In psychiatrically healthy individuals, overweight women but not men have lower tryptophan levels.

Gender differences in tryptophan (TRP) breakdown in obese individuals have been previously reported. This could be both contributory to, as well as a consequence of, gender differences in mood changes among obese people. To exclude the potential effect of depression on TRP breakdown and its levels in obesity, we replicated analyses in psychiatrically healthy individuals. In 1000 participants, plasma kynurenine (KYN), TRP, and the KYN/TRP ratio were compared between overweight/obese and normal-weight individuals using analysis of covariance, with adjustment for age and gender. Bivariate post hoc tests were also conducted. There were no significant relationships between KYN, TRP, or the KYN/TRP ratio and overall overweight/obese status. However, a significant gender by weight category interaction was identified for TRP only, with overweight/obese women having lower TRP than overweight/obese men (p = 0.02). No gender differences in TRP were found in non-obese participants. Our study in psychiatrically healthy individuals suggested that lower TRP levels in obese women were not secondary to depression, strengthening the possibility that TRP levels could mediate depression in vulnerable women. Thus experimental manipulations of TRP levels could be used to advance theoretical knowledge, prevention, and clinical control of depression in obese women.