Evaluation of Protein Quality in Humans and Insights on Stable Isotope Approaches to Measure Digestibility – A Review

Abstract

The recent Food and Agricultural Organization/World Health Organization/United Nations University expert consultations on protein requirements and quality have emphasized the need for the new Digestible Indispensable Amino Acid Score (DIAAS), as a measure of protein quality. This requires human measurements of the true ileal digestibility of individual indispensable amino acids (IAA) until the end of the small intestine. Digestibility is measured using standard oro-ileal balance methods, which can only be achieved by an invasive naso-ileal intubation in healthy participants or fistulation at the terminal ileum. Significant efforts have been made in last two decades to develop non-invasive or minimally invasive methods to measure IAA digestibility in humans. The application of intrinsically labeled (with stable isotopes like 13C, 15N and 2H) dietary proteins have helped in circumventing the invasive oro-ileal balance techniques and allowed for the differentiation between endo and exogenous protein digestibility. The non-invasive indicator amino acid oxidation (IAAO) technique, which is routinely employed to measure IAA requirements, has been modified to estimate metabolic availability (a sum of digestibility and utilization) of IAA in foods, but provide estimate for a single IAA at a time and burdensome for participants. The recently developed minimally invasive dual isotope tracer method measures small intestinal digestibility of multiple amino acids at once and is suitable for use in vulnerable groups and disease conditions. However, it remains to be validated against standard oro-ileal balance techniques. This review critically evaluates and compares the currently available stable isotope-based protein quality evaluation methods with a focus on the digestibility and metabolic availability measurements in humans. In the view of building reliable DIAAS database of various protein sources and subsequently supporting protein content claims in food labeling, a re-evaluation and harmonization of the available methods are necessary.

Analysis and Screening of Commercialized Protein Supplements for Sports Practice

“…On the other hand, antinutritional factors such as trypsin inhibitors, tannins, and lectins, present in some protein-rich foods such as soybean meal, peas, and beans, could increase protein losses in the terminal ileum, reducing protein hydrolysis and the absorption of amino acids. A more precise determination is proposed taking into account ileal digestibility [ 28 , 29 ].…”

“…Although this measure has been used for twenty years and has been very useful, its limitations have been recognized and should be reassessed. WHO/FAO recommends that amino acids in the diet be considered as individual nutrients, as PDCAAS could underestimate quality [ 28 , 29 , 30 ].…”

“…According to the reviewed literature, none of them have been able to accurately determine protein digestibility and bioavailability, however, the protein biological value has been one of the most successful, and the DIAAS method is proposed as a promising method that may be the reference method in the future, for the calculation of protein digestibility, due to the accuracy of its results. It can be concluded that there is a close relationship between the number of limiting EAA and the quality of the protein in terms of digestibility and bioavailability [ 28 , 29 , 30 , 31 ].…”

Comparison of True Ileal Amino Acid Digestibility between Adult Humans and Growing Pigs

“…The overall mean true ileal digestibility of AAs was 0.92 compared with 0.95 for WPI and 0.63 compared with 0.60 for zein [Calvez et al (17) and data reported in this article]. Both nasoileal intubation and the use of ileostomates to determine true ileal AA digestibility rely on assumptions (6). They are, however, the only 2 models available to directly determine true ileal AA digestibility in the human.…”

“…Less invasive indirect methods to determine AA digestibility or availability in humans, such as the dual-isotope method (4) and indicator AA oxidation (IAAO) method (5), are also options. However, these methods, although showing much promise, cannot be considered routine methods and either are still under development or have not been fully validated (3,6). With the IAAO method, relatively costly intrinsic labeling of the food is not required, which is the case for the dualisotope approach, but AA availability observations are limited to one AA per assay period, which requires multiple testing days.…”