Proteins from dairy products such as milk or cheese were more easily digested than plant proteins, such as soya and wheat.
They were an excellent or high quality source of protein for children over six months outscoring plant-based alternatives.
Professor Dr Hans Stein at the University of Illinois said: “Plant proteins are the primary sources of amino acids in many parts of the world, whereas animal proteins are the primary sources in other parts of the world.
“However, the composition and digestibility of these types of proteins differ.”
So the study used pigs to calculate how easily different proteins were absorbed by the body.
It tested whey protein isolate, whey protein concentrate, milk protein concentrate, skimmed milk powder, pea protein concentrate, soya protein isolate, soya flour, and whole-grain wheat.
Dairy scored more than 100 while soya while ranked a ‘good’ source of protein only scored between 75 and 100.
Pea protein and wheat scored less than 75 so the study could not recommend it “regarding protein quality.”
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.
For the past two decades scientists had used a protein score known as protein digestibility-corrected amino acid scores or PDCAAS.
PDCAAS is calculated using the total tract digestibility of crude protein. However, this method has certain shortcomings.
Prof Stein said: “The total tract digestibility fails to take into account nitrogen excretion in the hindgut.
“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.
Dr Greg Miller, chief science officer at the National Dairy Council, who funded the research said: “The results of this pilot study indicate that dairy proteins may be an even higher quality source of protein compared to vegetable-based protein sources than previously thought.
“While using DIAAS is a newer concept and more research will be needed, one thing rings true - milk proteins are high quality and milk as a beverage has protein plus eight other essential nutrients, which is especially important when it comes to kids, because they need quality nutrition to help support their growth and development.”
Prof Stein concluded: “Compared with DIAAS, PDCAAS calculations tended to underestimate the protein value of high quality protein sources, and overestimate the value of lower quality sources.
“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.”
The study was published in the British Journal of Nutrition.