The relationship between plasma tryptophan levels and depression

L-tryptophan is an essential amino acid and a raw material for serotonin production through the kynurenine pathway. Serotonin and kynurenine are involved in the pathophysiology of major depressive disorder (MDD).

A study published in The Journal of Clinical Psychiatry in 2014 examined the relationship between plasma tryptophan levels and MDD.

To investigate this relationship, researchers reviewed studies of serotonin and depression while also conducting their own study.

In a randomized controlled study, tryptophan levels were measured in 66 participants diagnosed with MDD and 82 controls. Fifty-three study participants were receiving medication.

The MDD group had a lower mean plasma tryptophan level, 53.9 μmol/L, compared with 57.2 μmol/L in the control group.

The meta-analysis reviewed 106 studies and selected 24 that measured plasma tryptophan in participants and controls. It included 744 participants with depression and 793 controls.

The review found low tryptophan levels among people diagnosed with MDD. Among participants not receiving medication, plasma tryptophan levels affected depressive symptoms: the lower the level, the more severe the symptoms.

Researchers’ conclusion:

The meta-analysis provided compelling information linking low plasma tryptophan to severe depressive symptoms, particularly in patients not receiving medication.