Many peptides are released in the jejunum through the digestion of milk proteins in humans

Cow milk contains two major protein fractions: casein and whey soluble protein (SP) that behave differently during digestion in the tract. SP are rapidly evacuated from the stomach, whereas casein clots under the acidic gastric pH, which delays its gastric emptying and results in a slower delivery of amino acids to the gut. The differences in kinetics digestion have metabolic consequences: SP transiently stimulate postprandial anabolism, whereas casein is not associated with a stimulation of protein synthesis. To understand the metabolic consequences of each protein, we identified peptides in the jejunum in humans fed a meal containing either casein or SP. The peptides have been identified using proteomic tools: chromatography coupled on line with mass spectrometry. We analysed the peptides in the range 450 - 3000 Da. We show evidence of the presence of many diverse peptides in the intestinal tract in humans fed dairy proteins. Peptides appeared continuously in the jejunum during 5 and 3 hours for casein and SP meal, respectively. The identified peptides mainly originated from -casein and -lactoglobulin for casein and SP meal, respectively. The same peptide sequences were found throughout the digestion in the ileal effluents from the whole volunteers fed the same meal. Thus 15 peptides from -lactoglobulin were present as well throughout the 3 hours digestion as in ileal effluent from all the volunteers fed the SP meal. Similarly for casein meal, 14 peptides from -casein were present throughout the 5 hours digestion for all the volunteers. Among the latter were bioactive peptides acting as opioid agonist, immuno-modulant, etc. Our results emphasize the presence of many diverse peptides in the human jejunum throughout the digestion of dairy proteins. The opened question is to demonstrate the functional role of these (bioactive) peptides.