Biochemical assessment of the nutritional status of vitamin B6 in the human

Abstract

It has been found that the human body stores only a small amount of Vitamin-B6 and can quickly be depleted of this vitamin. Pyridoxal phosphate is the active form of Vitamin-B6. Normal adults have blood plasma levels of Vitamin-B6 in excess of 50 ng/ml whereas in those with a deficiency the levels fall below 25 ng/ml. In a study of 172 alcoholic patients 25% of those with liver cirrhosis had low serum Vitamin-B6 levels. In a group of New York City schoolchildren a serum level of 36 ng/ml of B6 was determined. The vitamin appears in the urine mainly as pyridoxal but also as pyridoxamine and pyridoxine. A rapid decrease in excretion has been noted when patients received only .16 mg of Vitamin-B 6 daily. A daily supplement of 1.5 mg of pyridoxine corrected this. Age differences in excretion have been shown with children excreting more than adults. The ingestion of oral contraceptives did not alter the excretion of Vitamin-B6. Normal subjects excrete .5-1.3 mg of 4-pyridoxic acid a day. This represents 40% of daily ingested Vitamin-B6. Thus a 2 mg daily intake of Vitamin-B6 would result in a daily excretion of .8 mg of 4-pyridoxic acid. Tryptophan metabolism has been shown to be influenced by Vitamin-B6 dietary restriction. Following a loading dose of tryptophan excretion of xanthurenic acid and other tryptophan metabolites have been increased. This test has been used for detecting Vitamin-B6 deficiency. Prolonged fasting modifies interpretation of this test and oral contraceptives containing estrogen and progsterone increase the amounts of tryptophan metabolites in the urine. Large doses of pyridoxine supplements correct this increase in urinary metabolites. Transaminase activities in erythrocytes leukocytes and plasma have been used as measurements of Vitamin-B6 nutritional status. Other tests have been suggested but have not been shown to be better.