Vitamin B-6 metabolism was studied as a function of dietary protein intake. Subjects were 29 young adults (29 +/- 1 y old) and 26 elderly adults (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, according 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 micrograms/g protein) and 1.47 +/- 0.05 mg/d (23.81 +/- 0.08 micrograms/g protein), respectively, during consumption of Diet A and 1.79 +/- 0.07 mg/d (14.49 +/- 0.11 micrograms/g protein) and 1.73 +/- 0.05 mg/d (16.24 +/- 0.06 micrograms/g protein) during consumption of Diet B. Plasma pyridoxal phosphate (PLP), pyridoxal 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 higher 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 effect 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 concentrations were not influenced by the amount of protein intake in young and elderly subjects.(ABSTRACT TRUNCATED AT 250 WORDS)