Abstract Xanthurenic acid (XA) is a secondary product of the biosynthesis of nicotinamide from L ‐tryptophan. Disturbances in this pathway have been observed in depression. An enhanced glucocorticoid secretion is considered to induce this pathway. This could result in reduced levels of L ‐tryptophan (L‐TRP) in the plasma. The present study investigates whether the synthesis of XA from L‐TRP is distrubed in depression, and also whether the synthesis of XA is intercorrelated with L‐TRP or measures of glucocorticoid secretion. To this end the XA excretion in 24 hour urine following L‐TRP loading was determined in 166 psychiatric inpatients. There were no significant differences in XA excretion between non‐depressed psychiatric controls, minor depression and major depression. Among the depressed patients there was a significant ( p <0·01) negative correlation between the excretion of XA and total L‐TRP levels in plasma, with abnormally decreased ( p <0·0006) L‐TRP levels occurring in the depressed patients with a disturbed XA excretion (≥ 106·8 m̈mol/24 hour).
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