Table of Contents


Vitamin K is a fat-soluble organic compound that the body needs to remain healthy. Although bacteria in the human intestine make some vitamin K, it is not nearly enough to meet the body's needs, so people must get most of their vitamin K from foods in their diet.


The liver needs vitamin K to make factors that regulate blood clotting. Vitamin K may also play a role in maintaining strong bones and preventing osteoporosis.


Vitamin K is not a single substance but a collection of chemically similar compounds called naphtho-quinones. Vitamin K1, called phylloquinone, is the natural form of vitamin K. It is found in plants and is the main source of vitamin K in the human diet. Vitamin K2 compounds, called menaquinones, are made by bacteria that live in the human intestine. Researchers originally thought that bacteria in the gut provided a substantial percentage of human vitamin K needs, but more recent research suggests that these bacteria provide only a small amount and that people should get most of their vitamin K from diet. Vitamin K1 is manufactured synthetically and sold many brand names as a dietary supplement. Vitamin

Vitamin K

AgeAdequate intake (mcg/day)
Children 0–6 mos.2
Children 7–12 mos.2.5
Children 1–3 yrs.30
Children 4–8 yrs.55
Children 9–13 yrs.60
Children 14–18 yrs.75
Men 19≥ yrs.120
Women 19≥ yrs.90
Pregnant women 18≤ yrs.75
Breastfeeding women 18≤ yrs.75
Pregnant women 19≥ yrs.90
Breastfeeding women 19≥ yrs.90
FoodVitamin K (mcg)#
Kale, cooked, ½ cup530
Spinach, cooked, ½ cup445
Swiss chard, cooked, ½ cup285
Turnip greens, cooked, ½ cup265
Parsley, fresh, 2 tbsp.120
Brussels sprouts, cooked, ½ cup110
Broccoli, cooked, ½ cup77
Asparagus, cooked, ½ cup46
Celery, raw, ½ cup18
Carrots, raw, ½ cup8
Milk, 2%, 1 cup5
Miso, 1 oz.4
mcg = microgram 

(Illustration by GGS Information Services/Thomson Gale.)

K is also included in many multivitamins. In addition, a synthetic water-soluble form of vitamin K called K3 or menadione is not allowed in dietary supplements in the United States because of its association with serious side effects.

Normal vitamin K requirements

The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97–98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.

The IOM has not set RDA values for vitamin K because of incomplete scientific information. Instead, in 2000, it set AI levels for all age groups. AI and levels for vitamin K are measured in by weight (micrograms or


Coenzyme—Also called a cofactor, a small non-protein molecule that binds to an enzyme and catalyzes (stimulates) enzyme-mediated reactions.

Dietary supplement—A product, such as a vitamin, mineral, herb, amino acid, or enzyme, that is intended to be consumed in addition to an individual's diet with the expectation that it will improve health.

Enzyme—A protein that change the rate of a chemical reaction within the body without themselves being used up in the reaction.

Fat-soluble vitamin—A vitamin that dissolves in and can be stored in body fat or the liver.

Osteoporosis—A condition found in older individuals in which bones decrease in density and become fragile and more likely to break. It can be caused by lack of vitamin D and/or calcium in the diet.

Vitamin—A nutrient that the body needs in small amounts to remain healthy but that the body cannot manufacture for itself and must acquire through diet.

mcg). No UL levels have been set for vitamin K. Large amounts of vitamin K1 do not appear to cause blood clotting or other side effects. However, K3 is associated with health risks especially to children. It is banned by the United States Food and Drug Administration.

The following are the AIs for vitamin K for healthy individuals:

  • children birth-6 months: 2 mcg
  • children 7-12 months: 2.5 mcg
  • children 1-3 years: 30 mcg
  • children 4-8 years: 55 mcg
  • children 9-13 years: 60 mcg
  • children 14-18 years: 75 mcg
  • men age 19 and older: 120 mcg
  • women age 19 and older: 90 mcg
  • pregnant and breastfeeding women age 18 and younger: 75 mcg
  • pregnant and breastfeeding women age 19 and older: 90 mcg

Sources of vitamin K

Vitamin K is found in the largest quantities in green, leafy vegetables. The following list gives the approximate vitamin K1 content or some common foods. Little vitamin K is lost during cooking, but more is lost when foods are frozen.

  • parsley, fresh, 2 Tablespoons: 120 mcg
  • spinach, cooked ½ cup: 445 mcg
  • kale, cooked, ½ cup: 530 mcg
  • turnip greens, cooked, ½ cup: 265 mcg
  • Swiss chard, cooked, ½ cup: 285 mcg
  • brussels sprouts, cooked ½ cup: 110 mcg
  • broccoli, cooked, ½ cup: 77 mcg
  • asparagus, cooked, ½ cup: 46 mcg
  • celery, raw, ½ cup: 18 mcg
  • carrots, raw, ½ cup: 8 mcg
  • miso, 1 ounce: 4 mcg
  • milk, 2% 1 cup: 5 mcg
  • dietary supplements: 10-120 mcg

Vitamin K's role in health

Vitamin K is necessary for normal blood clotting (coagulation). In the liver, it is converted into more than half a dozen coenzymes that are essential to the complex cascade of events that result in the formation of a blood clot.

