The term vitamin B6 includes three pyridines: pyridoxine, pyridoxal and pyri-doxamine and their 5'-phosphorylated derivatives, which are metabolically interconvertible. The liver is the main tissue responsible for converting pyridoxine and pyridoxamine phosphates into pyridoxal 5'-phosphate (PLP). PLP is the main coenzyme form of the vitamin and the co-factor for a large number of enzymes catalysing reactions of amino acids (Department of Health, 1991). PLP is also the main extra-cellular form of the vitamin, being transported in plasma bound to albumin. PLP, which is not enzyme-bound, is oxidised to pyridoxic acid and excreted in urine (Powers, 1999).
The vitamin is widely distributed in foods (Table 3.2), although much of the vitamin B6 in some vegetables may be present as glycosides, which are unavailable. The intestinal bacteria are also able to synthesise relatively large amounts of pyridoxine (Department of Health, 1991). Intestinal absorption takes place mainly in the jejunum by non-saturable, passive diffusion of the non-phosphorylated forms of the vitamin. Post-absorptive phosphorylation of all three forms is catalysed by pyridoxal kinase and the phosphorylation constitutes a form of cellular trapping in intestinal cells and other tissues (liver, muscles and brain) as the charge on the phosphate hinders efflux through the cell membrane. Pyridoxine exhibits greater stability than other forms of the vitamin. As the hydrochloride salt, it is used in dietary supplements and food fortification because of its stability, comparative ease of manufacture and low cost (Gregory, 2001).
The total body pool of vitamin B6 is about 15 mmol/kg body weight. Isotope tracer studies have indicated a turnover of about 0.13%/day hence the estimated minimum requirement for replacement is 0.02 mmol (5 mg)/kg body weight or 350 mg/day for a 70 kg adult. Vitamin B6 is extensively required in protein metabolism hence depletion studies have shown that deficiency develops more quickly on high protein intakes (80-160 g/day) and repletion is faster at lower intakes of protein (30-50g/day). Therefore RNIs are based on protein intakes and estimated at 15-16 mg/g protein (Department of Health, 1991). At average intakes of about 100g protein/day, this gives an RNI of 1.5-1.6mg vitamin B6 (Table 3.1). Average intakes in Britain are between 20 and 30 mg/g protein (Thurnham, 2000).
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