The following is an excerpt from Jamie Hale’s book, Protein Essentials.

A variety of methods are used to test protein quality. Different methods yield different ratings for the tested source. In other words, a particular food or supplement may score very high with one test but score lower on other tests. Obviously, supplement companies choose to publicize the tests that rank their products the highest. Protein quality is also dependent on the individual. A professional bodybuilder probably has different protein requirements than an endurance athlete. So saying that an ultimate, high quality protein exists is like saying that the perfect training regimen exists. It doesn’t exist.

The quantity of protein ingested alters the importance of protein quality. The large amount of protein consumed by most athletes and bodybuilders will outweigh the differences in protein qualities. Once you reach a certain level of quality in a protein, increasing it further won’t significantly enhance its effectiveness to pack on muscle. However, the quality of the protein becomes increasingly more important the less protein you consume.

Below, we’ll take a look at some of the most common methods used to determine protein quality. Keep in mind that tests used to determine protein quality use the lower end of protein requirements. This creates a metabolic atmosphere greatly different from that seen in athletes and bodybuilders.

Chemical score

The chemical score was introduced by Block and Mitchell (1946). It’s a method for rating protein based on its indispensable amino acid levels. This method first assigns a score of 100 to a reference protein, often an egg. Investigators then compare the essential amino acid profile of a number of proteins to the amount of each essential amino acid in the egg. The essential amino acid, which is lowest in quantity in the protein of interest, is compared to the quantity of that amino acid in the egg protein, and its chemical score is calculated.

The indispensable amino acid in the lowest quantity (relative to what is required) is defined as the first limiting amino acid. The second lowest indispensable amino acid relative to the requirements is the second limiting amino acid, and so on. The limiting amino acid typically determines the capacity that the protein has for being utilized by the body. For example, in a classic study by Munavor and Harper (1959), animals were fed wheat protein, which is lacking in the amino acid lysine. The animals were fed a diet ranging from 10–80 percent wheat protein. It was found that the group of animals who ingested 70 percent protein had the greatest amount of growth. However, when their diet was supplemented with lysine (the amino acid that was lacking), they grew an equivalent amount with only 20 percent protein in their diet! This implies that supplementing the limiting amino acid or combining proteins with different limiting amino acids might increase the quality of protein.

Chemical scoring is advantageous because it’s easy and inexpensive to use. It can also be useful for rating proteins based on their composition. The disadvantages are that it can’t tell you anything about the protein’s digestibility, and it involves a procedure that may destroy certain amino acids, which may lead to inaccurate values. It’s also insensitive to substances in a given protein that can adversely affect digestibility. For these reasons, it’s rarely the only method used when looking at protein quality.

Biological value

To determine the relative utilization of a given protein by the body, it’s necessary to measure urinary and fecal losses of nitrogen when that protein is fed to human beings under test conditions. Even under these conditions, small additional losses from sweat, hair, and fingernails will be missed. This type of test, one used internationally, measures the biological value (BV) of proteins.

The BV test involves two nitrogen balance studies. In the first, no protein is ingested and fecal and urinary nitrogen losses are measured. It’s assumed that under these conditions nitrogen lost in the urine is the amount the body always necessarily loses by filtration into the urine each day, regardless of what protein is ingested. The nitrogen lost in the feces is the amount the body loses into the intestine each day whether or not food protein is ingested. In the second test, the test protein is fed at amounts slightly under requirements, and a nitrogen balance study is conducted. The BV can be expressed as BV=N retained/N absorbed X 100.

Supplement companies often state that their protein powder has a BV higher than 100. For example, a protein with a BV of 125 would indicate that 1.25 grams of nitrogen were retained for every one gram absorbed. Remember, the body’s only source of nitrogen is dietary protein. How can you derive 1.25 grams from one gram? You can’t. When you see this type of claim, don’t buy the product.

The BV method has the advantage because it’s based on experiments with human beings and measures actual nitrogen retention. It’s a fair estimate of how well or poorly a given protein supports the body’s needs. On the other hand, there are various problems with this testing method. It doesn’t tell us which tissues are being affected. It only tells us what is happening on a whole body level. The protein may be effective at promoting protein synthesis in one tissue but not other tissues. This makes it hard to know which tissues are benefiting and which ones aren’t. The test is also time-consuming, expensive, and sometimes impractical. It’s also based on assumptions that might be invalid.

Keep in mind when looking at the BV rating that this value can be affected by a number of factors. One factor affecting the BV rating is caloric intake. A high energy intake will improve nitrogen balance at any protein intake level. This will result in a higher BV rating. A diet deficient in calories will result in a lower BV rating.

Another factor to consider is exercise. Exercise, particularly resistance training, increases nitrogen retention, which will result in a higher BV rating. Also, the amount of protein ingested needs to be considered. The BV is measured at levels under maintenance requirements. As protein intake increases, the BV of the protein decreases. Researchers have indicated that protein is utilized more effectively at suboptimal levels than at maintenance levels or near maintenance levels. Thus, the BV has little value for individuals with protein intakes exceeding the requirements.

Protein efficiency ratio

When measuring the protein efficiency ratio (PER), immature rats were fed a measured amount of protein and weighed periodically as they grew. The PER was then calculated by dividing the weight gained (in grams) by the protein ingested (in grams). The advantages of this method are that it’s simple and cheap. The disadvantages are that it’s a time-consuming process. Also, 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.

A recent study compared the protein quality of different animal foods and their mixtures with vegetable foods at the 30:70 animal–vegetable protein proportion with experiments performed under the same circumstances. The protein concentrations in the raw and cooked substances were analyzed. The digestibility and PER were determined in Fisher 344 weanling rats. Based on the corrected PER, the foods with the best protein quality (from highest to lowest ) were eggs, lamb, chicken breasts, milk powders, beef livers, beef rounds, ham, pork loins, casein, and sausages.

In most of the mixtures of animal and vegetable protein (30:70), the PER was similar to or higher than that of the animal food alone. This was probably due to proteins combining (with varying amino acid concentrations) to decrease the effect of limiting amino acids. Those who wish to decrease animal protein intake can combine specific plant proteins with animal proteins and achieve equal or greater PER ratings.

Protein digestibility corrected amino acid score

Some people have suggested that the protein digestibility corrected amino acid score (PDCAAS) is superior to other methods for evaluating the protein quality of food proteins for humans. The PDCAAS is directly applicable to humans and considers factors for more real life variables than the chemical score, PER, or BV.

The amino acid pattern for humans aged 2–5 years is used as the reference for determining the PDCAAS. Corrections for digestibility of protein are also taken from human data. Scores range from 1.0–0.0 with 1.0 being the upper limit of protein quality.

Some foods contain anti-nutritional factors. These factors sometimes occur naturally or are a result of heating and/or cooking. They inhibit the body’s ability to digest and thus absorb certain amino acids. Some research has shown that the PDCAAS method of scoring protein often overestimates the quality of foods containing anti-nutritional factors. It’s also questionable whether using PDCAAS to rate proteins for athletes and bodybuilders is reliable because the physiology of exercise changes the requirements.

None of the methods mentioned above are sufficient for rating proteins for athletes or bodybuilders because the methods weren’t meant to determine protein quality for either group.