Proteins differ greatly in their nutritive value, and there are numerous methods used in nutrition science to establish protein quality and bio-availability.  Protein quality refers to a protein’s digestibility as well as its amino acid profile and how well the protein is used by the body to perform specific metabolic functions.  One way that protein quality can be evaluated is by categorizing proteins as complete (also referred to as high-quality) or incomplete (also referred to as low-quality).  Complete proteins contain all the essential amino acids that the body requires from food whereas incomplete proteins lack one or more of the essential amino acids.

Methods To Evaluate Protein Quality

Different methods are used to evaluate protein quality: biological value (BV) and net protein utilization are two of them.  These methods not only look at a food’s protein profile, but they also look at how the body utilizes the protein in a specific food.  Biological value is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism’s body.  BV uses nitrogen to establish how readily the digested protein can be used in protein synthesis.  Proteins are the major source of nitrogen in our diet, and BV assumes protein to be the only source of nitrogen.  The difference between the amount of nitrogen ingested and the amount of nitrogen excreted tells us how much nitrogen has been incorporated in an organism’s body.  The ratio of nitrogen incorporated into the body over nitrogen absorbed gives the biological value.  Unlike other measures of protein usability, BV does not consider how readily the protein can be digested and absorbed. 

What Is Net Protein Utilization?

Net protein utilization (NPU) also estimates nitrogen retention.  Unlike BV, though, this method estimates nitrogen retention by determining the difference between body nitrogen content of animals fed no protein and those fed a test protein.  This value divided by the amount of protein consumed is the NPU, which is defined as the “percentage of the dietary protein retained”.   While both NPU and BV estimate retained nitrogen, in the calculation of NPU the denominator is the total protein eaten whereas in the calculation of BV it is the amount absorbed.

Protein and Kidney Health As Used In Nutritional Science

Biological value and net protein utilization are commonly used in nutrition science as a guideline for protein choice in diseased states that need to restrict protein intake.  Kidney disease is one of these conditions.  People suffering from kidney disease need to restrict protein intake.  Eliminating protein altogether is not an option, as protein malnutrition would cause even more harm.  Therefore, people suffering from kidney disease need to focus on protein foods of high biological value that the body can metabolize efficiently and that yield very little waste.  The diet of a person suffering from kidney disease not in dialysis needs to provide around 0.6 to 0.8g per kg of body weight of proteins.  At least 50% of proteins needs to be of high biological value (eggs, meat and poultry, fish, and dairy).  People on dialysis have higher protein needs that those who are not on dialysis; therefore, patients on peritoneal dialysis need 1.3g per kg of body weight; patients in hemodialysis are recommended 1.2g per kg of body weight. 

Sarwar G, Blair R, Friedman M, Gumbmann MR, Hackler LR, Pellett PL, et al. Inter- and Intra-laboratory Variability in Rat Growth Assays for Estimating Protein Quality of Foods. Journal of AOAC INTERNATIONAL. 1984 Sep 1;67(5):976–81

Stipanuk MH, Caudill MA, editors. Biochemical, physiological, and molecular aspects of human nutrition. 4th ed. St. Louis, Mo: Elsevier; 2019. 959 p.

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