BackgroundProtein quality, evaluated using Digestible Indispensable Amino Acid Score (DIAAS), requires ileal digestibility values of individual indispensable amino acids (IAAs) in each protein. However, true tryptophan (Trp) digestibility has rarely been quantified in humans. ObjectiveTo measure the true Trp digestibility and DIAAS of 2H-intrinsically labeled plant and animal protein sources in humans, using the dual isotope tracer technique. MethodsThe true Trp digestibility of 2H intrinsically labeled plant proteins such as whole mung bean (n = 6) and dehulled mung bean (n = 6), chickpea (n = 5), and yellow pea (n = 5), and protein from animal source foods such as egg white (n = 6), whole egg (n = 6), chicken meat (n = 6), and goat milk (n = 7) was determined against the known digestibility of U-13C spirulina whole cell protein as reference, except for goat milk protein that was measured against free crystalline 13C-Trp as reference. Banked samples from earlier studies conducted to determine true IAA digestibility of different protein sources were used for the analysis. DIAAS was calculated for each test protein using digestibility corrected IAA scores (mg IAA/g of protein) in comparison with the IAA requirement score for adults. ResultsThe true Trp digestibility of whole mung bean, dehulled mung bean, chickpea, yellow pea, egg white, whole egg, chicken meat, and goat milk were 67.6 ± 3.7%, 74.5 ± 4.4%, 72.6 ± 2.3%, 72.5 ± 2.2%, 89.7 ± 2.5%, 91.4 ± 2.6%, 95.9 ± 2.2%, and 92.8 ± 2.9%, respectively. The true Trp digestibility of plant protein sources was significantly lower than that of animal protein sources (P < 0.05). Trp was not a limiting IAA in all the tested proteins. ConclusionThe true Trp digestibility determined in this study ranged from 67.6 ± 3.7% to 95.9 ± 2.2% for whole mung bean and chicken meat, respectively, and adds to the database of individual true IAA digestibility of different protein sources. Trial registration numberThis study was registered in Clinical Trials Registry of India (CTRI) with registration numbers CTRI/2017/11/010468, CTRI/2020/04/024512, and CTRI/2018/03/012265.