Protein Quality–The 4 Most Important Metrics


The quality of a protein is determined by its essential amino acid composition and the digestibility and bioavailability of its amino acids, according to the Food and Agriculture Association and the World Health Organization (FAO/WHO). Several metrics have been developed in an effort to condense these qualitative terms into easy-to-understand scores:

  • Protein Efficiency Ratio (PER)
  • Biological Value (BV)
  • Net Protein Utilization (NPU)
  • Protein Digestibility Corrected Amino Acid Score (PDCAAS)

As a reference, the list below outlines how the four most popular protein sources fare in each of these metrics:

  • Whey: PER (3.2), BV (104), NPU (92), PDCAAS (1.00)
  • Casein: PER (2.5), BV (77), NPU (76), PDCAAS (1.00)
  • Soy: PER (2.2), BV (74), NPU (61), PDCAAS (1.00)
  • Egg: PER (3.9), BV (100), NPU (94), PDCAAS (1.00)

Protein Ranking Scales

Protein Efficiency Ratio determines protein quality by measuring animal growth. In this rating, rats are fed a test protein and are measured for weight gain vs. every gram of consumed protein. The computed value is then compared to a standard value of 2.7, the value corresponding to casein protein’s effect on growth. Any value higher than 2.7 indicates an excellent protein source. However, this measure of growth in rats does not strongly correlate to human growth. As other methods (below) were developed, this ratio has become increasingly outdated.

Biological Value determines protein quality by measuring how efficiently the human body uses dietary protein. Specifically, BV measures the nitrogen (largely obtained from dietary protein) that is retained in the body and theoretically used in tissue and muscle formation, and divides it by total amount of nitrogen absorbed from dietary protein. Since BV is a function of how much protein is absorbed and how much ends up being utilized, the theoretical top score is 100. However, since whole egg (the best whole food source of protein content) was originally set as the protein digestibility standard, it is possible for processed protein sources like whey protein concentrate to exceed this value.

Net Protein Utilization aims to determine the percentage of amino acids consumed that are eventually converted into proteins and utilized by the body. To maximize NPU values, dietary protein sources must both be easy to digest and provide an effective ratio of essential amino acids. NPU values are usually measured indirectly using protein intake vs. nitrogen excretion.

Protein Digestibility Corrected Amino Acid Score (PDCAAS) measures protein quality based on human essential amino acid requirements and our ability to digest it. The test protein is compared to a standard amino acid profile and is given a score from 0-1, with a score of 1.0 indicating maximum amino acid digestibility. Common protein supplements (whey, casein, and soy) all receive 1.0 scores. Meat and soybeans (0.9), vegetables and other legumes (0.7), and whole wheat and peanuts (0.25-0.55) all provide diminished protein digestibility. PDCAAS is currently considered the most reliable score of protein quality for human nutrition.

Putting It All Together

These metrics are not intended to serve as definitive measures of protein quality, and are known to depend on several factors, such as age, activity level, and overall dietary intake. Each methodology suffers its own drawbacks, but serves as an approximate estimate of protein quality, particularly under dietary conditions. Currently, both the Biological Value and PDCAAS are held in high regard.

Understanding these metrics is a good way to make informed decisions about the protein supplements you choose for your needs. High PDCAAS scores, for example, indicate that a protein will provide close to 100% of the essential amino acids, including the branched-chain amino acids (BCAA’s) known to have the greatest effect of protein synthesis. But a lower biological value or net protein utilization value may indicate that not all of the amino acids absorbed or ingested, respectively, will be efficiently used by the body. For example, while all of these proteins share excellent essential amino acid profiles, the amino acids obtained from whey and egg protein appear to be better utilized by the body then casein or soy, especially under dietary conditions.

All protein sources are not created equal. The quality of a protein and its absorption tendencies are important when developing a diet tailored to your specific health requirements.