True digestibility of tryptophan in plant and animal protein.

ABSTRACT

BACKGROUND: Protein quality, evaluated using Digestible Indispensable Amino Acid Score (DIAAS) requires ileal digestibility values of individual indispensable amino acids (IAA) in each protein. However, true tryptophan (Trp) digestibility has rarely been quantified in humans. OBJECTIVE: To measure the true Trp digestibility and DIAAS of 2H-intrinsically labelled plant and animal protein sources in humans, using the dual isotope tracer technique. METHODS: The true Trp digestibility of 2H intrinsically labelled 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 which 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 to the IAA requirement score for adults. RESULTS: The 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. CONCLUSION: The true Trp digestibility determined in the present 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. This study was registered in Clinical Trials Registry of India (CTRI) with registration number CTRI/2017/11/010468, CTRI/2020/04/024512, CTRI/2018/03/012265.