Define the Bioavailability of Vitamin K?
Very little is known about the bioavailability of the K vitamins from different foods. It has been estimated that the efficiency of absorption of phylloquinone from boiled spinach (eaten with butter) is no greater than 10% compared with an estimated 80% when phylloquinone is given in its free form. This poor absorption of phylloquinone from green leafy vegetables may be explained by its location in chloroplasts (organelles in plant cells that conduct photosynthesis) and tight association with the thylakoid membrane (a phospholipid bilayer membrane-bound compartment internal to chloroplasts). In comparison, the bioavailability of MK-4 from butter artificially enriched with this vitamin was more than two-fold higher than that of phylloquinone from spinach. The poor extraction of phylloquinone from leafy vegetables, which as a category represents the single greatest food source of phylloquinone, may place a different perspective on the relative importance of other foods with lower concentrations of phylloquinone (e.g. those containing soybean and rapeseed oils) but in which the vitamin is not tightly bound and its bioavailability is likely to be greater.
Vitamin K availability varies directly with fat intake and any condition of fat malabsorption reduces its bioavailability. In healthy adults, absorption of phylloquinone has been estimated to be 80 percent when phylloquinone is administered in its free form as discussed above, but decreases significantly when absorbed from foods. Cooking has no effect, but addition of fat increases absorption multifold.