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

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.