The research, conducted by Massey University in New Zealand and the R&D department of dairy firm Fonterra, compared two methods of measuring protein quality: the traditional protein digestibility–corrected amino acid score (PDCAAS) and the FAO-recommended 'newbie' called the digestible indispensable amino acid score (DIAAS).
The researchers used both methods to measure 14 different dietary protein sources - whey- and soy-protein isolates, milk-, whey-, rice- and pea- protein concentrates, cooked kidney beans, roasted peanuts, cooked peas, corn-based breakfast cereal, cooked rice, cooked rolled oats and wheat bran - consumed as part of a wheat starch- based diet by growing male rats.
The methods produced different results for 11 out of the 14 protein sources. Products by Fonterra, Axiom Foods, Solae and Roquette were included in the study.
The paper showed PDCAAS generally 'overestimated' the scores of sources of lower quality protein, while higher sources came out with lower scores compared to DIAAS. For example, DIAAS values for milk protein concentrate and whey protein isolate were 15% and 8% higher, respectively, than that found using PDCAAS. Meanwhile PDCAAS overvalued corn-based breakfast cereals by about 574% compared to the DIAAS score.
"Untruncated PDCAAS values were generally higher than a DIAAS values, especially for the poorer quality proteins; therefore, the reported differences in the scores are of potential practical importance for populations in which dietary protein intake may be marginal," the researchers wrote in the Journal of Nutrition.
Commenting on the paper, Suzane Leser, head of nutrition at Volac Human Nutrition, told us the results reiterated the importance of the DIAAS method being made law.
She said currently firms could cherry pick which method made their protein source look better.
"We would like protein quality to be regulated so that it becomes obligatory for the industry to use the same method. And having protein quality recognised in legislation is an important step for every party in the protein industry, because it gives us the ability to differentiate the value of foods by evolving the messages on to protein quality, which today is impaired by the way legislation has developed in Europe to treat all proteins as equal."
What's the difference?
DIAAS measures the oro-ileal nitrogen balance by calculating the digestibility of individual amino acids in a section of the small intestine called the ileum. Meanwhile PDCAAS uses crude faecal digestibility values to measure the oro-faecal nitrogen balance.
A fundamental criticism of PDCAAS is that it uses fecal rather than ileal digestibility as well as nitrogen digestibility rather than the digestibility of individual amino acids to correct amino acid scores, which can lead to under or overestimate of protein quality.
The Food and Agriculture Organisation (FAO) has called the DIAAS method "preferable" to PDCAAS and called for more research to be conducted.
Past research has shown the greater quality of dairy proteins using DIAAS. Dairy players like Volac have backed DIAAS and called for it to replace PDCAAS, which was established 20 years ago. Leser called PDCAAS "obsolete".
Other non-dairy sectors remained sceptical.
Roquette said it still used PDCAAS. Dr Catherine Lefranc-Millot, corporate scientific communications manager for the firm, told us: "[W]ith such methodology and analytical possible changes that could apply, it takes time."
Therefore she said the firm would be sticking with the long-standing PDCAAS method for now.
Leser said: "For the food industry, the ultimate aim is to adopt protein quality in food legislation, which can only be achieved with an accurate method. The concrete outcome is for protein nutrition claims to include a measure of protein quality, so that the use of claims becomes restricted to truly valuable, high quality proteins.
"This resolves many of the issues we have today around the definition of protein, such as preventing non-protein sources of nitrogen from making protein claims, as seen in the most recent cases of ‘protein spiking’."
Source: Journal of Nutrition
Published online ahead of print, doi:10.3945/jn.114.195438
"Protein Digestibility-Corrected Amino Acid Scores and Digestible Indispensable Amino Acid Scores Differentially Describe Protein Quality in Growing Male Rats"
Authors: S. M. Rutherfurd, A. C. Fanning, B. J. Miller and P. J. Moughan