Biochemical Assessment of Vitamin B6 Status in Man

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Abstract
Biochemical assessment of vitamin B6 status was investigated in three adult males using the following experimental protocol: 7 days of self-selected diet, vitamin B6 depletion with liquid formula diet, vitamin B6 repletion with liquid formula diet, and self-selected diet with 2.0 mg pyridoxine hydrochloride (1.6 mg pyridoxine) supplement. Each subject was clinically examined: body weight, oral temperatures, triceps skin folds, arterial blood pressures, hematocrits, hemoglobins; and urinary glucose, ketones, and protein were monitored throughout the investigation. The 24-hr urinary excretion of oxalate did not change nor reflect vitamin B6 status. Increased xanthurenic acid excretion following an oral load of tryptophan was observed in each subject; the appearance of this excretion was proportional to "excess" dietary protein and to initial body weight. With oral administration of 2.0 mg pyridoxine hydrochloride per day, xanthurenic excretion after tryptophan returned to normal within 24–48 hours. Erythrocyte glutamic-pyruvic transaminase (EGPT) and glutamic-oxaloacetic transaminase (EGOT) activities decreased in response to vitamin B6 depletion. However, restoration of activities to normal required 3–4 weeks of vitamin B6 supplementation. Response of transaminase activities to pyridoxal phosphate (PALPO) stimulation in vitro suggests that EGPT is a more sensitive measure of vitamin B6 status in man than is EGOT, at least in the situation of a mild deficiency. From these various observations, it would seem that the measurement of erythrocyte transaminase activities is superior to the tryptophan load test in assessment of long-term vitamin B6 status in man. Specifically, the stimulatory effect of PALPO stimulation in vitro in the measurement of EGPT activity appears to afford the most sensitive index.