Up until recently, another method of protein quality evaluation was commonly used; the protein digestibility-corrected amino acid score (PDCAAS). This method functions similarly to DIAAS but has several drawbacks that make it less accurate and no longer appropriate.
The assessment of the nutritional value of a protein should reflect its ability to satisfy the metabolic demands for nitrogen and amino acids. Its capacity to do so is determined by amino acid availability and digestibility . There are several reasons why PDCAAS does not adequately account for these factors.
Firstly, PDCAAS is taken from faecal samples, where the presence of amino acids will determine their relative digestibility. This can lead to an inaccurate evaluation, as many processes in the gut can introduce amino acids into the faeces, such as the synthesis of methionine and lysine and microbial metabolic functions. This can result in “false positives” when measuring quantities of amino acids to determine absorption .
On the other hand, DIAAS samples are taken from the ilium. This leads to greater accuracy when assessing digestibility, as amino acid absorption practically ceases once it reaches this point in the digestive tract . Measurement at the ilium also precedes the processes in the gut that can introduce additional amino acids.
Another factor to consider is that the PDCAAS system only allows for a maximum score of 1. It does not permit extra nutritional value to be evaluated in higher quality proteins. Under the DIAAS method, a protein that exceeds the human requirements for all essential amino acids can score higher than 1. This makes it a more effective way to evaluate and compare high-quality protein sources .
In 2011, the Food and Agriculture Organisation (FAO) recommended using DIAAS over PDCAAS for protein quality evaluation .