THE INFLUENCE OF PROTEIN-INTAKE ON VITAMIN-B-6 METABOLISM DIFFERS IN YOUNG AND ELDERLY HUMANS

Vitamin B-6 metabolism was studied as a function of dietary protein in take. Subjects were 29 young adults (29 +/- 1 y old) and 26 elderly ad ults (70 +/- 1 y old) who consumed standardized diets containing 12% ( Diet A) and 21% (Diet B) of total energy as protein for 3 wk each, acc ording to a randomized crossover design. Vitamin B-6 intake for young and elderly subjects was 1.52 +/- 0.08 mg/d (21.74 +/- 0.45 mu g/g pro tein) and 1.47 +/- 0.05 mg/d (23.81 +/- 0.08 mu g/g protein), respecti vely, during consumption of Diet A and 1.79 +/- 0.07 mg/d (14.49 +/- 0 .11 mu g/g protein) and 1.73 +/- 0.05 mg/d (16.24 +/- 0.06 mu g/g prot ein) during consumption of Diet B. Plasma pyridoxal phosphate (PLP), p yridoxal and total vitamin B-6 concentrations were significantly lower in the elderly subjects compared with the young adult subjects during both diet periods. In the elderly subjects, PLP was significantly hig her during Diet B consumption (Diet A: 27 +/- 3 nmol/L; Diet B: 32 +/- 3 nmol/L), whereas the level of protein intake had no significant eff ect on plasma PLP in the young adults (Diet A: 47 +/- 6 nmol/L; Diet B : 45 +/- 5 nmol/L). Plasma pyridoxal and plasma total vitamin B-6 conc entrations were not influenced by the amount of protein intake in youn g and elderly subjects. Relative urinary pyridoxic acid excretion did not differ significantly between diet periods in the elderly subjects (Diet A: 37 +/- 3%; Diet B: 43 +/- 3%), whereas pyridoxic acid excreti on was lower in young adults when Diet B was consumed (Diet A: 46 +/- 3%; Diet B: 38 +/- 2%; P < 0.001). The results of this study suggest a n age-dependent difference in the protein intake-related vitamin B-6 n eeds, whereby elderly subjects apparently need less vitamin B-6 at a h igher protein intake as compared with young adults.