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Howz it Work?: Protein Ratings
by Bryan Haycock MSc., CSCS
Protein is of major importance to bodybuilders and anyone else who exercise a lot. A brief discussion of the different methods of determining protein quality should help you better understand the nature of the RDA recommendations, ratings you see on the side of protein canisters and what it all means to you. It may also help you in your food vs. supplement decisions.
Chemical Scoring
The most obvious way to determine the quality of a given protein is to break it down into its individual amino acids. This amino acid profile is then compared to a standard profile. This method is called Chemical Scoring. Egg protein is the standard that is used in a chemical scoring scale for protein quality and has a rating of 100. Take for example a protein that has a limited amount of a specific amino acid (such as a vegetable protein). This amount is then compared to the amount found in egg protein. If the profile in the test protein is 75% of that found in egg then the test protein gets a rating of 75.
Although it is relatively easy and inexpensive to do a chemical scoring of any protein, it does not always accurately predict how well the body can utilize it. So the advantage of chemical scoring in determining the quality of protein is that it is easy and inexpensive. Its drawback is that it cannot tell you anything about the digestibility of the protein. Chemical scoring also involves a procedure that may destroy certain amino acids and this may lead to inaccurate values. It is also insensitive to substances in a given protein that can affect digestibility. To discover this variable the test would have to utilize living animals.
Biological Value (BV)
Biological Value scoring does utilize in-vivo testing. To determine the actual amount of a given protein that will be used by the body it is necessary to measure not only urinary, but also fecal losses of nitrogen when that protein is fed to human beings. The BV testing method is used internationally.
In testing for BV, two nitrogen studies are done. The first study determines how much nitrogen is lost from the body even when no protein is fed. This amount of nitrogen loss is assumed to be inevitable and that the body will naturally lose it regardless of the amount of nitrogen in the diet. In the second study an amount of the protein is fed that is slightly below what is required. As before, the nitrogen losses are then measured, but this time they are compared to the amount of nitrogen consumed.
To determine the actual BV of the protein the results are then derived using this formula:
BV = (Nitrogen retained / Nitrogen absorbed) x 100.
The obvious advantage of this method over chemical scoring is that it uses human beings. There are a few problems with this method however. One, there are inter-individual physiological differences that can effect the results. Two, the test subject does not always represent the people that will be consuming the protein in the real world. Finally, just because nitrogen is being retained does not mean that it is being effectively utilized. There is considerable exchange of proteins among tissues that is hidden from view when only nitrogen intake and output are measured. One tissue could be shorted and a test of biological value would not detect this.
Net Protein Utilization (NPU)
Net Protein Utilization is another test of protein quality. Like Biological Value testing, NPU tests involve two nitrogen balance studies. One involves measurements on zero intakes, and the other on sub-maximal intake. The formula is:
NPU = (Nitrogen retained / Nitrogen intake) x 100.
This method often involves animal test subjects and is more frequently used. It's draw backs are that if a low NPU is obtained, it is impossible to know if it is because of a poor amino acid profile or low digestibility.
Protein Efficiency Ratio (PER)
One last test for protein quality that I would like to discus is the Protein Efficiency Ratio. Protein efficiency ratio is the best-known procedure for evaluating protein quality and is used in the United States as the basis for regulations regarding food labeling and for the protein RDA. This method involves rats that are fed a measured amount of protein and weighed periodically as they grow. The PER is expressed as:
PER = weight gain (g) / protein intake (g).
The benefits of this method are its expense and simplicity. It's drawbacks are that it is time consuming; the amino acid needs of rats are not those of humans; and the amino acid needs of growing animals are not those of adult animals (growing animals and humans need more lysine, for example).
The PER is used to qualify statements about daily protein requirement in the United States. You are assumed to eat protein with a PER that is equal to or better than that of the milk protein casein; if the protein's PER is lower, you must eat more of it to meet the RDA. Food labels have to take protein quality into consideration, using the PER of casein as a reference point. If a food has a protein quality equal or better than that of casein, the RDA is 45 grams. If the protein quality is less than casein you need 65 grams for the RDA.
Protein Digestibility-Corrected Amino Acid Score (PDCAA)
As outlined above, protein quality can be measured by the profile (ratio) of the indispensable amino acids they contain. If a protein contains all the amino acids essential for life, it is called a complete protein and is given a high score on the Chemical Scoring test. Because some proteins are not as efficiently digested (bioavailable) there arose a need to not only test for the amino acid composition of proteins but also for digestibility. This type of testing is called Protein Digestibility-Corrected Amino Acid Score. It is now a federally accepted standard for determining protein quality for preschool aged children.
PDCAA of a protein is calculated by the following equation:
True protein digestibility x lowest amino acid ratio (limiting amino acid based on chemical scoring)
Some foods however, contain anti-nutritional factors. These factors sometimes occur naturally like in some beans, or are a result of heating and/or cooking, and inhibit the ability of the body to digest and thus absorb certain amino acids. Research has shown the PDCAA method of scoring protein often over estimates the quality of foods containing anti-nutritional factors (Sarwar G. The Protein Digestibility-Corrected Amino Acid Score method overestimates quality of proteins containing antinutritional factors and of poorly digestible proteins supplemented with limiting amino acids in rats. J. Nutr. 127: 758-764, 1997). This is especially true for vegetable proteins found in legumes.
You may be wondering if it makes any difference if you eat your protein from a supplement or from food. Aside from how quickly it is digested, by the time it gets absorbed into the blood stream, all your body knows is how much of each amino acid was present in the food you ate. If you have the money, it is certainly convenient to just drink down a high quality protein supplement. Beyond that, it makes no difference in what form you get your protein from as long as it is a complete protein. However, an over-reliance on "fast proteins" such as whey hydrosolates is probably not a good idea.
by Bryan Haycock MSc., CSCS
Protein is of major importance to bodybuilders and anyone else who exercise a lot. A brief discussion of the different methods of determining protein quality should help you better understand the nature of the RDA recommendations, ratings you see on the side of protein canisters and what it all means to you. It may also help you in your food vs. supplement decisions.
