Key learnings on the DIAAS and measuring protein quality
The following text is a non-exhaustive highlight of main insights derived from the research article "Protein quality as determined by the Digestible Indispensable Amino Acid Score evaluation of factors underlying the calculation (2016)." The core of the discussion is regarding methodology and implications of the Digestible Indispensable Amino Acid Score (DIAAS) in assessing protein quality.
Key Acronyms
AA - Amino Acid
DAA - Dispensable Amino Acids
DIAAS - Digestible Indispensable Amino Acid Score
EAR - Estimated Average Requirement
FAO - Food and Agriculture Organization of the United Nations
IAA - Indispensable Amino Acids
IAAR - Indispensable Amino Acid Requirement
PDCAAS - Protein Digestibility Corrected Amino Acid Score
About the PDCAAS
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was developed as a method to quantify dietary protein quality. It is based on both the amino acid composition and relative amounts of dietary Indispensable Amino Acids (IAAs) in the test protein, corrected for digestibility using a single value for true fecal crude protein digestibility, and expressed relative to a profile of amino acid requirements.
A PDCAAS of 1.0 means that all minimal requirements for IAA intake would be met if the amount of test protein consumed was equivalent to the estimated average requirement (EAR) for protein—commonly set at 0.66g/kg/day for adult males. This represents the minimal nitrogen intake that can enable the attainment of zero nitrogen balance (N-balance) in 50% of the population. However, there is no way to differentiate between two high-quality test foods that both deliver IAA intake in excess of the criteria.
About the DIAAS
The DIAAS (Digestible Indispensable Amino Acid Score) represents the percentage of the total daily requirement of the most limiting dietary IAA contained in an amount of protein equivalent to the EAR for total daily protein intake of the test protein. Unlike PDCAAS, it is not capped at 100%, allowing food sources to achieve rankings higher than 100% and enabling better comparison between foods that both exceed dietary IAA values.
The DIAAS is also measured using ileal digestibility, making it a more accurate reflection of absorption compared to what may be metabolized in the colon by bacteria.
Both scores are determined by the most limiting IAA in the composition. DIAAS simply allows for a closer representation of what is actually absorbed and better comparison of foods that exceed IAA values.
A DIAAS of 90% means that 90% of the daily requirement is provided by ingesting 0.66 g/kg/day (EAR) of the test protein.
Dependence on IAAR Accuracy
Requirements for individual IAAs depend on physiological state, such as growth or maintenance phases. Circumstances such as aging, athletic performance, and serious illness may also influence both the amount and profile of optimal IAA intake.
The basis for the dietary essentiality of IAAs stems from their role in protein synthesis and the lack of enzymes necessary for their synthesis from other amino acids and compounds.
The IAAs (indispensable amino acids) are:
Histidine (His)
Isoleucine (Ile)
Leucine (Leu)
Valine (Val)
Lysine (Lys)
Threonine (Thr)
Phenylalanine (Phe)
Methionine (Met) + Cysteine (Cys) = Sulfur-containing Amino Acids (SAA)
Tryptophan (Trp)
Ingestion of IAAs alone stimulates muscle protein synthesis as much as a mixture of the same amount of IAAs plus additional Dispensable Amino Acids (DAAs).
The Need for DAAs
However, the efficiency of IAA utilization has been shown to be enhanced by the amount of DAAs given concurrently. Both IAAs and DAAs are required for protein synthesis, and if IAAs are given in abundance without DAAs, the resulting stimulation of protein synthesis may deplete DAA availability. If only IAAs are ingested, the availability of certain DAAs could become rate-limiting for protein synthesis.
The amount of DAAs needed to prevent rate-limitation has not been determined. Agricultural science literature indicates that the ideal feed composition for maximum growth and muscle development in farm animals consists of approximately two-thirds amino nitrogen in the form of IAAs, but comparable human data are not available. Dietary protein contains between 30% and 50% IAAs, meaning that DAA contribution to amino acid composition of proteins is more than adequate to meet requirements if animal literature can be extrapolated to human diets.
How IAAR Has Been Determined
IAA requirements for maintenance are assumed to be the same for all individuals 18 years or older. The amount and pattern of dietary IAAs required for maintenance are based on data derived from experiments using the general approach of deleting one IAA from an otherwise complete diet, then progressively adding back greater amounts of that amino acid until the selected endpoint indicates that a breakpoint in response has been achieved.
