Urine 3-hydroxykynurenine is higher during the postovulatory phase than in the preovulatory phase indicating a higher vitamin B6 requirement.

The relationship between l-tryptophan to nicotinamide metabolism and the menstrual cycle of Japanese women was investigated. Nine metabolism intermediates from urine samples collected during the preovulatory and postovulatory phases were measured. Only urine 3-hydroxykynurenine was higher in the postovulatory phase than in the preovulatory phase. This increase in 3-hydroxykynurenine suggests a decreased reaction of 3-hydroxykynurenine → 3-hydroxyanthranilic acid catalyzed by kynureninase, a vitamin B6 enzyme.