Determination of Indispensable Amino Acid Digestibility of the Red Kidney Bean in Humans Using a Dual Stable Isotope Tracer Method

Abstract

BackgroundProtein quality of the red kidney bean (RKB), a common source of dietary protein, has been assessed using the protein digestibility-corrected amino acid score (PDCAAS) determined in animal models using mainly oro-fecal digestibility. More recently, the FAO recommended to use digestible indispensable amino acid score (DIAAS) instead of PDCAAS but highlighted insufficient data on true ileal indispensable amino acid (IAA) digestibility of proteins because amino acids are absorbed in the ileum. ObjectivesUsing a recently developed dual stable isotope tracer method, we aimed to measure each IAA digestibility as representation of true ileal digestibility of the RKB, Phaseolus vulgaris, in humans consuming a typical Jamaican meal. MethodsRKB-IAAs were intrinsically labeled by adding 2H2O to the plants. Uniformly labeled-[13C]-spirulina (standard protein) was added to a meal prepared with the labeled RKB and fed to 10 healthy adults (5 males, 5 females) in a nonrandomized trial as primed/intermittent doses to achieve a steady state IAA enrichment in plasma. Enrichment of 2H- and 13C-labeled IAA in plasma and the bean was measured by mass spectrometry. Each IAA digestibility (except tryptophan and histidine) was calculated as the ratio of plasma 2H-IAA to meal 2H-IAA divided by the ratio of plasma 13C-IAA to meal 13C-IAA adjusted for loss of 2H-atom during transamination and digestibility of spirulina. ResultsAdequate IAA labeling (>1000 ppm 2H excess) and plasma plateau isotopic enrichment were achieved. Mean RKB-IAA digestibility (%) was 79.4 ± 0.5, ranging from 69.0 ± 1.2 (threonine) to 85.7 ± 1.7 (lysine). ConclusionThe dual stable isotope tracer digestibility data are similar to published oro-fecal digestibility supporting substantial nitrogen exchange in the colon. The individual IAA digestibility is useful to derive DIAAS to replace PDCAAS. Using published RKB-IAA composition, extrapolated DIAAS was 0.63 based on the lowest score for methionine. Clinical trial registrationhttps://register.clinicaltrials.gov; ID: NCT-04118517.