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.

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.2-5Pregnant and postmenopausal women are also sometimes deficient in this vitamin.6-8In 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 antibioticsmay 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.17-20Alcoholismcan also lead to vitamin K deficiency.21