Effect of Protein Quality on Vitamin B-6 Status in the Rat

Although it is well established that the requirement for vitamin B-6 is increased by high levels of dietary protein, little is known regarding the effect of protein quality on this requirement. We therefore compared the effect on vitamin B-6 status of diets containing amino acid mixtures equivalent to low quality (LQ) or good quality (GQ) protein. The effect of protein quality was tested at two levels of vitamin B-6, 0.2 and 7.0 mg/kg diet. Food intake was controlled by pair-feeding so that no statistically significant difference in food intake existed among the groups. After 6 weeks, vitamin B-6 status was evaluated by determining urinary 4-pyridoxic acid, plasma pyridoxal phosphate and total vitamin B-6 in liver. At both levels of vitamin B-6 intake, mean values for all three indices of vitamin B-6 status were lower in rats fed LQ protein than in rats fed GQ protein, but these differences were not statistically significant (Scheffé's multiple-range test). However, a strong and highly significant linear relationship (P < 0.0001) existed between the mean values for each parameter of vitamin B-6 status and protein quality—specifically, LQ protein appeared to have an adverse effect on vitamin B-6 status of rats fed either suboptimal or ample vitamin B-6. These results suggest a minor but consistent deleterious effect of LQ protein on vitamin B-6 status in rats, regardless of vitamin B-6 intake.