Vitamin K functions in metabolism as a substrate for a microsomal enzyme
that catalyzes the post translational conversion of specific glutamyl residues
in a limited number of proteins to g-carboxyglutamyl
(Gla) residues. The proteins involved include clotting factors II (prothrombin),
VII, IX, and X, plasma proteins C and S, osteocalcin, and matrix Gla protein.
Phylloquinone (2-Me-3-phytyl-1,4-naphthoquinone) from plants and a series of
bacterial menaquinones (2-Me-3-polyisoprenyl-1, 4-naphthoquinone) are natural
The classical symptom of a vitamin K deficiency is a defect in blood coagulation
measured by a one-stage prothrombin time (clinical PT). Uncomplicated deficiencies
are rare. The hemorrhagic disease of the newborn is a potential risk, particularly
for breast-fed infants, but is preventable by vitamin K prophylaxis at birth.
The current Recommended Dietary Allowances (RDAs) for vitamin K are:
Past estimates of vitamin K intake were in the range of a few hundred µg/day;
current data suggest that a range of intake of 75-125 mg of phylloquinone/day
is more accurate. The contribution of menaquinones in the lower bowel to satisfying
the human requirement is unknown, but it is probably minor.
Excellent sources of phylloquinone in the diet are dark green vegetables such
as spinach, broccoli, and kale. These foods may provide more than one RDA in
a single serving. Other green vegetables also furnish significant amounts. Bioavailability
of vitamin K from various food sources has not been established. Meats, grains,
and fruits contribute little vitamin K to the diet, but soybean oil, canola
oil, and olive oil furnish appreciable amounts.
Large amounts of phylloquinone can be consumed over extended periods with no
toxic effects. Menadione (2-Me-1,4-naphthoquinone) is currently used in animal
feeds but is not longer administered to infants because of resulting hemolytic
anemia, hyperbilirubenemia, and kernicterus.
A number of reports point to a relationship between vitamin K status and skeletal
health of the elderly. If these findings turn out to be correct, they will open
an exciting new area of research in the field of vitamin K metabolism.
For Further Information:
Suttie, J.W. (1992) Vitamin K and human nutrition. J. Am. Diet. Assoc. 92:
Suttie, J.W. (1993) Synthesis of vitamin K-dependent proteins. FASEB J. 7:
Booth, S.L., Pennington, J.A.T. & Sadowski, J.A. (1996) Food sources and
dietary intakes of vitamin K-1 (phylloquinone) in the American diet: J. Am.
Diet. Assoc. 96: 149-154.
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