Vitamin K is a group of compounds that share the same biological activity. Vitamin K1 is also called phylloquinone and is synthesised by plants. Menaquinones are a group of structurally related compounds synthesised by bacteria, and also called vitamin K2. A man-made form of vitamin K is also available called menadione (vitamin K3). Nutritionally important sources of vitamin K include the brassica family vegetables such as kale, broccoli, cabbage, watercress, brussel sprouts, mustard seeds and turnips. Absorption of vitamin K occurs in the small intestine, but the colon may also be an important location of absorption because colonic gut bacteria can synthesise vitamin K, and this appears to be an important source of the vitamin in humans. The traditional view of vitamin K is that it is required for post translational modification of glutamic acid residues to form γ-carboxyglutamic acid, a process that activates certain proteins. Such post translational carboxylation is required for a number of blood clotting factors (II, VII, IX and X).
This explains the classic example of vitamin K and blood clotting. Without adequate vitamin K the glutamic acid residues on the clotting factors are not able to undergo posttranslational modification and there are therefore not activated, resulting in a blood clotting disorder that could be said to be a vitamin K deficiency. Such post translational modification of glutamic acid residues is also required in bone tissue on the protein osteocalcin, which upon post translational carboxylation by vitamin K is able to bind 3 molecules of calcium. This pathway explains the role of vitamin K in bone formation. For example, individuals with the lowest intakes of vitamin K1 have an increased risk of experiencing hip fractures compared to individuals with the highest intakes of vitamin K11. Although such observational studies are interesting, it is not clear if dietary vitamin K can prevent hip fractures or if high intakes of the vitamin are just a marker for a healthy diet, which may be the causative agent.
The role of vitamin K in cancer prevention was first researched in the late 1940’s when menadione was investigated for its anticancer effects. Results from such studies have shown a detrimental effect from menadione on cancer cell lines2. In particular menadione shows pronounced anticancer effects when combined with traditional chemotherapeutic agents and with vitamin C. Phylloquinone is much less studied (presumably because it is a natural agent and not a synthetic drug) but has also shown detrimental effects to cell lines grown in culture. Most of the work on vitamin K has been performed in cell culture on cancer cell lines, but some human trials have been performed2. The results from such studies have been positive, but vitamin K therapy is no anticancer miracle, although survival rates do increase. However, the vegetables that phylloquinone is contained within, brassica family vegetables, do possess potent anticancer effects in themselves and so eating vitamin K rich foods will provide protection from cancer.
RdB
