General Description
Vitamin K is a fat-soluble nutrient essential to the health of the
human body, where it plays crucial roles in blood coagulation and
bone formation. There are three major forms of vitamin K: vitamin
K-1 (phylloquinone), the natural form found in plants; vitamin K-2
(menaquinone), produced by bacteria in the intestines; and vitamin
K-3 (menadione), a synthetic form available only by prescription.
The preferred form used in multivitamin and individual supplements
is vitamin K-1. There is also a water-soluble form of vitamin K
(phytonadione) which is used in some topical formulations.
Food sources
Some of the richest dietary sources of vitamin K are seaweed and
green leafy vegetables. Other good sources include liver, cauliflower,
soybeans, cheese, egg yolk, strawberries, tomatoes, and potatoes.
Health applications
- Prenatal nutrition
- Celiac disease
- Bleeding disorders
- Osteoporosis
Functions and uses
The most important function of vitamin K is the activation of blood-clotting
factors such as prothrombin and clotting factors VII, IX, and X.
These factors are essential for coagulating blood to stop wounds
from bleeding. The primary symptom of vitamin K deficiency is uncontrolled
bleeding, or hemorrhaging. Because it is found in a wide variety
of foods and is produced by gut bacteria, vitamin K deficiencies
are rare. Infants are usually given vitamin K injections at birth
to prevent deficiency because their intestinal tracts have not yet
been colonized with the bacteria that produce the vitamin and breast
milk does not provide it. Some research suggests that prenatal vitamin
K supplementation may be able to replace administration at birth;1,2
however, other studies indicate that the vitamin is absorbed through
the placenta slowly and to a limited degree.3
Vitamin K deficiencies may also occur in people with malabsorption
disorders (such as celiac disease),4,5 in patients
taking antibiotics that destroy intestinal bacteria,6-9
or in people taking drugs that inhibit blood clotting, such as warfarin
(CoumadinÂ).10,11 In addition to its role
in blood clotting, vitamin K appears to play an important part
in maintaining bone strength, presumably through its effect on
the bone protein known as osteocalcin. This protein must go through
a process called carboxylation in order for normal bone formation
to occur, and vitamin K mediates that process. Studies indicate
that low vitamin K intake may increase the risk of hip fracture
in women, and that vitamin K supplementation may help prevent
bone mineral loss.11-15 Further research may
indicate whether vitamin K should join calcium and vitamin D as
nutritional weapons in the battle against osteoporosis.
Dosage/toxicity
The reference daily intake (RDI) for vitamin K is 80 mcg/day for
adults, an amount which can easily be obtained through intake
of green leafy vegetables. While excessive consumption of synthetic
vitamin K (K-3) may cause liver damage, there are no known side
effects or toxicity associated with consumption of natural-source
vitamin K (K-1 or K-2). Because vitamin K may counteract the activity
of some prescription blood-thinners (such as warfarin), anyone
taking these drugs should not supplement vitamin K without consulting
a physician.
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