Chemical Scoring
The most obvious way to determine the quality of a given protein is to break it down into its individual amino acids. This amino acid profile is then compared to a standard profile. This method is called Chemical Scoring. Egg protein is the standard that is used in a chemical scoring scale for protein quality and has a rating of 100. Take for example a protein that has a limited amount of a specific amino acid (such as a vegetable protein). This amount is then compared to the amount found in egg protein. If the profile in the test protein is 75% of that found in egg then the test protein gets a rating of 75.
Although it is relatively easy and inexpensive to do a chemical scoring of any protein, it does not always accurately predict how well the body can utilize it. So the advantage of chemical scoring in determining the quality of protein is that it is easy and inexpensive. Its drawback is that it cannot tell you anything about the digestibility of the protein. Chemical scoring also involves a procedure that may destroy certain amino acids and this may lead to inaccurate values. It is also insensitive to substances in a given protein that can affect digestibility. To discover this variable the test would have to utilize living animals.
Biological Value (BV)
Biological Value scoring does utilize in-vivo testing. To determine the actual amount of a given protein that will be used by the body it is necessary to measure not only urinary, but also fecal losses of nitrogen when that protein is fed to human beings. The BV testing method is used internationally.
In testing for BV, two nitrogen studies are done. The first study determines how much nitrogen is lost from the body even when no protein is fed. This amount of nitrogen loss is assumed to be inevitable and that the body will naturally lose it regardless of the amount of nitrogen in the diet. In the second study an amount of the protein is fed that is slightly below what is required. As before, the nitrogen losses are then measured, but this time they are compared to the amount of nitrogen consumed.
To determine the actual BV of the protein the results are then derived using this formula:
BV = (Nitrogen retained / Nitrogen absorbed) x 100.
The obvious advantage of this method over chemical scoring is that it uses human beings. There are a few problems with this method however. One, there are inter-individual physiological differences that can effect the results. Two, the test subject does not always represent the people that will be consuming the protein in the real world. Finally, just because nitrogen is being retained does not mean that it is being effectively utilized. There is considerable exchange of proteins among tissues that is hidden from view when only nitrogen intake and output are measured. One tissue could be shorted and a test of biological value would not detect this.
Net Protein Utilization (NPU)
Net Protein Utilization is another test of protein quality. Like Biological Value testing, NPU tests involve two nitrogen balance studies. One involves measurements on zero intakes, and the other on sub-maximal intake. The formula is:
NPU = (Nitrogen retained / Nitrogen intake) x 100.
This method often involves animal test subjects and is more frequently used. It's draw backs are that if a low NPU is obtained, it is impossible to know if it is because of a poor amino acid profile or low digestibility.
Protein Efficiency Ratio (PER)
One last test for protein quality that I would like to discus is the Protein Efficiency Ratio. Protein efficiency ratio is the best-known procedure for evaluating protein quality and is used in the United States as the basis for regulations regarding food labeling and for the protein RDA. This method involves rats that are fed a measured amount of protein and weighed periodically as they grow. The PER is expressed as:
PER = weight gain (g) / protein intake (g).
The benefits of this method are its expense and simplicity. It's drawbacks are that it is time consuming; the amino acid needs of rats are not those of humans; and the amino acid needs of growing animals are not those of adult animals (growing animals and humans need more lysine, for example).
The PER is used to qualify statements about daily protein requirement in the United States. You are assumed to eat protein with a PER that is equal to or better than that of the milk protein casein; if the protein's PER is lower, you must eat more of it to meet the RDA. Food labels have to take protein quality into consideration, using the PER of casein as a reference point. If a food has a protein quality equal or better than that of casein, the RDA is 45 grams. If the protein quality is less than casein you need 65 grams for the RDA.
Protein Digestibility-Corrected Amino Acid Score (PDCAA)
As outlined above, protein quality can be measured by the profile (ratio) of the indispensable amino acids they contain. If a protein contains all the amino acids essential for life, it is called a complete protein and is given a high score on the Chemical Scoring test. Because some proteins are not as efficiently digested (bioavailable) there arose a need to not only test for the amino acid composition of proteins but also for digestibility. This type of testing is called Protein Digestibility-Corrected Amino Acid Score. It is now a federally accepted standard for determining protein quality for preschool aged children.
PDCAA of a protein is calculated by the following equation:
True protein digestibility x lowest amino acid ratio (limiting amino acid based on chemical scoring)
Some foods however, contain anti-nutritional factors. These factors sometimes occur naturally like in some beans, or are a result of heating and/or cooking, and inhibit the ability of the body to digest and thus absorb certain amino acids. Research has shown the PDCAA method of scoring protein often over estimates the quality of foods containing anti-nutritional factors (Sarwar G. The Protein Digestibility-Corrected Amino Acid Score method overestimates quality of proteins containing antinutritional factors and of poorly digestible proteins supplemented with limiting amino acids in rats. J. Nutr. 127: 758-764, 1997). This is especially true for vegetable proteins found in legumes.
You may be wondering if it makes any difference if you eat your protein from a supplement or from food. Aside from how quickly it is digested, by the time it gets absorbed into the blood stream, all your body knows is how much of each amino acid was present in the food you ate. If you have the money, it is certainly convenient to just drink down a high quality protein supplement. Beyond that, it makes no difference in what form you get your protein from as long as it is a complete protein. However, an over-reliance on "fast proteins" such as whey hydrosolates is probably not a good idea.
