Improved in vitro gastrointestinal digestion protocol mimicking brush border digestion for the determination of the Digestible Indispensable Amino Acid Score (DIAAS) of different food matrices

The Digestible Indispensable Amino Acid Score (DIAAS) is the new gold standard method for the assessment of protein nutritional quality. The DIAAS is evaluated with in vivo models, that are complex, constraining and costly. There is still no established method to assess it in vitro. In this study, we proposed to add a jejunal-ileal digestion phase to the standardized in vitro gastrointestinal digestion protocol developed by the International Network of Excellence on the Fate of Food in the Gastrointestinal Tract (INFOGEST protocol) to mimic brush border digestion and to enable DIAAS assessment in vitro in a more physiologically relevant manner. This jejunal-ileal digestion phase was performed with a porcine intestinal aminopeptidase as an alternative to brush border membrane extract, which is more difficult to obtain in a standardized way. This modified INFOGEST protocol was applied to various food matrices (faba bean, pea and soy flours, whey protein isolate and caseins) and the results were compared to published in vivo data to assess the model's physiological relevance. The addition of the jejunal-ileal digestion phase lead to a significant (p < 0.05) increase of 31 and 29 % in free and total amino acid digestibility, respectively, and of 83 % on average for the in vitro DIAAS values for all food matrices. Although the in vitro DIAAS remained underestimated compared to the in vivo ones, a strong correlation between them was observed (r = 0.879, p = 0.009), stating the relevance of this last digestion phase. This improved digestion protocol is proposed as a suitable alternative to evaluate the DIAAS in vitro when in vivo assays are not applicable.