Infections and persistent immunological activation are linked to increased kynurenine (KYN) and the KYN-to-Tryptophan (TRP) or KT ratio and may be critical factors in undernutrition. We sought to determine the association between the KT ratio and adult malnutrition, as well as investigate if nutritional supplementation had any influence on the decrease of the KT ratio. A total of 525 undernourished adults aged 18–45 years were recruited and provided a nutrition intervention for 60 feeding days. TRP and KYN concentrations were determined from plasma samples using LC-MS/MS. At baseline, the median (interquartile range (IQR)) TRP, KYN and KT ratios were 24.1 (17.6, 34.3) µmol/L, 0.76 (0.53, 1.18) µmol/L and 30.9 (24.5, 41.7), respectively. Following intervention, the median (IQR) KYN and KT ratios were significantly reduced to 0.713 (0.46, 1.12) µmol/L and 27.5 (21.3, 35.8). The KT ratio was found to be inversely linked with adult BMI (coefficient: −0.09; 95% CI: −0.18, 0.004; p-value = 0.06) but not statistically significant. Additionally, Plasma CRP was correlated positively, while LRP1 was inversely correlated with the KT ratio. Our data suggest that in Bangladeshi adults, the KT ratio is not related to the pathophysiology of malnutrition but correlated with inflammatory and anti-inflammatory biomarkers, and the ratio can be reduced by a nutrition intervention.