Vitamin K is a fat-soluble vitamin.
Originally identified for its role in the process of blood clot
formation ("K" is derived from the German word
"koagulation"), vitamin K is essential for the functioning
of several proteins involved in physiological processes that
encompass, but are not limited to, the regulation of blood clotting
(coagulation). Naturally, occurring forms of vitamin K include a
number of vitamers known as vitamin K1 and vitamin K2. Vitamin K1 or
phylloquinone is synthesized by plants and is the predominant form in
the diet. Vitamin K2 includes a range of vitamin K forms collectively
referred to as menaquinones. Most menaquinones are synthesized by
human intestinal microbiota and found in fermented foods and in
animal products. Menaquinones differ in length from 1 to 14 repeats
of 5-carbon units in the side chain of the molecules. These forms of
vitamin K are designated menaquinone-n (MK-n), where n stands for the
number of 5-carbon units (MK-2 to MK-14). Widely used in animal
husbandry, the synthetic compound known as menadione (vitamin K3) is
a provitamin that needs to be converted to menaquinone-4 (MK-4) to be
active.
Vitamin K functions as a cofactor for
the enzyme, γ-glutamylcarboxylase (GGCX), which catalyzes the
carboxylation of the amino acid glutamic acid (Glu) to
γ-carboxyglutamic acid (Gla). Vitamin K-dependent γ-carboxylation
that occurs only on specific glutamic acid residues in identified
vitamin K-dependent proteins (VKDP) is critical for their ability to
bind calcium.
Although vitamin K is a fat-soluble
vitamin, the body stores very small amounts that are rapidly depleted
without regular dietary intake. Perhaps because of its limited
ability to store vitamin K, the body recycles it through a process
called the vitamin K-epoxide cycle. The vitamin K cycle allows a
small amount of vitamin K to be reused many times for protein
carboxylation, thus decreasing the dietary requirement. Briefly,
vitamin K hydroquinone (reduced form) is oxidized to vitamin K
epoxide (oxidized form). The reaction enables γ-glutamylcarboxylase
to carboxylate selective glutamic acid residues on vitamin
K-dependent proteins. The recycling of vitamin K epoxide (oxidized
form) to hydroquinone (reduced form) is carried out by two reactions
that reduce vitamin K epoxide (KO) to vitamin K quinone and then to
vitamin K hydroquinone (KH2). Additionally, the enzyme vitamin K
oxidoreductase (VKOR) catalyzes the reduction of KO to vitamin K
quinone and may be involved — as well as another yet-to-defined
reductase — in the production of KH2 from vitamin K quinone. The
anticoagulant drug warfarin acts as a vitamin K antagonist by
inhibiting VKOR activity, hence preventing vitamin K recycling (see
Coagulation).
The ability to bind calcium ions (Ca2+)
is required for the activation of the several vitamin K-dependent
clotting factors, or proteins, in the coagulation (clotting) cascade.
The term, coagulation cascade, refers to a series of events, each
dependent on the other, that stop bleeding through clot formation.
Vitamin K-dependent γ-carboxylation of specific glutamic acid
residues in those proteins makes it possible for them to bind
calcium. Factors II (prothrombin), VII, IX, and X make up the core
of the coagulation cascade. Protein Z appears to enhance the action
of thrombin (the activated form of prothrombin) by promoting its
association with phospholipids in cell membranes. Protein C and
protein S are anticoagulant proteins that provide control and balance
in the coagulation cascade; protein Z also has an anticoagulatory
function. Control mechanisms for the coagulation cascade exist since
uncontrolled clotting may be as life threatening as uncontrolled
bleeding. Vitamin K-dependent coagulation factors are synthesized in
the liver. Consequently, severe liver disease results in lower blood
levels of vitamin K-dependent clotting factors and an increased risk
for uncontrolled bleeding (hemorrhage).
|
Life Stage |
Age |
Males (μg/day) |
Females (μg/day) |
|---|---|---|---|
|
Infants |
0-6 months
|
2.0
|
2.0
|
|
Infants |
7-12 months
|
2.5
|
2.5
|
|
Children |
1-3 years
|
30
|
30
|
|
Children |
4-8 years
|
55
|
55
|
|
Children |
9-13 years
|
60
|
60
|
|
Adolescents |
14-18 years
|
75
|
75
|
|
Adults |
19 years and older
|
120
|
90
|
|
Pregnancy |
18 years and younger
|
-
|
75
|
|
Pregnancy |
19 years and older
|
-
|
90
|
|
Breast-feeding |
18 years and younger
|
-
|
75
|
|
Breast-feeding |
19 years and older
|
-
|
90
|
Phylloquinone (vitamin K1) is the major
dietary form of vitamin K in most diets. Green leafy vegetables and
some plant oils (soybean, canola, olive, and cottonseed) are major
contributors of dietary vitamin K. However, phylloquinone
bioavailability from green vegetables is lower than in oil and
supplements. Also, the phylloquinone content of green vegetables
depends on their content in chlorophyll (green pigment), so that
outer leaves have more phylloquinone than inner leaves. The
efficiency of phylloquinone intestinal absorption varies among plant
sources and is increased with the addition of a fat source to a meal.
Finally, the hydrogenation of vegetable oils may decrease the
absorption and biological effect of dietary phylloquinone. If you
wish to check foods for their nutrient content, including
phylloquinone, search the USDA food composition database. A number
of phylloquinone-rich foods are listed in Table 2, with their content
in phylloquinone expressed in micrograms (μg).
|
Food |
Serving |
Phylloquinone (μg) |
|---|---|---|
|
Kale, raw |
1 cup (chopped)
|
472
|
|
Swiss chard, raw |
1 cup
|
299
|
|
Parsley, raw |
¼ cup
|
246
|
|
Broccoli, cooked |
1 cup (chopped)
|
220
|
|
Spinach, raw |
1 cup
|
145
|
|
Watercress, raw |
1 cup (chopped)
|
85
|
|
Leaf lettuce (green), raw |
1 cup (shredded)
|
46
|
|
Soybean oil |
1 Tablespoon
|
25
|
|
Canola oil |
1 Tablespoon
|
10
|
|
Olive oil |
1 Tablespoon
|
8
|
|
Cottonseed oil |
1 Tablespoon
|
3
|
