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Abstract
Dietary protein quality is influenced by several factors and especially amino acid composition as well as the bioavailability of the protein. The method to assess the dietary protein quality recommended by the FAO/WHO (1985, 1990) is based on the ability of the protein to satisfy the indispensable amino acid requirements. The Protein Digestibility Corrected Amino Acid Score (PD-CAAS) has been proposed as a quality index and takes into account both the indispensable amino acid composition and the protein digestibility. This index can easily be used routinely, but some conceptual and methodological limits must be considered, such as the determination of both nitrogen and indispensable amino acid requirements, the bioavailability of dietary protein and the validation of the quality indexes. Another level in the evaluation of protein quality considers more specific activities related to specific protein-derived components. The compounds responsible for these activities include enzymes, immunoglobulins, mediator and hormone-like substances. These actions are linked to native proteins or to peptides cleaved from protein during digestion.
Key teaching points:
• Cow’s milk represents a major source of dietary protein in young and adult humans. High milk protein quality arises both from its nutritional value and its physiological properties.
• The assessment of protein nutritional value raises methodological problems due to the difficulty of quantifying nitrogen and amino acid requirements and of defining the protein characteristics that have to be taken into account.
• The association of the amino acid content of a protein source and of its digestibility in the rat is the current index (PD-CAAS) recognized by the FAO (1990) for the assessment of protein nutritional quality. This index reduces the differences between high (such as milk) and poor quality protein sources.
• When considering the postprandial nitrogen retention as a criteria for nutritional value, milk proteins demonstrate a high metabolic utilization by the organism compared with other protein sources. These data obtained in vivo in humans could serve as reference for the validation of routine tools of protein quality evaluation.
• Several milk protein-derived compounds confer a functional quality to dairy products. These proteins and bioactive peptides are either found in the milk, released from protein digestion or from the processing of dairy products.
• Physiological properties of milk protein include acute regulatory effects on nutrient bioavailability or on immune mechanisms and longer term potential benefits for cardiovascular-system or tissue development. These effects have to be further confirmed.
Key teaching points:
• Cow’s milk represents a major source of dietary protein in young and adult humans. High milk protein quality arises both from its nutritional value and its physiological properties.
• The assessment of protein nutritional value raises methodological problems due to the difficulty of quantifying nitrogen and amino acid requirements and of defining the protein characteristics that have to be taken into account.
• The association of the amino acid content of a protein source and of its digestibility in the rat is the current index (PD-CAAS) recognized by the FAO (1990) for the assessment of protein nutritional quality. This index reduces the differences between high (such as milk) and poor quality protein sources.
• When considering the postprandial nitrogen retention as a criteria for nutritional value, milk proteins demonstrate a high metabolic utilization by the organism compared with other protein sources. These data obtained in vivo in humans could serve as reference for the validation of routine tools of protein quality evaluation.
• Several milk protein-derived compounds confer a functional quality to dairy products. These proteins and bioactive peptides are either found in the milk, released from protein digestion or from the processing of dairy products.
• Physiological properties of milk protein include acute regulatory effects on nutrient bioavailability or on immune mechanisms and longer term potential benefits for cardiovascular-system or tissue development. These effects have to be further confirmed.
