterol, but they also reduce fat absorption and the absorption of fat-soluble vitamins. Again, if you take one of these drugs, your doctor might recommend that you take a vitamin K supplement.[5]

Orlistat (Xenical, Alli) and Olestra: The weight-loss drug Orlistat and food additive olestra prevent the body from absorbing as much fat — and thus as many fat-soluble vitamins, including K. That’s why the U.S. Food and Drug Administration requires fat-soluble vitamins to be added to foods that contain olestra. It’s also why doctors who prescribe Orlistat often tell their patients to take a multivitamin.[5]

Phenytoin (Dilantin): If you’re taking the anticonvulsant Phentoin, which inhibits the body’s ability to use vitamin K, your doctor also might prescribe a supplement. In addition, anticonvulsants taken by pregnant women might lower the level of vitamin K in their babies at birth.[5]


Other precautions for certain people taking vitamin K include:

  • Diabetes: If you’re diabetic (type I or II), monitor your blood sugar closely because K1 might lower it.[19]
  • Kidney disease: too much vitamin K can cause problems for people on dialysis because of kidney disease.[19]
  • Liver disease: people with severe blood clotting problems due to liver disease should not expect K1 to help, and they might find that it makes those problems worse.[19]
  • Reduced bile secretion: People taking vitamin K who also have reduced bile secretion might need to take bile salts to make sure the vitamin is absorbed adequately.[19]

Considerations When Choosing A Supplement

Most multivitamins contain vitamin K, in an amount that is 75 percent or less of the recommended daily value. Supplements that contain only vitamin K, or vitamin K combined with other nutrients, often contain a much higher dose of vitamin K — up to 4,050 mcg (5,063% of the recommended daily value).[31]

Supplements may contain phylloquinone (natural K1) or phytonadione (synthetic K1) and MK-4 or MK-7 (K2).[31] Data are scarce in how bioavailable different forms of vitamin K are. However, according to one study, the body readily absorbs phytonadione and MK-7 supplements, but MK-7 lasts longer in the body.[32]

When choosing a supplement, look for one that has both K1 and K2. K2 comes in two primary forms, both of which a good supplement should contain: menaquinone-4 (MK-4) and menaquinone-7 (MK-7). K1 and MK-7 are the most beneficial as food supplements, with MK-7 having several advantages over K1. In particular, MK-7, which is obtained from natto, is more fat-soluble and bioavailable, and has a longer half-life, than K1. Regularly supplementing with MK-7 thus increases the amount of vitamin K in the blood 7-8 times more than supplementing with K1 alone. Moreover, compared with K1, MK-7 is distributed in bodily tissues more effectively.[33, 34]

In addition, because vitamin K is fat-soluble, eating fat along with a vitamin K supplement will aid in absorption.

Finally, there is some evidence that vitamin K and vitamin D can work together to improve bone health. For example, one study found that adding vitamin K to supplements containing vitamin D and calcium improved bone mineral density more than just taking vitamin D and calcium alone.[35] As a result, consider a supplement that contains both K and D.

Which is the Best Vitamin K Supplement?

Check out our unbiased recommendation of the best products on the market using our rigorous methodology. We screen products for the right formulation, bioavailability, safety, and efficacy to bring you only the best supplements available in 2019.

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