Tracking dietary proteins and peptides throughout digestion by omic approaches

The digestion process provides nutrients and energy essential to the survival and growth of the organisms. But degradation of food constituents is also a major source of biological signals which can be beneficial or deleterious to human health. In this context, little is known about the influence of food structure on the digestibility and nutritional properties of food constituents.Food digestion can be investigated by submitting samples to either static and dynamic in vitro models simulating what happens in the GI or through in vivo experiments performed on animal or human. Digested products are collected and their composition in proteins and peptides determined using proteomic tools, like mass spectrometry, molecular immunology...Two examples of the characterization of digested samples by proteomic approaches will be presented. First, skim milk powders were manufactured at a pilot plant scale and submitted to in vitro digestion. Samples were collected throughout digestion and characterized using immunoassays (ELISA, antibody arrays). These proteomic tools showed that hydrophobic, phosphorylated or glycosylated areas were the most resistant to digestion. It is interesting to notice that these areas carrying post-translational modifications have been proposed for playing a key role in the development of allergy to milk in infant. Our results tend to prove that the ability of these sequences to elicit an allergic response might be due to their ability in resisting to the GI tract.Then, a second experiment was conducted using 14 human volunteers whose protein intake was constituted of 30 g of either casein or whey proteins, and 40 g of maltodextrine dissolved in water to 0.5 litre. Jejunal effluents were continuously collected every 30 min for 6 hours using a double lumen nasogastric tube. Peptides present in the jejunum were identified using mass spectrometry (LC-MS-MS). More than 4700 peptides were detected and their sequence determined. Results showed that large protein fragments were present for both meals thus demonstrating that protein digestion was incomplete in the jejunum. The peptides derived from casein were smaller than the ones from whey proteins and among the common peptides identified in effluents from volunteers who had ingested the same meal some were bioactive peptides.Proteomic approaches are therefore essential for making progress in understanding how dietary proteins are broken down in the gut and identifying the release of bioactive molecules. They will allow increasing our knowledge and the impact of food on health.