Vitamin K plays a major role in the body's blood clotting system. There are three forms of vitamin K: K 1 (phylloquinone), found in plants; K 2 (menaquinone), produced by bacteria in your intestines; and K 3 (menadione), a synthetic form.

Vitamin K is used medically to reverse the effects of "blood-thinning" drugs, such as warfarin (Coumadin). Growing evidence suggests that it may also be helpful for osteoporosis.

Requirements/Sources

Vitamin K is an essential nutrient, but you need only a tiny amount of it. The official U.S. recommendations for daily intake have been set as follows:

  • Infants
    • 0-6 months: 2 mcg
    • 7-12 months: 2.5 mcg
  • Children
    • 1-3 years: 30 mcg
    • 4-8 years: 55 mcg
  • Males
    • 9-13 years: 60 mcg
    • 14-18 years: 75 mcg
    • 19 years and older: 120 mcg
  • Females
    • 9-13 years: 60 mcg
    • 14-18 years: 75 mcg
    • 19 years and older: 90 mcg
  • Pregnant Women
    • 18 years or younger: 75 mcg
    • 19 years and older: 90 mcg, preferably the K 1 variety (phylloquinone)
  • Nursing Women
    • 18 years or younger: 75 mcg
    • 19 years and older: 90 mcg, preferably the K 1 variety (phylloquinone)

Vitamin K (in the form of K 1) is found in green leafy vegetables. Kale and turnip greens are the best food sources, providing about 10 times the daily adult requirement in a single serving. Spinach, broccoli, lettuce, and cabbage are very rich sources as well, and you can get perfectly respectable amounts of vitamin K in such common foods as oats, green peas, whole wheat, and green beans, as well as watercress and asparagus.

Vitamin K (in the form of K 2) is also manufactured by bacteria in the intestines and is a major source of vitamin K. Long-term use of antibiotics can cause a vitamin K deficiency by killing these bacteria. However, this effect seems to be significant only in people who are deficient in vitamin K to begin with.2-5 Pregnant and postmenopausal women are also sometimes deficient in this vitamin.6-8 In addition, children born to women taking anticonvulsants while pregnant may be significantly deficient in vitamin K, causing them to have bleeding problems and facial bone abnormalities.9-11 Vitamin K supplementation during pregnancy may be helpful for preventing this.

The blood-thinning drug warfarin (Coumadin) works by antagonizing the effects of vitamin K. Conversely, vitamin K supplements, or intake of foods containing high levels of vitamin K, block the action of this medication and can be used as an antidote.12

Cephalosporins and possibly other antibiotics may also interfere with vitamin K-dependent blood clotting.13-16 However, this interaction seems to be significant only in people who have vitamin K-poor diets.

People with disorders of the digestive tract, such as chronic diarrhea, celiac sprue, ulcerative colitis, or Crohn's disease, may become deficient in vitamin K.17-20 Alcoholism can also lead to vitamin K deficiency.21

Therapeutic Dosages

In one study of osteoporosis described below, vitamin K was taken at the high dose of 1 mg daily, more than 10 times the necessary nutritional intake.

Therapeutic Uses

Growing, but not yet definitive, evidence suggests that vitamin K should be added to the list of nutrients helpful for preventing osteoporosis.22-32,50,52,57-58

Based on its ability to help blood clot normally, vitamin K has been proposed as a treatment for excessive menstrual bleeding.33 However, the last actual study testing this idea was carried out more than 55 years ago.34 Vitamin K has also been recommended for nausea, although there is as yet no meaningful evidence that it really works.

Preliminary evidence suggests that vitamin K supplementation may help prevent liver cancer.51 Very high doses of intravenous vitamin K have also been used to treat advanced liver cancer, with, perhaps, marginal benefits.55

What Is the Scientific Evidence for Vitamin K?

Vitamin K plays a known biochemical role in the formation of bone. This has led researchers to look for relationships between vitamin K intake and osteoporosis.

Observational studies have found that people with osteoporosis often have low levels of vitamin K,35-38 and that people with higher intake of vitamin K have a lower incidence of osteoporosis.39,40

Research also suggests that supplemental vitamin K can reduce the amount of calcium lost in the urine.41-43 This is indirect evidence of a beneficial effect on bone.

However, while these studies are interesting, only double-blind, placebo-controlled trials can actually prove a treatment effective. (For the reasons why, see Why Does This Database Rely on Double-blind Studies?). Several such studies have been performed on vitamin K for osteoporosis, with generally positive results.50,53, 56,57-58

One of these was a 3-year, double-blind, placebo-controlled trial of 181 women; it found that vitamin K significantly enhanced the effectiveness of supplementation with calcium, vitamin D, and magnesium.50 Participants, postmenopausal women between the ages of 50 and 60, were divided into three groups: receiving either placebo, calcium plus vitamin D plus magnesium, or calcium plus vitamin D plus magnesium plus vitamin K 1 (at the high dose of 1 mg daily). Researchers monitored bone loss by using a standard DEXA bone density scan. The results showed that the study participants using vitamin K along with the other nutrients lost less bone than those in the other two groups.

Benefits were seen in other studies as well.53,56,57-58 However, another placebo-controlled trial involving 452 older men and woman with normal levels of calcium and vitamin D failed to demonstrate any beneficial effects of 500 mcg per day of vitamin K supplementation on bone density and other measures of bone health over a3-year period.60

If there is a favorable effect, it is appears to be quite modest. Vitamin K may show its influence most strongly when, instead of DEXA scan alone, more complex tests of bone strength are used.57

Some evidence hints that vitamin K works by reducing bone breakdown, rather than by enhancing bone formation.54

For more information, see the Osteoporosis article.

Safety Issues

Vitamin K is quite safe at the recommended therapeutic dosages.

Note: Vitamin K directly counters the effects of the anticoagulant warfarin (Coumadin). If you are taking warfarin, you should not take vitamin K supplements or alter your dietary intake of vitamin K without doctor supervision.44,45 (One study suggests a novel way of using this effect deliberately.59 Researchers gave people on warfarin a fixed daily dose of vitamin K in order to override the changes in warfarin action caused by the natural variation in day-to-day dietary vitamin K consumption. The results were positive: INR values—the standard measurement of warfarin’s blood thinning effect—became more stable. Needless to say, however, this method should not be used except under close physician supervision.)

Newborns are commonly given vitamin K 1 injections to prevent bleeding problems. Although some have suggested that this practice may increase the risk of cancer,46 enormous observational studies have found no such connection (one such trial involved more than one million participants).47,48

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