Bioavailability of dietary proteins and amino acids in humans
Résumé
In humans, dietary proteins are the only bioavailable source of nitrogen. The amino acids they contain are necessary to sustain protein synthesis and turnover. Among the 20 amino acids that make up proteins, 9 of them are indispensable because they can’t be de novo synthetized. Together with amino acid composition, protein bioavailability is an important criteria of protein quality. Efficiency of protein digestion is a critical step that impacts protein bioavailability. In healthy people, the protein source is a determinant factor of protein bioavailability. Milk proteins are the highest digestible (95 %) followed by meat (94 %) while most purified plant proteins are digestible at a level of 90-91 % (Oberli et al, 2015). Other nutrients that are present in the food matrices, such as fibers or polyphenols, can decrease protein digestion efficiency. After intestinal absorption, metabolic losses through urea production also influences protein bioavailability. This depends not only on the amino acid composition but also on the speed of digestion. For instance, whey proteins are less retained postprandially (67 %) than caseins (73 %) due to their rapid digestion kinetic, while their amino acid composition is excellent (Lacroix et al, 2006). Wheat proteins exhibit a lower postprandial retention (65 %) than other plant sources such as soy or pea (73 %), as a result of their poor lysine content (Bos et al, 2005).
In vivo measurement of protein bioavailability faces with important methodological barriers. Because peptide and amino acid absorption occurs in the small intestine and that proteins and peptides entering to the colon are fermented by microbiota, digestibility should be measured at the ileal level. However, this implies invasive procedures, such as cannula in pigs or intestinal tubes in humans, to collect ileal effluents. As a consequence, measurements are often performed at the fecal level, especially in rats, to calculate a value of PD-CAAS (Protein Digestibility Corrected Amino Acid Score) that has been recommended as a criteria of protein quality by the FAO. Fecal measurements overestimate protein digestibility because nitrogen that is transferred to ammonia by microbiota is partly absorbed in the colon, but the error is considered as acceptable. More recently (FAO, 2011), FAO has recommended to assess the digestion of each indispensable amino acid, instead of the total protein, to calculate a DIAAS (Digestible Indispensable Amino Acid Score). For milk and soy proteins, ileal digestibility has revealed differences of 4-5 % among amino acids in humans (Gaudichon et al, 2002). In people for which protein intake is limited and/or of poor quality, these variations can result in non-adequate intake of some amino acids compared to requirements. To provide DIAAS values, fecal measurements are not reliable anymore because the impact of microbial metabolism on individual amino acid content is high.
Another important methodological issue related to protein bioavailability is the discrimination between endogenous and exogenous proteins in the digestive and metabolic losses. Endogenous losses can be assessed using protein free diets but the use of stable isotopes is the most accurate to follow the digestive and metabolic fate of dietary proteins and amino acids. Among labelling methods, 15N has been widely used to intrinsically label all amino acids of the protein, especially in plants and ruminants. 13C is often used to label a single amino acid that is considered to be representative of all the other amino acids in the protein. Multitracer methods were recently proposed (FAO, 1994) as an alternative method to remove the methodological barriers related to DIAAS calculation, by throwing off the invasive procedures to collect effluents. Nevertheless, these methods are complex and they need to be developed and validated.
In conclusion, bioavailability of protein and amino acids is an important criteria of protein quality. It is an important issue in the view of finding alternative protein sources to preserve environment and to feed humans over the world in the near future. However, it must be kept in mind that its assessment in humans faces with methodological complexity.
*Bos C et al. Postprandial metabolic utilization of wheat protein in humans. Am J Clin Nutr. 2005;81(1):87-94.
*FAO. Dietary protein quality evaluation in human nutrition. Rome: 2011.
*FAO. Research approaches and methods for evaluating the protein quality of human foods. Rome: 2014.
*Gaudichon C et al. Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements. Gastroenterology. 2002;123(1):50-9.
*Lacroix M et al. Compared with casein or total milk protein, digestion of milk soluble proteins is too rapid to sustain the anabolic postprandial amino acid requirement.
*Oberli M et al. High True Ileal Digestibility but Not Postprandial Utilization of Nitrogen from Bovine Meat Protein in Humans Is Moderately Decreased by High-Temperature, Long-Duration Cooking. J Nutr. 2015 Oct;145(10):2221-8.