Assessing dietary protein quality

Protein quality has been identified as a critical question by international authorities (FAO). Protein nutritional quality is related to the capacity of the different food sources of protein to achieve the different functions associated to the supply of nitrogen and amino acids in the body. The nutritional efficiency of a protein can be determined from the extent to which dietary protein nitrogen is absorbed and retained by the organism and is able to balance daily nitrogen losses. The capacity to provide an adequate profile of bioavailable indispensable amino acid is considered as a limiting factor for protein quality.

The Amino Acid Scoring approach considers the capacity of a protein source ingested at the level of the mean protein requirement derived from nitrogen balance (0.66 g/kg/d in adult) to meet indispensable amino acid needs. The Protein Digestibility Corrected Amino Acid Score (PD-CAAS) corrects the content of each indispensable amino acid of the protein by the faecal digestibility of the protein in order to evaluate the bioavailable part of these amino acids in comparison to a reference amino acid profile. The new discussed Digestible Indispensable Amino Acid Score (DIAAS) considers the specific ileal digestibility of each indispensable amino acid separately. The application of DIAAS faces to several methodological difficulties for measurement of amino acid ileal digestibility particularly in humans.

The definition of “true” digestibility is the proportion of the dietary amino acids that have disappeared from the intestinal lumen at the terminal ileum. By using ileal cannulation the apparent digestibility, ie the net disappearance of amino acid as a proportion of dietary intake can be measured. This includes endogenous secretions from mucus, enzymes and microbial biomass as well.

Digestibility can be determined at the faecal level, but the value are underestimated due to the colon stasis and the microbiota metabolism of proteins. Ileal digestibility is the more accurate measurement because amino acids are absorbed in the small intestine and those that reach the colon can be transformed by the microbiota. However, such determinations cannot be done easily on various sources of proteins with the currently available methods and there is a need to develop alternative methods to assess protein and amino acid bioavailability and nutritional quality in humans.

The overall aim is to develop a meal/plasma dual stable isotope-based approach using intrinsic labelled proteins for the non-invasive evaluation of protein digestibility, amino acid bioavailability and protein nutritional quality for humans. The present project focusses on the characterization of the labelling and use of intrinsically-labelled milk protein as a reference-protein for development of the non-invasive method that can be used in human subjects.

More research: Nutrition, obesity and the metabolic syndrome