Jtilization of vitamin Bs
Several studies have indicated that the intestinal aborption of the three Bs-vitamers is equally efficient and akes place by passive diffusion over avide range of uminal concentrations. A recent human study suggests, however, that the absorption of PM is either slower han that ofPL or PN, or it is metabolized differently from II, or PN, or both. The phosphorylated forms are hydrozed prior to absorption; however, some may be aborbed intact. Even though the free vitamers are Ihosphorylated in the mucosal cell, the rate of phosIborylation apparently does not affect the rate of transIOrt of the vitamin from the lumen to the serosa. lespite an increased mucosal activity of pyridoxal kinase 11 children with acute celiac disease, the net absorption fvitamin B is reduced. The availability of vitamin Bs rom foods is highly variable; little is known about the possible influence on the absorption of vitamin B by other JOd components. The effects on bioavailability of vitamin by food processing and storage also vary and probably depend on the composition offood and the conditions of processing and storage.
Cellular uptake of vitamin B in tissues occurs most rapidly as the free vitamers, but each form can be phosphorylated by the cell. The phosphorylated forms of pyridoxine (PNP) (PMP) can be converted to PALP to provide the cells with the active coenzyme form of the vitamin. Although PMP can activate some enzymes, most depend on PALP.
Both the free bases and the phosphorylated forms of pyridoxal and pyridoxamine are found in the blood, but no pyridoxine or its phosphate have been detected. The plasma and erythrocyte levels of pyridoxal and P ALP are influenced by dietary intake ofthe vitamin and have been used in research studies to assess the nutritional status and requirements of vitamin B.
Vitamin Bs is excreted in the urine in the form of free and phosphorylated vitamers and as 4-pyridoxic acid, the major metabolite of the vitamin. The excretion levels of total vitamin Bs and 4-pyridoxic acid reflect the adequacy of intake and provide another means of assessing vitamin Bs nutriture.
