Dietary sources, bacterial synthesis and metabolism
Phylloquinone has a role in photosynthesis, and therefore it is found in all green leafy vegetables; the richest sources are spring (collard) greens, spinach, and Brussels sprouts. In addition, soybean, rapeseed, cottonseed, and olive oils are relatively rich in vitamin K, although other oils are not.
About 80% of dietary phylloquinone is normally absorbed into the lymphatic system in chylomicrons, and is then taken up by the liver from chylomicron remnants and released into the circulation in VLDLs.
Intestinal bacteria synthesize a variety of menaqui-nones, which are absorbed to a limited extent from the large intestine, again into the lymphatic system, cleared by the liver, and released in VLDLs. It is often suggested that about half of the requirement for vitamin K is met by intestinal bacterial synthesis, but there is little evidence for this, other than the fact that about half of the vitamin K in liver is phylloquinone and the remainder a variety of menaquinones. It is not clear to what extent the menaquinones are bio-logically active. It is possible to induce signs of vitamin K deficiency simply by feeding a phylloquinone-defi-cient diet, without inhibiting intestinal bacterial action.
The synthetic compound menadiol is absorbed largely into the hepatic portal system, and undergoes alkylation in the liver to yield menaquinone-4, which is released together with phylloquinone and other menaquinones in VLDLs.
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