Understanding the Protein Digestibility Corrected Amino Acid Score (PDCAAS)

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Nancy Collins, PhD, RD, LD/N, FAPWCA

     Protein, defined as a complex nitrogenous compound made up of amino acids in peptide linkages, is essential for life. Proteins carry out the work of the cell by serving as enzymes, receptors, transporters, hormones, antibodies, or communicators that build, maintain, and repair body tissues.

     Wound care professionals and registered dietitians (RDs) understand that patients often require increased amounts of protein for healing. Protein is a hot topic today. This has resulted in a proliferation of new products on the market, such as whole proteins, partially hydrolyzed proteins, or targeted amino acids, all available in a variety of forms and flavors, including powders, liquids, and even chicken soup. The challenge is deciding what type of protein is best.

Amino Acids Classification

     A polypeptide chain is comprised of three categories of amino acids. Indispensable amino acids (IAAs), also known as essential amino acids, are not synthesized by humans and must come from the diet. Dispensable amino acids (DAA), also known as nonessential amino acids, are produced by the body in sufficient amounts under normal, healthy conditions. Conditionally indispensable amino acids (CIAA) are produced in sufficient amounts by healthy individuals. However, in the presence of certain disease states or underlying physiological stress such as nonhealing wounds, supplementation often is required to achieve an adequate supply of CIAAs. Table 1 lists the amino acids that fall into each category.1

Systems of Evaluating Protein

     Protein assessment has evolved through the years. Some of the past methods included protein efficiency ratio (PER), biological value (BV), and net protein utilization (NPU). Although each of these grading systems used different inputs, they all sought to rate the utilization and digestibility of protein and amino acids.

     In 1991, the Food and Agriculture Organization/World Health Organization2 (FAO/WHO) proposed a new system, which subsequently was adopted by the Food and Nutrition Board of the Institute of Medicine. This method, the protein digestibility corrected amino acid score (PDCAAS), is determined by comparing the amino acid profile of the food in question against a standard amino acid profile, with 100 as the highest possible score. Scores above 100 are not allowed and are truncated to 100.2 Figure 1 shows the PDCAAS formula.

     The purpose of the PDCAAS is to rate the overall quality of a protein based on the amount of IAAs contained therein. The score is based on the amino acid that is in the shortest supply when compared to a reference (scoring) pattern. This scoring pattern is based on the IAA requirements of preschool-age children. Digestibility of the protein also is factored into the score by way of fecal digestibility. For example, common digestibility percentages include 98% for egg protein; 95% for casein, soy, and collagen; and 91% for wheat protein. This system, although criticized,3,4 is widely used today.

PDCAAS in Action

     If a protein is completely missing just one IAA, the PDCAAS is 0. If a protein with a low amount of one IAA has a PDCAAS of 20, we would say that the protein provides only 20% of that particular amino acid when compared to the scoring pattern; so, in theory, only 20% of this protein is possibly available for protein synthesis in the human body. In other words, to meet the IAA requirement for the day, without a source of any other protein in the diet, a person would need to consume five times the quantity of the protein with a PDCAAS of 20 than for one with a PDCAAS of 100.

References: 

1. Castellanos VH, Litchford MD, Campbell WW. Modular protein supplements and their application to long-term care. Nutr Clin Pract. 2006;21(5):485–504.
2. Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr. 2000;130(7):1865S–1867S.
3. National Academy of Sciences, Institute of Medicine, Food and Nutrition Board. Protein and amino acids. In: Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids: Food and Nutrition Board: National Academy of Sciences. Washington, DC: National Academy Press;2005.
4. FAO/WHO Expert Consultation. Protein Quality Evaluation: Food and Agriculture Organization of the United Nations, FAO Food and Nutrition Paper No. 51. Rome, Italy: Food and Agriculture Organization;1991.




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