The effect of dietary protein on the metabolism of vitamin B-6 in humans.

Eight men, aged 21-31 yr, were fed semipurified diets containing 0.5 (low), 1.0 (medium) and 2.0 (high) g protein/kg body weight; vitamin B-6 intake was kept constant at 1.6 mg/d. Each level of protein was fed for 15 d. Urinary vitamin B-6 (UB-6), urinary 4-pyridoxic acid (4-PA), plasma total vitamin B-6 (PB-6) and plasma pyridoxal 5'-phosphate (PLP) were determined every third day. Means are reported for all subjects of values determined during the second half of each period. Concentration of urinary and plasma vitamin B-6 compounds were negatively correlated with protein intake: the correlation coefficient of nitrogen intake with 4-PA was -0.69 (P less than 0.01); with PLP, -0.45 (P less than 0.05); and with PB-6, -0.48 (P less than 0.05). The decrease in UB-6 was not statistically significant. These results indicate that with increased intake of dietary protein, vitamin B-6 is retained in the body for increased catabolism of amino acids. When evaluating vitamin B-6 requirements or status in humans, protein intake must be considered.