Gut amino acid absorption in humans: Concepts and relevance for postprandial metabolism

Jorn Trommelen,Daniel Tomé,Luc J.C Van Loon

doi:10.1016/j.nutos.2020.12.006

Abstract

<h2>Summary</h2> Dietary amino acid absorption kinetics are an important determinant of protein quality. The term "amino acid digestibility" is commonly used to refer to the amount of ingested amino acids that become available following absorption. However, one should differentiate between the subsequent processes of converting protein into smaller constituents (protein digestion) and luminal amino acid uptake (amino acid absorption). Amino acid "absorbability" or "bioavailability" is assessed by quantifying the disappearance of amino acids across (part of) the gastrointestinal tract. The assessment of fecal, apparent ileal (AID), standardized ileal (SID), and true ileal disappearance (TID), reflect amino acid absorbability with increasing accuracy, due to correction for microbial metabolism in the large intestine, basal gut endogenous amino acid losses, and total gut endogenous amino acids losses, respectively. A substantial amount of absorbed amino acids undergo first-pass splanchnic extraction, but the majority is immediately released in the circulation and becomes available for peripheral tissues. The assessment of amino acid "bioavailability" or "absorbability" is used in protein quality ranking systems such as the Digestible Indispensable Amino Acid Score (DIAAS). However, such scores neglect that the rate of absorption is also an important determinant of postprandial metabolism. In addition, amino acid absorption and/or its rate are highly dependent on factors such as the duration of the postprandial assessment period. Therefore, amino acid absorption kinetics should be assessed under the relevant experimental conditions. To this end, an oral-intravenous dual tracer approach can be applied to assess dietary protein derived amino acid release into the circulation and allows the assessment of the subsequent impact on postprandial whole-body protein metabolism.

Highlights

Proteins in all living biological tissues are in a constant turnover as a result of the balance between protein synthesis and protein breakdown rates [1,2]
The purpose of this review is to describe the processes by which dietary proteinderived amino acids are made available to the organism, their impact on postprandial protein metabolism, and to critically assess how these processes are described
Protein digestion is the process of cleaving protein into smaller fragments while amino acid absorption is the process of luminal uptake of protein constituents

Summary

Introduction

Proteins in all living biological tissues are in a constant turnover as a result of the balance between protein synthesis and protein breakdown rates [1,2]. The capacity of dietary protein to stimulate protein synthesis appears to largely depend on both its amino acid profile and its digestion and amino acid absorption kinetics [9,10]. The latter two are, the main components of dietary protein quality ranking systems such as the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and Digestible Indispensable Amino Acid Score (DIAAS) [11]. Accurate assessment of protein digestion and amino acid absorption is essential to understand postprandial protein metabolism and to define dietary protein intake recommendations. The purpose of this review is to describe the processes by which dietary proteinderived amino acids are made available to the organism, their impact on postprandial protein metabolism, and to critically assess how these processes are described

Protein digestion and intestinal amino acid absorption

Splanchnic extraction and exogenous amino acid release in the circulation

Conclusions

Declaration of interests