Protein quality in food is primarily measured by assessing the amino acid composition and digestibility of the protein source.
Several methods are used to evaluate protein quality, with the two most prominent being the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).
Protein Digestibility-Corrected Amino Acid Score (PDCAAS)
PDCAAS has been a widely used method for evaluating protein quality. Here's how it works:
-
Amino Acid Profile: It compares the amino acid profile of a food protein to a standard amino acid profile based on human requirements. Specifically, it focuses on essential amino acids (also called indispensable amino acids), which the body cannot produce on its own.
-
Limiting Amino Acid: It identifies the 'limiting amino acid,' which is the essential amino acid present in the lowest amount relative to the reference protein.
-
Digestibility: It incorporates a protein digestibility value, which accounts for how well the protein is absorbed during digestion.
-
Calculation: The PDCAAS is calculated as follows:
PDCAAS = (mg of limiting amino acid in 1g of test protein / mg of the same amino acid in 1g of reference protein) x Fecal True Digestibility Percentage -
Score Range: The PDCAAS ranges from 0 to 1.0, with 1.0 indicating that the protein source meets or exceeds human amino acid requirements after accounting for digestibility.
Limitations of PDCAAS:
- PDCAAS truncates values above 1.0 to 1.0, potentially underestimating the quality of some proteins.
- It relies on fecal digestibility, which can be less accurate than ileal digestibility.
- It may not accurately reflect the availability of amino acids for specific tissues.
Digestible Indispensable Amino Acid Score (DIAAS)
DIAAS is a newer and more refined method for assessing protein quality:
-
Ileal Digestibility: DIAAS uses ileal digestibility, which measures amino acid absorption at the end of the small intestine (ileum). This is considered more accurate than fecal digestibility because it accounts for amino acids absorbed earlier in the digestive process.
-
Amino Acid Profile: Similar to PDCAAS, DIAAS compares the amino acid profile of a food protein to a reference protein.
-
Calculation: DIAAS is calculated for each essential amino acid and then summed. The lowest amino acid DIAAS score becomes the DIAAS for that food protein.
DIAAS % = 100 x [(mg of digestible indispensable amino acid in 1 g of the dietary protein) / (mg of the same indispensable amino acid in 1 g of the reference protein)] -
Interpretation: DIAAS values above 100 indicate excellent protein quality, meaning the protein source provides a sufficient amount of digestible essential amino acids. DIAAS values between 75 and 100 suggest good protein quality, while values below 75 indicate poor protein quality.
Advantages of DIAAS:
- Uses ileal digestibility for greater accuracy.
- Does not truncate values, allowing for better differentiation between high-quality proteins.
- More accurately reflects amino acid availability for the body.
Other Methods
While PDCAAS and DIAAS are the most commonly used methods, other techniques exist, including:
- Biological Value (BV): Measures the efficiency with which the body utilizes absorbed protein.
- Net Protein Utilization (NPU): Measures the proportion of ingested nitrogen that is retained in the body.
- Protein Efficiency Ratio (PER): Measures weight gain in growing animals per gram of protein consumed.
Conclusion
Protein quality in food is evaluated through methods like PDCAAS and DIAAS, which assess amino acid composition and digestibility. DIAAS is increasingly favored due to its use of ileal digestibility and its ability to provide a more accurate reflection of amino acid availability. These scores help determine how well a protein source meets human nutritional needs.
