The modern mode of thinking that digested protein is transported from the intestinal lumen into enterocytes and then into the circulation both as amino acids (AAs) and di/tripeptides dates back to the mid-1970s. Today, we know that di/tripeptides are taken up by a single transporter known as PEPtide Transporter 1 (PEPT1). However, some questions remain unanswered. For instance, although PEPT1 knock-out models have been generated [1,2], it is not clear whether the intestinal peptide transporter plays a role in the up- or downregulation of animal growth. Indeed, by deleting the intestinal peptide transporter pept1 (previously known as pep-2) body size is reduced in an invertebrate animal model, such as the nematode Caenorhabditis elegans [1,3], whereas PepT1 null mice grow to normal size, body weight and organ weight [2].
In recent years, interest in piscine PEPT1 has grown rapidly [4,5]. This is owing to the recurring observation that fish can efficiently use dietary di/tripeptides to sustain development, growth and metabolism [6–14].