Chemistry Reference
In-Depth Information
Food composition data for vitamin K derived from HPLC are generally
lower than earlier data derived from the chick bioassay (Booth et al., 1993).
The use of these HPLC-derived data on the vitamin K content of foods allows
for a more accurate determination of the phylloquinone content of a typical
Western diet.
12.5.2.
Absorption, Metabolism and Excretion
Dietary vitamin K, mainly as phylloquinone, is absorbed into the
lymphatic system from the proximal intestine after solubilization into
mixed micelles composed of bile salts and the products of pancreatic lipo-
lysis (Shearer et al., 1974). In healthy adults, the efficiency of absorption of
phylloquinone is about 80% (Shearer et al., 1974). Intestinal bacteria can
synthesize a variety of menaquinones which are absorbed to a limited extent
from the large intestine, transported into the lymphatic system, cleared by
the liver and released in VLDL. However, it is not fully clear to what extent
intestinal menaquinone contributes to the vitamin K requirement. Approxi-
mately 50% of vitamin K is carried in the plasma in the form of VLDL,
about 25% in LDL (low-density lipoprotein) and about 25% in HDL (high-
density lipoprotein). Once in the circulation, phylloquinone is cleared
rapidly at a rate consistent with its continuing association with chylomi-
crons (Kohlmeier, 1996). Vitamin K is extensively metabolized in the liver
and excreted in the urine and bile. It has been demonstrated in tracer
experiments that about 20% of an injected dose of phylloquinone is recov-
ered in urine whereas about 40-50% is excreted in the faeces via the bile
(Shearer et al., 1974) and the proportion excreted was the same regardless of
whether the injected dose was 1000 or 45 mg. It seems likely, therefore, that
about 60-70% of the amount of phylloquinone absorbed from each meal
will ultimately be lost to the body by excretion. These results suggest that the
body's stores of phylloquinone are constantly replenished. Vitamin K itself
is too lipophilic to be excreted in the bile and is excreted as side chain-
shortened carboxylic acid metabolites.
There is no evidence that phylloquinone or menaquinone are toxic.
However, high intakes of phylloquinone can negate the effects of the anti-
coagulant warfarin. The synthetic form of vitamin K, menadione, can interfere
with the function of glutathione, one of the body's natural antioxidants,
resulting in oxidative damage to cell membranes. Menadione given by injec-
tion has induced liver toxicity, jaundice and haemolytic anemia (due to the
rupture of red blood cells) in infants and is no longer used for treatment of
vitamin K deficiency (Suttie, 1996). No tolerable upper level (UL) of intake
has been established for vitamin K.
