True ileal amino acid digestibility and protein quality of 15 N-labeled faba bean in healthy humans.

BACKGROUND: The recommended transition towards more plant-based diets, particularly containing legumes, requires a wider knowledge of plant protein bioavailability. Faba beans are cultivated at different latitudes and are used increasingly in human nutrition.

OBJECTIVE: We aimed to assess the nutritional quality of faba bean protein in healthy volunteers equipped with an intestinal tube to implement the ileal 15 N balance method.

METHODS: Nine volunteers completed the study (7 men, 2 women, aged 33±10 years, BMI 24.7±2.6 kg/m2 ). They were equipped with a naso-ileal tube. After fasting overnight, they ingested a test meal consisting of cooked mash of dehulled faba bean seeds (20 g protein in ca. 250 g portion) intrinsically labeled with 15 N. Samples of ileal contents, plasma, and urine were collected over an 8-h postprandial period. Undigested nitrogen (N) and amino acids (AAs) were determined using isotopic mass spectrometry, and subsequently, ileal digestibility and digestible indispensable amino acid score (DIAAS) were calculated. The measurement of postprandial deamination allowed calculation of the net postprandial protein utilization (NPPU).

RESULTS: The ileal N digestibility was 84.1±7.7%. Postprandial deamination represented 19.2±3.6% of ingested N, and the NPPU was 64.7±9.7%. The ileal digestibility of individual AAs varied from 85.1±13.7% for histidine to 94.2±3.6% for glutamine + glutamate. The mean AA digestibility was approximately 6 percentage points higher than digestibility of N, reaching 89.8±5.9%, whereas indispensable AA digestibility was 88.0±7.3%. Histidine and tryptophan were the first limiting AAs [DIAAS=0.77 (calculated by legume-specific N-to-protein conversion factor 5.4): 0.67 (by default factor 6.25)]. Sulfur AAs were limiting to a lesser extent [DIAA ratio=0.94 (N*5.4); 0.81 (N*6.25)].

CONCLUSIONS: Protein ileal digestibility of cooked, dehulled faba beans in humans was moderate (<85%) but that of AAs was close to 90%. Overall protein quality was restricted by the limited histidine and tryptophan content. This trial was registered at clinicaltrials.gov as NCT05047757.