PDCAAS
Encyclopedia
Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method of evaluating the protein quality based on both the amino acid
requirements of humans and their ability to digest it. The PDCAAS rating is a fairly recent evaluation method; it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization
(FAO/WHO) in 1993 as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.
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 noticeably differs from that 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.
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
of the protein
, it provides per unit of protein 100 percent or more of the indispensable amino acids required.
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.
in the body are less likely to be absorbed for use in protein synthesis. They may pass out of the body, or may be absorbed by bacteria
, and thus will not be present in the faeces, and will appear to have been digested. The PDCAAS takes no account of where the proteins have been digested.
Similarly, amino acids that are lost due to antinutritional factor
s present in many foods are assumed to be digested according to the PDCAAS.
The PDCAAS method may also still be considered incomplete, since human diets, except in times of famine
, almost never contain only one kind of protein
. However, calculating the PDCAAS of a diet solely based on the PDCAAS of the individual constituents is impossible. This is 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
. On the other hand, it contains more than enough methionine
. White bean
protein (and that of many other pulses) 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 (which contains virtually no tryptophan
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
, threonine
and methionine
. Further, according to a recent 2000 study by scientist Gerjan Schaafsma, "The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ideal 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.
In addition the fact that four protein
s, 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.
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
requirements of humans and their ability to digest it. The PDCAAS rating is a fairly recent evaluation method; it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...
(FAO/WHO) in 1993 as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.
PDCAAS value of protein
A PDCAAS value of 1 is the highest, and 0 the lowest. The table shows the ratings of selected foods.| 1.00 | casein (milk protein) Casein Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk.... |
| 1.00 | egg white Egg (food) Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes... |
| 1.00 | soy protein Soy protein Soy protein is a protein that is isolated from soybean. It is made from dehulled, defatted soybean meal. Dehulled and defatted soybeans are processed into three kinds of high protein commercial products : soy flour, concentrates, and isolates. Soy protein isolate has been used since 1959 in foods ... |
| 1.00 | whey (milk protein) Whey protein Whey protein is a mixture of globular proteins isolated from whey, the liquid material created as a by-product of cheese production. Some preclinical studies in rodents have suggested that whey protein may possess anti-inflammatory or anti-cancer properties; however, human data is lacking... |
| 0.92 | beef Beef Beef is the culinary name for meat from bovines, especially domestic cattle. Beef can be harvested from cows, bulls, heifers or steers. It is one of the principal meats used in the cuisine of the Middle East , Australia, Argentina, Brazil, Europe and the United States, and is also important in... |
| 0.91 | soybeans |
| 0.78 | chickpeas |
| 0.76 | fruits |
| 0.73 | vegetables |
| 0.70 | Other legumes |
| 0.59 | cereals and derivatives |
| 0.42 | whole wheat Wheat Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice... |
Methodology
The formula for calculating the PDCAAS percentage is: (mg of limiting amino acid 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)
Protein efficiency ratio
Protein efficiency ratio is based on the weight gain of a test subject divided by its intake of a particular food protein during the test period....
and the biological value
Biological Value
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. It summarises how readily the broken down protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of...
(BV) methods. The PER was based upon the amino acid requirements of growing rats, which noticeably differs from that 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.
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
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
profile with the highest possible score being a 1.0. This score means, after digestion
Digestion
Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....
of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
, it provides per unit of protein 100 percent or more of the indispensable amino acids required.
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
Amino acids that move beyond the terminal ileumIleum
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 may be used instead of ileum.The ileum follows the duodenum...
in the body are less likely to be absorbed for use in protein synthesis. They may pass out of the body, or may be absorbed by bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, and thus will not be present in the faeces, and will appear to have been digested. The PDCAAS takes no account of where the proteins have been digested.
Similarly, amino acids that are lost due to antinutritional factor
Antinutritional factor
Anti-nutritional factors are natural or synthetic substances found in the human diet or animal feed that have the potential to adversely affect health and growth by preventing the absorption of nutrients from food.-Classes of Anti-nutrients:...
s present in many foods are assumed to be digested according to the PDCAAS.
The PDCAAS method may also still be considered incomplete, since human diets, except in times of famine
Famine
A famine is a widespread scarcity of food, caused by several factors including crop failure, overpopulation, or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every continent in the world has...
, almost never contain only one kind of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
. However, calculating the PDCAAS of a diet solely based on the PDCAAS of the individual constituents is impossible. This is 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
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
. On the other hand, it contains more than enough methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
. White bean
Bean
Bean is a common name for large plant seeds of several genera of the family Fabaceae used for human food or animal feed....
protein (and that of many other pulses) 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 (which contains virtually no tryptophan
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
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
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
, threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
and methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
. Further, according to a recent 2000 study by scientist Gerjan Schaafsma, "The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ideal 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.
In addition the fact that four protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, all with different amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
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.
See also
- Biological valueBiological ValueBiological value is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body. It summarises how readily the broken down protein can be used in protein synthesis in the cells of the organism. Proteins are the major source of...
- Net protein utilizationNet protein utilizationThe net protein utilization, or NPU, is the ratio of amino acid converted to proteins to the ratio of amino acids supplied. This figure is somewhat affected by the salvage of essential amino acids within the body, but is profoundly affected by the level of limiting amino acids within a...
- Protein efficiency ratioProtein efficiency ratioProtein efficiency ratio is based on the weight gain of a test subject divided by its intake of a particular food protein during the test period....
- Nitrogen balanceNitrogen balanceNitrogen balance is the measure of nitrogen output subtracted from nitrogen input.Blood urea nitrogen can be used in estimating nitrogen balance, as can the urea concentration in urine....