The researchers behind a new study calculated protein scores for eight sources of protein, derived from both plants and animals. Protein scores compare the amount of digestible amino acids in a food with a "reference protein," a theoretical protein which contains fully digestible amino acids in the proportions required for human nutrition at a particular stage of life.
The score which has been used for more than 20 years is the protein digestibility-corrected amino acid score, or PDCAAS. PDCAAS is calculated using the total tract digestibility of crude protein. However, this method has certain shortcomings.
"The total tract digestibility fails to take into account nitrogen excretion in the hindgut," says Dr. Hans H. Stein, professor of animal sciences at University of Illinois. "The PDCAAS also assumes that all amino acids in a foodstuff have the same digestibility as crude protein, but in reality, amino acid digestibilities differ."
These flaws led to the development of a new measure, called the digestible indispensable amino acid score (DIAAS). The DIAAS is calculated using ileal digestibility values, because all absorption of amino acids takes place in the small intestine. It also uses values calculated individually for each amino acid.
Stein and his team determined standardized ileal digestibility of crude protein and amino acids in eight sources of animal and plant protein: whey protein isolate, whey protein concentrate, milk protein concentrate, skimmed milk powder, pea protein concentrate, soy protein isolate, soy flour, and whole-grain wheat. They derived DIAAS scores from those ileal digestibility values. They also calculated PDCAAS-like scores by applying the total tract digestibility of crude protein in the ingredients to all amino acids.
All dairy proteins tested in the study met Food and Agriculture Organization (FAO) standards as "excellent/high"-quality sources of protein for people six months of age or older, with DIAAS values of 100 or greater. Soy protein isolate and soy flour qualified as "good" sources of protein, with a score between 75 and 100. With scores below 75, pea protein concentrate and wheat did not qualify to make recommendations regarding protein quality.
"Compared with DIAAS, PDCAAS calculations tended to underestimate the protein value of high quality protein sources, and overestimate the value of lower quality sources," says Stein. "Thus, to better meet protein requirements of humans, especially for people consuming diets that are low or marginal in digestible amino acids, DIAAS values should be used to estimate protein quality of foods.
Stein acknowledged certain limitations in the study. "The protein sources used in this experiment were fed raw, and foods processed as they typically are for human consumption might well have different protein values."