BACKGROUND:: METHODS:: Serum concentrations of kynurenine and tryptophan, calculated kynurenine to tryptophan ratios (kyn trp-1) as an indirect estimate of IDO activity, and neopterin levels reflecting IFN- γ mediated immune activation, were assessed before and after bariatric surgery. The study population included 22 morbidly obese individuals and 20 normal-weight volunteers. RESULTS:: Median weight loss after 24.4±5.1 months was 40.6 kg resulting in a reduction of BMI from 44.1 kg/m to 29.9 kg/m ( P <0.001). Preoperative kyn trp-1 in morbidly obese patients was significantly increased compared to the control group (41.6±20.1 mmol/mol vs 26.5±5.1 mmol/mol; P <0.001). Postoperative weight reduction did not lead to normalization of kyn trp-1 (37.9±14.0 mmol/mol). As a consequence, tryptophan levels were significantly lower in morbidly obese patients (pre-: 51.5±9.2 μmol L and postoperatively: 46.9±7.6 μmol L) when compared with those of normal-weight controls (64.8±9.5 μmol L; P <0.001). In addition, neopterin levels were elevated in the study population pre- and postoperatively compared to normal-weight volunteers (both P <0.001). CONCLUSIONS