The quality of a protein relates to the proportion of consumed protein that can be used by the body, whilst minimising the proportion of protein which is oxidised and wasted. No protein is used 100%, and the amino acid profile of each type largely dictates that, proteins are not the same in their rate of utilisation. The body is unable to use approximately 30% of a high quality source of protein and up to 60% of a low quality source of protein.

Over the years, many scientific methods have been developed to assess the quality of a source of protein and many of them are cited by companies when they release a new product, but what are they and what do they entail?  The following article identifies and discusses the most frequently utilised and widely referenced methods of assessing protein quality in scientific research.

Amino Acid Scoring (AAS)

Amino Acid Scoring (AAS) involves the measurement of the essential amino acids (EEAs) present in a source of protein compared to a reference protein. The rating of the protein being measured is then determined by the most limiting essential amino acids (EEAs).

Protein Efficiency Ratio (PER)

PER is determined by measuring the ability of a source of protein to promote the growth of a laboratory rat. The PER represents the weight gain of the rat related to the amount of protein used. Sources of protein with a score of 2.5 or above are considered to be high quality sources of protein. The maximum score on the scale is 4.4 PER. The main problem of PER is that it cannot be applied to human children, because they do not grow at the same rate as rats, and it is difficult to practically apply to human adults because it measures growth and not maintenance, so it is difficult to determine their protein requirements.

Biological Value (BV)

Biological value measures the quantity of nitrogen retained in the body compared to the quantity of nitrogen absorbed in rats. It is calculated by measuring the amount of nitrogen utilised for the formation of muscle tissue (muscle protein synthesis) and dividing this value by the quantity of nitrogen absorbed from consumption. Full retention of nitrogen (i.e. none excreted via urine) indicates that the individual utilised all of the protein, resulting in a BV score of 100. Whole egg serves as the basis of the BV test, as it was considered to be the most useable source of protein when the method was developed, however since the test was developed more useable sources of protein have been found such as whey, which carry protein scores in excess of 100, between 106-159 depending on the type of whey protein.

Nitrogen (Net) Protein Utilisation (NPU)

The nitrogen protein utilisation test (NPU) is extremely similar to the BV test. The principal difference is that whilst BV is based on protein absorbed, NPU is based on the amount of protein consumed (and is therefore a measure of the amount of protein retained). It is usually assessed through nitrogen balance testing but the animal can be sacrificed for more accurate measures. An NPU score of 100 is indicative of 100% use of nitrogen (muscle protein synthesis). The main criticism of NPU is that the quantity of protein consumed in research varies widely from recommended daily intake.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

The PDCAAS is a more accurate modern method for assessing the quality of a protein based on a protein’s profile of essential and non-essential amino acids, in addition to their digestion within humans. It is the standard methodology accepted and used by the World Health Organisation as the most relevant method for human use. Its main limitation is that it does not take into consideration other factors which can influence protein utilisation, such as protein digestibility, the effect of other nutrients, and the value of proteins with higher essential amino acids (EAA’s).

Values greater than 1.0 for the PDCAAS show that the protein contains greater quantities of essential amino acids greater than is required for human consumption, and those protein sources have values rounded down to 1.0, (which fails to take into consideration a protein’s ability to complement a lower quality source of protein, which is lacking in quantities of essential amino acids).