Protein, and protein powder, quality can be classified using a number of different methods. The main issues are how much of the protein is broken down and enters the blood stream (digestibility) and once the protein peptides enter the body how much of this can used to build proteins in the body including the repair and building of muscle.
Sometimes lactose content is mistakenly associated with whey protein powder quality. However, even though whey protein powders with high lactose levels may cause gas and bloating this is a digestive issue not a protein quality problem.
So, to summarise, protein quality is about how much protein in a protein source is absorbed by the body and how well it is utilised.
A common way of measuring the ebb and flow of protein in the body is through tracking nitrogen. This works nicely because amino acids contain nitrogen and protein is made from amino acids.
Common Methods of Measuring Protein Quality.
Protein Efficiency Ratio (PER).
The protein efficiency ratio is based on weight gain in rats divided by the amount of the particular protein consumed. Therefore the higher the ratio the more efficient the protein source is at increasing bodyweight.
Biological Value (BV).
The biological value is a measure of the proportion of the source protein that is incorporated into proteins in the body. It has long been the method of choice for estimating the nutritive value of different protein sources. However, this method doesn’t take into account the digestibility of the protein which is the percentage of food nitrogen absorbed from the gut.
Therefore a better measure of the overall nutritive value of a protein would be BV x Digestibility which should be identical to the NPU.
Net Protein Utilization (NPU).
The net protein utilisation compares the amount of amino acids converted to protein in the body of an animal fed the protein source with animals that are fed no protein.
Protein Digestibility Correct Amino Acid Score (PDCAAS).
The protein digestibility corrected amino acid score assesses protein quality based on the amino acid requirements of humans and takes into account the digestibility of the protein source.
The formula is the amount of limiting amino acid in the test protein divided by the amount in the reference protein (egg white) multiplied by the fecal true digestibility percentage. The highest value is therefore 1.
The PDCAAS is the method preferred by the FDA and the WHO. The other methods may not be directly applicable to humans as the test animals are rats. However, they are interesting as they give an indication as to which of the high quality proteins may lead to better muscle growth and repair.
While egg white, whey protein powders and casein all have a PDCAAS of 1, their PER values differ with whey protein at 3-3.2 whereas egg white and casein have a PER of 2.8.
By comparison, the PDCAAS values for beef, chicken and fish are around 0.91 while their PER values are generally between 2 and 2.3.