This work was supported by the National Institutes of Health (NIH) 5D43TW009349–03 “Inter-American Training for Innovations in Emerging Infectious Diseases” (to GOL). F. S. was supported by FONDECYT-CONCYTEC (grant contract number 246–2015-FONDECYT), and the UJMT Fogarty Global Health Fellows Consortium comprising Johns Hopkins University, the University of North Carolina, Morehouse University, and Tulane University (NIH Research Training Grant # D43 TW009340 funded by the NIH Fogarty International Center, NINDS, NIMH, NHBLI and NIEHS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Early childhood enteric infections have adverse impacts on child growth and can inhibit normal mucosal responses to oral vaccines, two critical components of environmental enteropathy. To evaluate the role of indoleamine 2,3-dioxygenase 1 (IDO1) activity and its relationship with these outcomes, we measured tryptophan and the kynurenine–tryptophan ratio (KTR) in two longitudinal birth cohorts with a high prevalence of stunting. Children in rural Peru and Tanzania (N = 494) contributed 1,251 plasma samples at 3, 7, 15, and 24 months of age and monthly anthropometrics from 0 to 36 months of age. Tryptophan concentrations were directly associated with linear growth from 1 to 8 months after biomarker assessment. A 1-SD increase in tryptophan concentration was associated with a gain in length-for-age Z-score (LAZ) of 0.17 over the next 6 months in Peru (95% confidence interval [CI] = 0.11�...