Protein digestibility-corrected amino acid score (PDCAAS) is a method of evaluating the quality of a protein based on both the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

requirements of humans and their ability to digest it. The PDCAAS rating was adopted by the

US FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food s ...

and the

Food and Agriculture Organization of the United Nations The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level o ...

(FAO/WHO) in 1993 as "the preferred 'best'" method to determine

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

quality.Boutrif, E., Food Quality and Consumer Protection Group, Food Policy and Nutrition Division, FAO, Rome: "Recent Developments in Protein Quality Evaluation" ''Food, Nutrition and Agriculture'', Issue 2/3, 1991
/ref> In 2013, FAO proposed changing to Digestible Indispensable Amino Acid Score.

Methodology

Using the PDCAAS method, the protein quality rankings are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile with the highest possible score being a 1.0. This score means, after

digestion Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

of the protein, it provides per unit of protein 100% or more of the indispensable amino acids required. The formula for calculating the PDCAAS percentage is: (mg of

limiting amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life form ...

in 1 g of test protein / mg of same amino acid in 1 g of reference protein) x fecal true digestibility percentage. The PDCAAS value is different from measuring the quality of protein from the protein efficiency ratio (PER) and the biological value (BV) methods. The PER was based upon the amino acid requirements of growing rats, which differ significantly from those of humans. The PDCAAS allows evaluation of food protein quality based on the needs of humans as it measures the quality of a protein based on the amino acid requirements (adjusted for digestibility) of a 2- to 5-year-old child (considered the most nutritionally demanding age group). The BV method uses nitrogen absorption as a basis. However, it does not take into account certain factors influencing the digestion of the protein and is of limited use for application to human protein requirements because what is measured is maximal potential of quality and not a true estimate of quality at requirement level. Nevertheless, BV can be used to assess requirements of protein derived from foods with known quality differences and measure the proportion of absorbed nitrogen which is retained and presumably used for protein synthesis as an accurate indicator for protein measurement. The FDA gave two reasons for adopting the PDCAAS in 1993: 1) PDCAAS is based on human amino acid requirements, which makes it more appropriate for humans than a method based on the amino acid needs of animals. 2) The Food and Agricultural Organization/World Health Organization (FAO/WHO) had previously recommended PDCAAS for regulatory purposes.

Limitations

Digestion

Amino acids that move beyond the terminal

ileum The ileum () is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine m ...

in the body are less likely to be absorbed for use in

protein synthesis Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critical ...

. They may pass out of the body or become absorbed by

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

, thus appearing to have been digested instead of being present in the feces. The PDCAAS takes no account of where the proteins have been digested. Similarly, amino acids that are lost due to

antinutritional factor Antinutrients are natural or synthetic compounds that interfere with the absorption of nutrients. Nutrition studies focus on antinutrients commonly found in food sources and beverages. Antinutrients may take the form of drugs, chemicals that natura ...

s present in many foods are assumed to be digested according to the PDCAAS. Due to this, in 2013, the FAO proposed changing to Digestible Indispensable Amino Acid Score.

Focus on single proteins

The PDCAAS method may also still be considered incomplete, since human diets, except in times of

famine A famine is a widespread scarcity of food, caused by several factors including war, natural disasters, crop failure, population imbalance, widespread poverty, an economic catastrophe or government policies. This phenomenon is usually accompan ...

, almost never contain only one kind of

. However, calculating the PDCAAS of a diet solely based on the PDCAAS of the individual constituents is impossible, because one food may provide an abundance of an amino acid that the other is missing, in which case the PDCAAS of the diet is higher than that of any one of the constituents. To arrive at the final result, all individual amino acids would have to be taken into account, though, so the PDCAAS of each constituent is largely useless. For example, grain protein has a PDCAAS of about 0.4 to 0.5, limited by

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated − ...

. On the other hand, it contains more than enough

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ...

White bean ''Phaseolus vulgaris'', the common bean, is a herbaceous annual plant grown worldwide for its edible dry seeds or green, unripe pods. Its leaf is also occasionally used as a vegetable and the straw as fodder. Its botanical classification, alo ...

protein (and that of many other

pulses In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...

) has a PDCAAS of 0.6 to 0.7, limited by methionine, and contains more than enough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of the combined constituent is 1.0, because each constituent's protein is complemented by the other. A more extreme example would be the combination of

gelatine Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...

(which contains virtually no

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

and thus has a PDCAAS of 0) with isolated tryptophan (which, lacking all other essential amino acids, also has a PDCAAS of 0). Despite individual scores of 0, the combination of both in adequate amounts has a positive PDCAAS, with the limiting amino acids

isoleucine Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depr ...

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO� ...

, and

. Further, according to a 2000 study by Gerjan Schaafsma, "The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ileal digestibility correction, as well as the truncation of PDCAAS values warrant a critical evaluation of PDCAAS in its current form as a measure of protein quality in human diets." Also, the scientific community has raised critical questions about the validity of PDCAAS (the validity of the preschool-age child amino acid scoring pattern, the validity of the true fecal digestibility correction and the truncation of PDCAAS values to 100%).

Capped score

In addition, the fact that four proteins, all with different amino acid profiles, receive identical scores of 1.0 limits its usefulness as a comparative tool. Since they have different compositions, it is natural to assume that they perform differently in the human body and should have different scores. In short, this method, however, gives no distinction of their performance relative to each other, because after they pass a certain point, they are all capped at 1.0 and receive an identical rating. This is because in 1990 at a FAO/WHO meeting, it was decided that proteins having values higher than 1.0 would be rounded or "leveled down" to 1.0 as scores above 1.0 are considered to indicate the protein contains essential amino acids in excess of the human requirements.FAO/WHO

990 Year 990 ( CMXC) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. Events By place Europe * Al-Mansur, ''de facto'' ruler of Al-Andalus, conquers the Castle of Montemor-o-Velho (mode ...

Expert consultation on protein quality evaluation. Food and Agriculture Organization of the United Nations, Rome.

Reference pattern

This reference pattern is based on the essential amino acid requirements for preschool children aged 1–3 years as published i
Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005)
Adults aged 18+ will hav
slightly lower requirements

Example values

A PDCAAS value of 1 is the highest, and 0 the lowest. The table shows the ratings of selected foods. (others)

References

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Detailed Anaysis and Study of Collagen Protein Synthesis and Absorption
Nutrition Proteins