The protein in your porterhouse steak and the protein in the baked potato next to it are chemically very different. Proteins are complex polymers made up of about 20 different amino acids. Some of these amino acids are essential, meaning that you have to eat them in your diet to obtain them, and some amino acids are nonessential because your body can synthesize them from other nutrients. The amount of essential amino acids in a food, as well as how easily your body digests that food, can affect a protein's overall quality rating.
Biological Value
Until the 1980s, the most common method to index protein quality was to assess each food's biological value. Fabio Comana, a sports nutrition expert with the American Council on Exercise, notes that the biological value of a protein food is essentially the amount of protein from a food your body uses before excreting the remainder. This system ranks eggs as the comparative standard against which all other proteins are measured.
PDCAAS
In 1985, the World Health Organization reevaluated the indexing of protein foods because the biological value scale does not account for absorption rates from different protein structures. The new system is called the protein digestibility corrected amino acid score, or PDCAAS.
While an improvement over the biological value scale, the PDCAAS is far from a perfect indexing system. For example, a 2000 review of the PDCAAS appearing in the "Journal of Nutrition" points out that the standard used to evaluate protein digestibility is based upon studies performed on preschool children in the 1980s, and the margins of safety in the defining equation may not be similar in the numerator and denominator. This would result in an incorrect reference pattern to index proteins against.
Highest Protein Foods
Comparing protein sources using both the biological value and the PDCAAS scales, Comana ranks whey protein as the highest-quality protein you can consume. Whey is one of the protein types present in milk. It contains all the essential amino acids and digests quickly in your body. Whole egg, cow's milk, egg white and casein --- the main protein in milk --- complete the top five protein foods in Comana's index. Foods that do not contain all the essential amino acids, such as vegetables, legumes and wheat gluten, rank lower on both protein indexing scales.
Improving Protein Quality
Consuming lower-quality protein sources such as beans does not mean that your diet needs to be incomplete. Consuming a variety of protein sources, such as beans and rice together, can help you receive all the amino acids your body needs as if you were eating a higher-quality protein source. Additionally, lower-indexed protein sources like soy may be healthier for you than higher-ranked sources like beef because of other nutrients present with the protein, such as saturated fats.
References
- University of Arizona; The Chemistry of Amino Acids; September 2003
- Harvard University: Protein -- Moving Closer to Center Stage
- "Journal of Nutrition"; The Protein Digestibility--Corrected Amino Acid Score; Gertjan Schaafsma; 2000
- American Council on Exercise; The Latest Scoop: Current Supplement Research Overview; Fabio Comana; 2010
- World Health Organization; Protein and Amino Acid Requirements in Human Nutrition; 2007
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