EvaluationofProteinQualityinHumansand InsightsonStableIsotopeApproachestoMeasure Digestibility–AReview

Abstract

TherecentFoodandAgriculturalOrganization/WorldHealthOrganization/UnitedNationsUniversityexpertconsultationsonproteinrequirements andqualityhaveemphasizedtheneedforthenewDigestibleIndispensableAminoAcidScore(DIAAS),asameasureofproteinquality.Thisrequires human measurements of the true ileal digestibility of individual indispensable amino acids (IAAs) 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 fistulationattheterminalileum.Significanteffortshavebeenmadeoverthelast2decadestodevelopnoninvasiveorminimallyinvasivemethods to measure IAA digestibility in humans. The application of intrinsically labeled (with stable isotopes like 13C, 15N, and 2H) dietary proteins has helped in circumventing the invasive oro-ileal balance techniques and allowed the differentiation between endogenous and exogenous protein. The noninvasive 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 provides an estimate for a single IAA at a time and is 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 view of building a reliable DIAAS databaseofvariousproteinsourcesandsubsequentlysupportingproteincontentclaimsinfoodlabeling,are-evaluationandharmonizationofthe availablemethodsarenecessary