Vitamin K is a group of fat soluble vitamers that include phylloquinone (vitamin K1), menaquinone (vitamin K2), and menadione (a synthetic form of vitamin K). While phylloquinone is present in green leafy vegetables, menaquinone is a product of bacterial fermentation and is therefore found in fermented foods and also produced naturally in the gut of humans. Vitamin K is a cofactor for the enzyme γ-glutamyl carboxylase, an enzyme that is required for the carboxylation of specific glutamic acid residues into γ-carboxyglutamyl residues in some proteins. Because γ-carboxyglutamyl residues are present on proteins in the vascular wall and these proteins are inhibitors of vascular calcification, it has been suggested that vitamin K deficiency could reduce the activity of these proteins, and thus increase calcification of vascular tissue and increase the rate of atherosclerosis. Some evidence suggests that elderly individuals with calcification of the arteries have lower than average levels of plasma vitamin K.
The mechanism by which vitamin K could influence vascular calcification suggests that higher intakes of the vitamin may be associated with a reduced risk of cardiovascular disease or mortality. With regard the latter, researchers1 have investigated the correlation between vitamin K intake and mortality in a group of over 7000 individuals using a prospective cohort study (following a group of individuals over time). The dietary vitamin K intake of the subjects was calculated using food tables and the dietary intake was estimated using food frequency questionnaires. The results of the analysis showed that energy adjusted baseline dietary phylloquinone intake was inversely associated with a 46 and 36 % reduction in the risk of cancer and all cause mortality, respectively. Those who increased their phylloquinone or menaquinone intake from baseline had a 36 and 43 % reduction in the risk of cancer and all cause mortality, respectively, compared to those whose intake did not change or decreased.
Therefore higher levels of vitamin K are associated with a decrease in cancer and mortality risk. The reason for the association with cancer are not fully understood, but may relate to the antiproliferative effects of vitamin K that has been demonstrated in cell culture experiments. Although much of this work has been performed using synthetic menadione, phylloquinone does also possess antiproliferative effects in vitro. High doses of menadione given to viral cirrhosis sufferers have also been shown in a randomised trial to significantly increase survival chances at a 6 year follow up. Inverse associations between phylloquinone and prostate cancer risk have also been reported in the nutritional literature. Increases in phylloquinone, but not menaquinone, from baseline, were also associated with a reduced risk of cardiovascular disease. Vitamin K may therefore have a protective effect against cardiovascular disease. However, the intake of phylloquinone could just have been a marker for a higher quality plant based diet.
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