Vitamin K is the name given to a number of compounds that share the same biological activity and possess a 2-methyl 1,4-napthoquinone ring. These compounds are phylloquinone (vitamin K1), menaquinone (vitamin K2 and menadione (vitamin K3). Vitamin K is required for posttranslational carboxylation of specific glutamic acid residues to form γ-carboxyglutamate in 4 of the factors necessary for blood coagulation. Phylloquinones are produced by plants and sources include green leafy vegetables. In contrast, menaquinones are synthesised by bacteria, including bacteria present in the human gut. Good sources of menaquinones include meat and cheese. Menadione is not active biologically in humans until it is enzymatically alkylated within tissues. Cell culture studies using menadione have shown growth inhibitory effects via increase in levels of oxidative stress in cancer cells. Menaquinones and phylloquinones have shown the ability to up-regulate genes that cause cell cycle arrest and apoptosis in cancer cells.
Researchers1 have investigated the anti-carcinogenic activity of vitamin K in 24,340 subjects aged 35 to 64 years during an initial screening and a 10 year follow up. Diets were assessed using a food frequency questionnaires at baseline and cancer rates estimated from medical records at follow-up. After 10 years, 458 fatal and 1755 non-fatal cancer cases had been detected amongst the study participants. Analysis of the data showed that intakes of menaquinones were inversely associated with cancer incidence, but the results were not significant. Menaquinones were also inversely associated with cancer mortality, and even though the association was stronger than for cancer incidence, the results still did not reach statistical significance. For men, significant inverse associations were detected between menaquinones and both lung cancer and prostate cancer, and these results strengthened the non-significant associations between menaquinones and total cancer for men compared to women.
Vitamin K has not been extensively researched in epidemiological studies. Probably because until recently food compositional data for vitamin K relevant to humans was unavailable. Interestingly, in this study, no associations were detected for phylloquinones and cancer although previous studies have found cell growth inhibitory effects of phylloquinones. The stronger associations between vitamin K and cancer mortality (compared with cancer incidence) may reflect the fact that vitamin K is involved in cell cycle arrest and apoptosis which is more likely to have an impact on later stages of carcinogenesis. The statistically significant inverse association between lung and prostate cancer and menaquinones in men may suggest that the protective effects of vitamin K on cancer are site specific. The majority of cancers in women are in the breast tissue and it might be that vitamin K does not have the same effect in this tissue as others.
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