Protein quality

Amino Acid Score The amino acid score is based on the proportion of amino acids in a food, compared to nutritional requirements. As such, only essential amino acids are considered in the two most common measurements of quality, the PDCAAS and the DIAAS. Nevertheless, common protein analytical methods such as ISO 13903 can easily distinguish these pairs of amino acids. Additionally, glutamic acid is easilty interconvertible with glutamine and aspartic acid is easily interconvertible with asparginine. easily interconvertible, via acid hydrolysis. Selenocysteine is usually not measured as part of amino acid analysis. It is usually analyzed directly as the amount of selenium, which mostly occurs as selenomethionine and selenocysteine in food. • Semi-essential, under certain conditions • Branched-chain amino acid (BCAA) Protein digestibility For many foods, the quantity of amino acids absorbed by the body may differ significantly from the quantities of amino acids originally present in the food, as a result of various digestive processes. The digestion of proteins begins in the stomach and is largely complete by the time food exits the small intestine. However, digestion may be reduced by antinutritional factors or the presence of other food components such as dietary fiber. Gut microbes may also impact protein digestion due to their own digestion of protein. Digestibility may also differ between amino acids. While the fecal digestibility of the whole protein is likely a fair approximation of the digestibility of individual amino acids for non-legume (beans, peas, lentils) proteins with a maximum difference of 10%, with legume proteins, the digestibility of methionine, cystine, and tryptophan can be overestimated. As a result, it is recommended to use lysine as the limiting amino acid for any food likely to have undergone the Maillard reaction. The essential amino acid requirements for infants are based on the essential amino acid proportions in human breast milk. ==Measures==

Primitive measures of protein quality use relatively few measurements about the body, mainly mass measurements. • Protein efficiency ratio (PER) is the ratio of weight gain to the amount of protein ingested. It is usually tested with rats. • Biological value (BV) essentially estimates the proportion of food nitrogen kept in the body by subtracting out nitrogen found in urine and feces. Nitrogen is assumed to originate from protein. • Net protein utilization (NPU) is similar, except it only subtracts out urine nitrogen. They are methods based on nitrogen balance. Modern measurements analyze two separate aspects: protein digestibility and amino acid balance. The former is measured by comparing how much protein is found in the food before and after it goes through the digestive tract or a part of it. The latter is measured by taking the amino acid profile of a protein and comparing it to essential amino acid needs of an organism, typically humans. PDCAAS The earlier "modern" measurement is the PDCAAS of 1989. For protein digestibility it compares the amount of protein-nitrogen that goes into a rat and out of the rat through feces, with a correction for "metabolic fecal protein": the amount of protein that occurs in feces when the rat is on a protein-free diet. For the amino acid score, it considers the quantity of each essential amino acid in the food as a proportion of the food's protein content and compares this to nutritional recommendations for preschool-age kids. The amino acid score used is that of the lowest, or 'limiting', amino acid. The amino acid score is then multiplied with the fecal digestibility score to get an overall score between zero and one. Antinutrient factors like phytic acid and trypsin inhibitors may decrease absorption of protein, as does the absorption of protein by gut bacteria in the test subject's large intestine. In addition, older rats show lower PDCAAS-estimated fecal digestibility compared to young rats when the protein source contains anti-nutritional factors. It also considers age by using different reference patterns for infants, toddlers, and people over three. DIAAS is considered the superior method to PDCAAS. ==Protein sources==

Notes: • With scores greater or equal to 1.0 or 100%, the concept of "limiting amino acid" technically still applies as the amino acid with the lowest ratio compared to the amounts in the reference protein. It is hardly relevant, however, so such columns are parenthesized. • PD is determined per PDCAAS ("true fecal") unless otherwise stated. • AAS explicitly does not take into account digestibility. It compares the amino acid profile to a reference profile, which is the PDCAAS profile unless otherwise stated. • Limiting AA may be determined by either PDCAAS (or a similar profile-only method) or the DIAAS (which gives the limiting absorbed AA). If unstated it is more likely to be PDCAAS. • PDCAAS values are officially capped, but the limit can be removed by manually calculating PD × AAS. Examples of 5 uncapped values are seen in Schaafsma (2000). == Implications ==

Since their introduction, modern measures of protein quality have been used to justify nutritional advice on the selection of protein sources and related public policy. As typical values for plant sources of protein are often considerably lower than those of animal protein, Advocates of the importance of animal proteins point to the health benefits of such a diet, as well as the potential for protein insufficiency in a plant-based diet. The choice of protein quality measurement also has implications for debates around plant-based diets. Plant sources of protein are more likely than animal sources to have a lower DIAAS than PDCAAS, so the use of DIAAS rather than PDCAAS may increase nominal discrepancies in protein quality between plant and animal sources. On the regulatory scale, this may affect how plant sources of protein may be marketed. For example, the USDA allows foods with at least 10% of RDI of protein to be labeled as a "good source" of protein, and for a food to be labeled "high" in protein, it must contain at least 20% of RDI, accounting for quality. The use of DIAAS would change which plant foods may be marketed as high protein, and thus consumer perception of their dietary choices. they do not necessarily reflect protein quality or changes due to food preparation, so consumers may use measures of protein quality of the food as prepared as an additional source of nutritional guidance. In the developing world, malnutrition may be characterized by a lack of protein. Measuring protein quality may further detail the nature of this malnutrition and have implications for treatment. Additionally, in the context of global development, research on the relative merits of plant and animal proteins takes on an environmental significance, because the trajectory of dietary improvement in developing nations could significantly affect the environment. == Limitations and Future Directions ==

Determining ileal digestibility While more accurate, the determination of ileal digestibility of various protein sources requires human or porcine subjects, making research much more difficult than in mice. While the body of research on food ileal digestibilities has grown since the introduction of the DIAAS, as of 2025, more research is still needed, particularly on foods outside of Western or Eastern diets. Legumes, with the exception of soy, generally have protein qualities between 0.5 and 0.8 and are limited by methionine rather than lysine. When eaten together in the optimal ratio, the quality of the combined constituent may be as high as 1.0, because each constituent's protein is complemented by the other. A more extreme example would be the combination of gelatine (which contains virtually no tryptophan and thus has a PDCAAS close to 0) ==References==