Effects of oral contraceptives on tryptophan metabolism and vitamin B6 requirements in women.

To evaluate the effect of oral contraceptive usage on the nutritional requirement for vitamin B6, control women and oral contraceptive users were depleted of vitamin B6 for 1 month followed by a month of repletion with 0.8, 2.0, or 20.0 mg of pyridoxine hydrochloride per day. At weekly intervals a number of indices of vitamin B6 nutrition were measured. Marked elevation in excretion of tryptophan metabolites occurred in oral contraceptive users after tryptophan loads. However, other indices of vitamin B6 nutritional state, including urinary 4-pyridoxic acid excretion, urinary cystathionine excretion, plasma pyridoxal phosphate concentrations, and erythrocyte aspartate and alanine aminotransferases were not different between controls and oral contraceptive users. The excretion of metabolites after oral loading doses of L-kynurenine (which bypasses tryptophan oxygenase) was elevated in oral contraceptive users indicating that abnormal metabolism of tryptophan was not due only to induced tryptophan oxygenase. The data indicate that use of oral contraceptives does not generally change the requirement for vitamin B6 but rather produces a specific change in activity of enzymes beyond kynurenine in the pathway of tryptophan metabolism.