Vitamin K is routinely given to newborns in order to prevent bleeding known as hemorrhagic disease of the newborn (HDN) or vitamin K deficiency bleeding (VKDB) that can occur during the early weeks of life. Although this type of bleeding occurs only in 0.25-1.7% of untreated newborns, it can be fatal. Since 1961, the American Academy of Pediatrics has recommended that all newborns receive a single 0.5-1.0 mg injection of vitamin K1 immediately after birth. As of 2007, there was no equivalent oral (by mouth) supplement available in the United States. A few researchers have questioned whether this early injection of vitamin K increases the risk of developing childhood cancer. In the view of the American Academy of Pediatrics, well-designed research does not support this link.

There is some growing evidence that vitamin K plays a role in maintaining strong bones. Certain proteins that regulate the cells (osteoblasts) that deposit calcium and other minerals in bone appear to be dependent on vitamin K. If this is true, vitamin K may play a role in preventing osteoporosis. Clinical trials are currently underway to determine safety and effectiveness of vitamin K in a variety of situations. Individuals interested in participating in a clinical trial at no charge can find a list of open trials at <>

Vitamin K deficiency

Vitamin K deficiency is extremely rare in healthy people. It can, however, occur in individuals who have disorders that interfere with the absorption of nutrients from the intestine. Signs of vitamin K deficiency include easy bruising, excessive bleeding, and slow clotting. People who are at higher risk for vitamin K deficiency include:

  • people with gastrointestinal diseases such as Crohn's disease, cystic fibrosis, inflammatory bowel disease, or ulcerative colitis
  • people who have had part of their stomach or intestine surgically removed for weight loss or other reasons
  • people with liver damage
  • people with alcoholism
  • people who take high doses of antibiotics over a long period.


People who are taking blood-thinning drugs, especially warfarin (Coumadin), should discuss their vitamin K needs with their healthcare provider. They may need to restrict their intake of vitamin K. The purpose of blood-thinning drugs is to keep the blood from forming clots in the veins and arteries. Since vitamin K helps blood to clot, high levels of vitamin K in the diet may work against blood-thinning drugs and reduce their effect. Individuals taking these drugs are encouraged to keep their daily intake of vitamin K steady at or slightly below the IA level. In addition, they should have their international normalized ratio (INR) and prothrombin time (PT), both measures of blood clotting potential, checked regularly.

Injections of vitamin K3 (menadione) are banned in the United States because they can cause liver damage and rupture of red blood cells in infants and children.


In addition to interfering with blood-thinning drugs mentioned above, vitamin K may interact with the following:

  • Some broad-spectrum antibiotics (antibiotics that kill a wide variety of bacteria) may decrease the amount of vitamin K2 produced in the intestines.
  • Aspirin (salicylates) taken in high doses over a long time may increase the body's need for vitamin K.
  • Cholestyramine (Questran) and mineral oil may decrease vitamin K absorption.
  • Quinine may increase the body's need for vitamin K
  • Orlistat (Xenical, Alli) is likely to decrease Vitamin K absorption.
  • Vitamin K may decrease the effectiveness of blood thinning herbs such as American ginseng (P. quinque-folius), alfalfa (Medicago sativa), and angelica (Angelica archangelica).
  • Olestra, a compound that reduces fat absorption, decreases the absorption of vitamin K. The FDA requires all foods containing olestra to be fortified with the fat-soluble vitamins A, D, E, and K.


No complications are expected from vitamin K, especially when most of the vitamin K comes from dietary sources. However, pregnant and breastfeeding women should avoid taking vitamin K supplements. In addition, people taking blood-thinning drugs should carefully monitor their intake of vitamin K so that they do not increase the chance of developing blood clots.


Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Cooper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academy Press, 2001, pp. 162-177. <>

Gaby, Alan R., ed. A-Z Guide to Drug-Herb-Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together. New York: Three Rivers Press, 2006.

Lieberman, Shari and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007.

Pressman, Alan H. and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.

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American Cancer Society. 1599 Clifton Road NE, Atlanta GA 30329-4251. Telephone: 800 ACS-2345. Website: <>

Linus Pauling Institute. Oregon State University, 571 Weniger hall, Corvallis, OR 97331-6512. Telephone: (541) 717-5075. Fax: (541) 737-5077. Website: <>

Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892-7517 Telephone: (301) 435-2920. Fax: (301)480-1845. Website: <>


Agricultural Research Service, USDA.“Vitamin K: Another Reason to Eat Your Greens.” United States Department of Agriculture, February 23, 2007. <>

American Cancer Society.“Vitamin K.” American Cancer Society, March 23, 2006.

Higdon, Jane.“Vitamin K.”Linus Pauling Institute-Oregon State University, May 25, 2004. <>

Harvard School of Public Health.“Vitamins.” Harvard University, November 10, 2006. <>

Medline Plus.“Vitamin K.” U. S. National Library of Medicine, August 1, 2006. <http://www.nlm.nih/gov/medli-neplus/druginfo/natural/patient-vitamink.html>

Warren Grant Magnuson Clinical Center Drug-Nutrient Interaction Task Force.“Important Information to Know When You Are Taking Coumadin and Vitamin K.” National Institutes of Health, December 2003. <>

Tish Davidson, A.M.