Proteins can be obtained from various sources. But all proteins are not the same. The nutritional value of proteins with inferior content or poor digestibility is lower. It is important for us to know this and make the good proteins a part of our diet.
The joint FAO and WHO Expert Consultation on Protein Quality Evaluation proposed these parameters to assess protein quality.
Digestibility :
In order that the amino acids can be absorbed, the protein in the diet has to be digested. Plant proteins are poorly digested as compared to the animal proteins.
Biological Value (BV) :
This provides a measurement of how efficiently the body utilizes the protein in the diet.
A high BV correlates with a high supply of essential amino acids and better utilization of the protein by the body.
Animal based proteins have a higher BV than the plant based sources.
Net Protein Utilization
This value is similar to BV. But it measures the utilization of ingested protein not protein absorbed.
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
This has been adopted as the preferred method for assessing protein value. A dietary protein with a PDCAAS less than 100%, cannot meet all the essential amino acid requirement of the body.
Among plant sources only soy protein isolates have a PDCAAS of 100%. Plant proteins have a lower PDCAAS due to poor digestibility and because they lack some essential amino acids.
Digestible Indispensable Amino Acid Score (DIAAS)
Recently the FAO has recommended DIAAS over PDCAAS . Unlike PDCAAS, DIAAS allows scores higher than 100%. Using DIAAS scores whey protein scores higher than soy.
Here is how these measures fit our regular protein sources :
| Whey: | BV (104) | PDCAAS(100) | Digestibility (99) |
| Casein: | BV(77) | PDCAAS (100) | Digestibility (99) |
| Milk: | BV (91) | PDCAAS (100) | - |
| Meat: | BV (80) | PDCAAS (92) | - |
| Egg: | BV (100) | PDCAAS (100) | - |
| Soy protein isolate: | BV (74) | PDCAAS (100) | - |
| Soy flour : | - | PDCAAS (93) | - |
| Black beans: | - | PDCAAS (75) | - |
| Pea protein : | - | PDCAAS (89) | - |
| Peanuts : | - | PDCAAS (52) | - |
| Wheat : | - | PDCAAS (42) | - |
| Wheat gluten : | - | PDCAAS (25) | - |
| Rice : | - | PDCAAS (50) | - |
So what protein should be included in our diet?
A good protein should not only have a PDCAAS of 100% but also have a good digestibility. Whey protein, casein, milk, soy protein and eggs have a PDCAAS of 100%. But, which better among these? Biological Value (BV) helps to know this. BV gives an idea how well the body can use the protein. The BV of whey and egg is higher than that of casein and soy. Thus, whey and eggs are considered better sources of proteins than soy or casein
However, a meal does not consist of a single food item. Therefore, PDCAAS score of the combined meal has been considered. The low PDCAAS score of rice can be greatly improved by adding milk to it. Or a combination of cereals with pulses as in our dal chawal, or dal roti or khichadi improves protein quality in a meal by improving the PDCAAS of the meal.
This explains the advantage of a Thali, where we eat different variety of food during a meal. The vegetarians have to be especially careful of the food they eat. The non-vegetarians less so, because animal and dairy based proteins are complete proteins and have high PDCAAS.