A limitation of this method is that it assumes the amount of one IAA in the diet has no impact on the metabolism or function of other IAAs. No multi-variable analysis is performed, yet it is well established that interplay between IAAs is significant—administration of one will modulate absorption and turnover of others.
Examples of metabolic interactions:
Increasing dietary leucine activates branched-chain ketoacid dehydrogenase, which increases the oxidation of valine and isoleucine in addition to leucine. Increased dietary leucine may result in decreased availability of isoleucine and valine unless those amino acids are also increased. Isolated leucine dosing response may reflect rate-limiting decreases in valine and isoleucine availability rather than increased leucine availability.
Examples of transporter competition:
Branched-chain amino acids and aromatic amino acids compete for transport. Increased concentrations of branched-chain amino acids will decrease uptake of phenylalanine and tryptophan, with physiological consequences such as altered synthesis rates of serotonin and norepinephrine.
Non-amino acid factors:
Increased carbohydrate intake decreases the oxidation of concurrently ingested IAAs due to insulin stimulation.
How Valid is the IAA Requirement?
The IAAR defined by the FAO (0.184 g/kg/day) is easily exceeded by the EAR. However, this is consistent with modern research showing more ideal protein intakes of 1-2g daily are needed to maintain and build healthy amounts of muscle tissue for longevity and function.
An example of how FAO IAAR does not meet physiological ideals is the lysine gut loss case, where predicted total loss of gut-endogenous lysine accounts for almost 75% of the FAO's estimated lysine requirement, on top of the amount of lysine oxidized in muscle tissue.
The profile of amino acids within the total IAA has demonstrated significance as well. For example, older populations need higher leucine content to stimulate an equivalent anabolic response compared to younger populations.
The difference between EAR and IAAR is not driven by DAA demand, and EAR is not in excess but more likely under-specified. This indicates that the set IAA requirement is likely underspecified, which is consistent with the flaws in the single-variable breakpoint determination method that doesn't factor how increasing amounts of one amino acid increases oxidation of others, creating a lower rate-limiting step.
True Ileal Digestibility of IAAs
For PDCAAS, digestibility of amino acids is determined via total nitrogen measurement in feces. This has multiple issues, notably it does not account for metabolism in the colon by bacteria or the differing absorption rates of different amino acids.
Examples of specific amino acids with absorption issues include methionine (due to oxidation) and lysine (due to Maillard reaction browning/caramelization).
True ileal digestibility of amino acids presents measurement challenges in humans, with most research performed on persons with colons removed—obviously a non-normal condition. Another method involves a naso-ileal tube passed through the nasal cavity, esophagus, stomach, and small intestine down into the terminal ileum. While this can be performed in healthy individuals, the invasive nature means it must be performed in a hospital and can only be maintained for a few days. The person's diet also needs adjustment to prevent tube blockages.
The most popular method uses animal models, often pigs. Cross-species consistency mapping has demonstrated this is a viable and sufficiently accurate method to estimate ileal digestibility. The most common practice in pigs uses the T-cannulation method (a tube at the end of the ileum accessible externally for samples) or the slaughter method.
One variable that must be factored is the normal non-dietary amino acid flow through the ileum from endogenous breakdown. This can be moderately managed by measuring amino acid content separately from exogenous consumption.
Key Takeaways
The DIAAS is the best benchmarking tool currently available and is far more relevant than the PDCAAS system. However, DIAAS has room for improvement, mainly related to its dependence on the IAA index.
Areas for improvement include:
Raising the IAA requirement to a level more representative of healthy living, factoring in aging, gut health, and exercise
Multi-variable analysis for differing amino acid profiles to determine the IAA reference, rather than exclusively varying each IAA independently
Integration of distribution of meals and size of intake in each sitting
The influence of raising and decreasing DAA levels may change the DIAAS score. However, some DAA alongside IAA intake has demonstrated value that is not currently factored in. The current system is also not representative of real-world conditions, where it is often not either/or—DAAs can be added to meals at low additional cost. A person is not limited to total grams of amino acids but by what is available in a whole meal, which can be increased or decreased.
A good recommendation made for future optimization of the IAA index would be endpoint physiological testing such as sport performance, health outcomes, and body composition.
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First published for thomaskirk.pro 18th August 2025.