Protein quality and sources

Abstract

Protein quality is related to the capacity of dietary protein to provide nitrogen and dietary essential (or indispensable) amino acids (EAA) to achieve nitrogen balance and support body composition and functions. Protein concentration or density is a factor of a food's protein quality determined by measuring either nitrogen content or amino acid content, or by direct measurement of protein. An accurate assessment of protein quality for humans is through metabolic studies that measure nitrogen balance. Biological assays in laboratory animals have been used to assess food protein quality, based either on the ability to support growth in young rats (protein efficiency ratio (PER)) or on nitrogen retention (net protein utilization (NPU)). Protein quality can be assessed based on protein's constituent amino acids by chemical scores defined as the content of each of the 9 EAA in the test protein over the content of the same EAA in the amino acid scoring reference pattern. The calculation is corrected by the ileal or fecal protein's digestibility in the protein digestibility-corrected amino acid score (PDCAAS), or by the ileal amino acid digestibility in the digestible indispensable amino acid score (DIAAS). Protein quality matters because there are differences in the quality of protein from the different food sources and some forms of food storage and processing can also affect protein quality. Animal products are rich in protein (30–70% dry weight), among vegetables, pulses have the highest protein concentrations, ranging from 20% to 25% dry weight in most raw beans and peas to approximately 36% dry weight in soya beans, and cereal seeds have a protein content of 15–20% dry weight. Foods of animal origin, have well-balanced amino acid composition with a score of 100% and true protein or amino acid digestibility of 95–98%. Almost all vegetable foods have one or more limiting amino acids and proteins in natural vegetable foods have digestibility of 70–85%.