Vitamin K; sources, significance and bioavailability

THE SIGNIFICANCE OF VITAMIN K

Vitamin K is synthesized by plants and bacteria, which use it for electron transport and energy production. Animals, however, cannot synthesize the vitamin; still, they require it for blood clotting, bone formation, and other functions.

Vitamin K


These needs are critical to good health; yet the prevalent microbial synthesis of the menaquinones, including that occurring in the hindgut of humans and other animals (many of whom have coprophagous eating habits), results in frank deficiencies of this vitamin being rare. Nevertheless, vitamin K deficiency can occur in poultry and other monogastric animals when they are raised on wire or slatted floors and treated with certain antibiotics that reduce their hindgut microbial synthesis of the vitamin. Human neonates, particularly premature infants, can also be at risk of hemorrhagic disease by virtue of limited transplacental transfer of the vitamin.

The function of vitamin K in blood clotting is widely exploited to reduce risks to post-surgical thrombosis and cardiac patients. Coumarin-based drugs (e.g., warfarin, dicumarol) and other inhibitors of the vitamin K oxidation/ carboxylation/reduction cycle are valuable in this purpose. In addition, vitamin K has clear roles in the metabolism of both calcified and non-calcified tissues. It may well prove that vitamin K functions with other vitamins in the regulation of intracellular Ca2metabolism, in signal transduction, and in cell proliferation, which functions have profound effects on health status.

SOURCES OF VITAMIN K

Vitamers K

There are two natural and one synthetic source of vitamin K:

Phylloquinones (formerly, K) . Green plants synthesize the phylloquinones (2-methyl-3- phytyl-1,4-naphthoquinones) as a normal component of chloroplasts.

Menaquinones (MKs, formerly K). Bacteria (including those of the normal intestinal microflora) and some spore-forming Actinomyces spp. Synthesize the menaquinones. The predominant vitamers of this series contain 6–10 isoprenoid units; however, forms with as many as 13 units have been identified.

Menadione . The formal parent compound of the menaquinone series does not occur naturally, but is a common synthetic form called menadione (2-methyl- 1,4-naphthoquinone). This forms a water-soluble sodium bisulfite addition product, menadione sodium bisulfite, the practical utility of which is limited by its instability in complex matrices such as feeds. However, in the presence of excess sodium bisulfite it crystallizes as a complex with an additional mole of sodium bisulfite (i.e., menadione sodium bisulfite complex), which has greater stability and therefore is used as a supplement to poultry and swine feeds. A third water-soluble compound is menadione pyridinol bisulfite (MPB), a salt formed by the addition of dimethylpyridinol.

Dietary Sources

Green leafy vegetables tend to be rich in vitamin K, whereas fruits and grains are poor sources. The vitamin K activities of meats and dairy products tend to be moderate. Unfortunately, data for the vitamin K contents of foods are limited by the lack of good analytical methods. Nevertheless, it is clear that, because dietary needs for vitamin K are low, most foods contribute significantly to those needs.1 This is not true for breast milk, which has been found in most studies to be of very low vitamin K content and insufficient to meet the vitamin K needs of infants.

Bioavailability

Little is known about the bioavailability of vitamin K in most foods. It appears, however, that only about 10% of the phylloquione in boiled spinach is absorbed by humans. This may relate to its association with the thylakoid membrane in chloroplasts, as the free vitamer was well absorbed (80%). This suggests that vitamin K may be poorly bioavailable from the most quantitatively important sources of it in most diets: green leafy vegetables.

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