Insensitivity of the Tryptophan-Load Test to Marginal Vitamin B-6 Intake in Rats

The tryptophan-load test for vitamin B-6 nutritional status was administered to adult female Long-Evans rats fed graded levels of pyridoxine hydrochloride (PN·HCl) in two experiments, and its sensitivity to marginal vitamin B-6 intake was evaluated. In Experiment 1, rats were 4-h meal-fed an AIN-76A (20% casein) diet devoid of PN·HCl for 3 wk, then repleted (n = 12) for 6 wk with 4-h pair-fed meals of either 0.25, 0.5, 1.0 or 7.0 (control) mg PN·HCl/kg diet. In Experiment 2, rats (n = 16) were pair-fed for 10 wk either 0.0, 0.5, 1.0 or 7.0 (control) mg PN·HCl/kg diet, with 24-h access to food. Vitamin B-6 nutritional status was assessed at the end of each experiment. Except in rats fed 0 mg PN·HCl/kg diet, mean body weights were not significantly different among diet groups of either experiment. Plasma pyridoxal phosphate (PLP), pyridoxal and total vitamin B-6 concentrations, determined by HPLC, were very sensitive to gradations in dietary PN·HCl concentrations (P < 0.05). Red blood cell endogenous and PLP-stimulated alanine and aspartate aminotransferase activity did not statistically differentiate all levels of dietary vitamin B-6, although the calculated activity coefficient for each enzyme (stimulated/endogenous activity) did. Urinary xanthurenic acid excretion following a tryptophan load [24.5 µmol (5 mg) L-tryptophan/100 g body weight, injected intraperitoneally] was significantly (P < 0.05) elevated compared with controls only in the group fed 0 mg PN·HCl/kg diet. At the tryptophan dose used here, the tryptophan-load test was not useful in detecting marginal vitamin B-6 intake in rats.