Nutritional summary

Function: Vitamin B6 (B6) participates in more than one hundred transamination, decarboxylation, and other types of reactions, including the initial step of porphyrin synthesis, glycogen mobilization, amino acid transsulfuration. and neurotransmitter synthesis.

Food sources: Good sources include fortified cereals, organ meats, muscle foods, potatoes, and fruits other than citrus.

Requirements Current intake recommendations arc 1.3 mg/d for young women and men, slightly higher for older adults. Higher needs also are likely during pregnancy (1.9mg/d) and lactation (2,0mg/d).

Deficiency: Low intakes may cause microcytic anemia, brain current (EEG) abnormalities and epileptic seizures, depression, confusion, seborrheic dermatitis, possibly also platelet and clotting dysfunction.

Excessive intake Consumption of very high doses of pyridoxine (more than 100 mg/d in adults) may cause peripheral sensory neuropathy, and possibly dermatological lesions. Increased intakes from supplements may interact with the action of drugs including levodopa (Dopar, Larodopa. Smemet. Atamet). phenobarhital (Luminal. Solfoton), and phenytoin (Dilantin).

Dietary sources

Animal-derived foods contain mainly pyridoxal 5'-phosphate (PLP) and pyridox-amine phosphate (PMP).

Plant-derived foods contain mainly pyridoxine, pyridoxine 5f-phosphate I I'NP). 4 -O-t^l-D-glycopyranosyl) pyridoxine, 5 -O-(jS-D-glycopyranosyl) pyridoxine. pyndox-i rte-5' - D-cel lob i oside, pyridoxine-4'-oligosaccharides, pyridoxine-5' -o I igosaccharides. pyridoxine-5'-(/i-D-glucosyl-malonyl ester), pyndoxine-5'-(0-D-glueosyl-(>-hydrox-ymethylglutaryl ester), pyridoxine-5'-|8-D-ccllobiosyl-indoleacetyl ester, and others (Gregory. 1998).

Good B6 sources include bananas (6mgkg), potatoes (3.0mg/kg). and other tubers, watermelon (1.4 mg kg), and fonilied cereals, as well as liver (9nig kg) and

Pyridoxal 5-phosphate

Pyndoxamine S'-phosphate figure 10.22 Vitamin B6 in animal-derived foods

Py ridoxme Py ridoxi n e- 51 -tf -D-G lucoside

Figure 1(1.23 Vitamin B6 in p!anc-d«rrivpd foods other organ meats, beef (3.9mg/kg), pork (4.0mg/g), poultry l4.7mgkg). and lish (2.8mg/kg).

Median daily intake has been estimated to be 1.5 mg in American women, and 2 mg in men (food and Nutrition Board, Institute of Medicine. 1998}, but much lower in elderly women (1 Omg) and men (1.2 mgi.

Cooking may lead to losses due to leaching into discarded water. Heating or prolonged storage can promote the reaction of PLP or pyridoxal with unspecific protein lysyl residues and the formation of e-pyridoxy I lysine with low bioavailability. Ascorbate and other components in foods or in a meal can promote at modestly elevated temperatures (less than 50°C) the conversion of pyridoxinc to the inactive form 6-hydroxypyridoxme (Tadera et«/., 1986).

6-HydroxypyridOxine

Lysylpyridoxine c—o-n-c-c-c-c-c-cooh h h-3 h? h-, h;i h oh

6-HydroxypyridOxine

Lysylpyridoxine

Fleuri- 10.24 Inactive derivatives of vitamin 86

Digestion and absorption

More than 75% of B6 in a mixed meal is absorbed in the small intestine, if digestive and absorptive functions are normal.

PI P. PMP. and PNP are dephosphorylated in the intestinal lumen by alkaline phosphatase (EC3.1.3.1 > and other phosphatases. Dietary 5'-t)-(/i-.D-glycopyranosyl) pyn-doxine may be cleaved by an incompletely characterized mucosal glucosidase (Gregory, 1998). The nonphosphorylated and nonglycosylated forms can enter duodenal and jejunal enterocytes by nonsaturable diffusion through unidentified carriers. B6 is trapped metabolically by phosphorylation in tbecytosol (pyridoxal kinase, CC2.7.1.35) In addition. some of the pyridoxinc glycoside can enter mucosal cells intact, possibly via the sodium/glucose cotransporter 1 (SLC5A11. and be cleaved in mucosal cytosol of the

Brush border me mftra ne

Figure 10.2S Intestinal absorption of vitamin B6

Brush border me mftra ne

Basolateral Capillary membrane endothelium

Figure 10.2S Intestinal absorption of vitamin B6

jejunum by pyridoxine-beta-l)-glucoside hydrolase (Tnimbo et <j/.. 1990; MeMahon & a/.. 1997). 136 appears to cross the basolateral membrane mainly without a phosphate group attached; the mechanism of this transfer is still unknown.

The bioavailability of dietary B6 glucoside (about 50%) is low compared to other Bft vitamers (Gregory, 1998). Incomplete hydrolysis of the glucoside and inhibition of intcrconvcrsion reactions may be responsible. e-Pyridoxyllysine is a B6 form with low bioavailability that may be generated by heating or prolonged storage under reducing conditions. The normal SchilT base linkage with specific sites of B6-containing proteins readily dissolves in the inactive enzyme, especially upon hydrolysis of the protein. The covalent bond in e-pyridoxy I lysine, on the other hand, is less likely to dissociate in the intestinal lumen and thus lost to absorption. Bioavailability of B6 from foods can also be decreased by high fiber content of a meal due to slowed dephosphorylation.

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