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deuterium-labeled broccoli suggest that the bioavail-ability of endogenous vitamin K can be studied in humans by intrinsic stable isotope-labeling procedures.

Once absorbed, vitamin K is transported to the liver in the chylomicrons, where it becomes distributed among the triglyceride-rich chylomicron remnants (ca. 50%) and the low-density lipoprotein and high-density lipoprotein fractions of plasma (ca. 25% each). Plasma vitamin K concentrations, which are typically in the low nanomolar range in humans, are much lower than for the other fat-soluble vitamins (A, D, and E), and they are strongly correlated with the triglyceride content of the plasma. Indeed, some authorities prefer to express plasma vitamin K as a ratio to triglycerides instead of as a simple concentration. Differences between the apoE lipoprotein genetic variants affect plasma vitamin K, according to their different triglyceride clearance profiles. There is evidence for a major diurnal cycle of plasma vitamin K, with peak concentrations of both vitamin K and its associated triglycerides occurring in late evening and with lowest values in the morning. A kinetic study using radioactive vitamin K indicated that the turnover time of the exchangeable pool of the vitamin is quite short, approximately 1.5 days, and the first and second exponential decay curves had half-lives of 0.5-1 and 25-78 h, respectively. The exchangeable body pool size was only approximately 1 mg/kg body weight. The liver is an important repository of the vitamin for both plant-derived phylloquinone and the bacterially derived menaquinones. Depletion studies have indicated that the hepatic phylloqui-none stores seem much more labile than the mena-quinone stores, and that a functional deficiency accompanies the loss of the phylloquinone, which the remaining nondepleted menaquinones cannot prevent. Despite this, if menaquinones are given exogenously, they can be curative. Different tissues have different relative avidities for phylloquinone and menaquinones, and it has been suggested that they may have a different spectrum of functions from each other. Thus, in humans, phylloquinone is concentrated in liver, heart, and pancreas. The longer chain menaquinones, MK-6 to -11, are found mainly in liver with traces in heart and pancreas, but MK-4 is found especially in brain and kidney, where it exceeds phylloquinone concentrations. The tissue distribution in humans is similar to that in the rat.

The turnover of phylloquinone results in ca. 40-50% of the exchangeable body pool being transferred via the bile into the feces and 20% being excreted into the urine, the latter including the excretion of oxidized products that become conjugated as glucuronides.

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