What does protein quality mean, and how is it measured?

Protein quality tells you to what extent a protein meets the essential amino acid requirement and how much of those amino acids can be digested. Türkiye Beslenme Rehberi (TÜBER), Türkiye's national dietary guideline, expresses this with a score: the digestible indispensable amino acid score, DIAAS for short. When the score is 100 percent it means the protein fully meets the requirement for the essential amino acids.

What does the score measure?

The measurement asks two questions together. First, are the essential amino acids present in the protein in sufficient amounts? Second, can those amino acids really be absorbed? The guideline states that the digestibility of the essential amino acids in plant-based foods is low, and that in plant proteins these amino acids are found in a more limited way.

A reference pattern is needed for the comparison to be made. The guideline's amino acid scoring pattern table gives, for children over 3, adolescents and adults, the amounts of digestible essential amino acids recommended in every 1 gram of protein: histidine 16, isoleucine 30, leucine 61, lysine 48, sulfur-containing amino acids 23, aromatic amino acids 41, threonine 25, tryptophan 6.6 and valine 40 milligrams.

The score of the average diet in Türkiye

The guideline has made this measurement for Türkiye as well. The DIAAS value calculated from the one-day food consumption records of the Türkiye Nutrition and Health Survey (TBSA) was found to be 83 percent for lysine, the limiting amino acid. So the protein quality of the diet at the country average is below that of the reference protein.

The practical result of this score shows up as a second column in protein tables. While the need of a man aged 19-29 is 0.83 g/kg with a protein pattern whose quality is 100 percent, the same need requires 1.04 g/kg with the average diet in Türkiye. At a median weight of 71.9 kilograms this means 74.8 grams instead of 59.7 grams.

Which foods raise the quality?

The food the guideline points to as the highest quality is the egg; it is counted as the reference protein because its proteins are known to be converted into body proteins at a rate of 100 percent. That is also why it is an important source in situations where the amount of protein in the diet is restricted.

On the plant side quality rises through pairing. When dried legumes are cooked well and mixed with grains in a certain ratio, protein quality increases. Pseudocereals such as quinoa, amaranth and buckwheat stand apart in carrying a balanced amino acid pattern and a protein rich in sulfur-containing amino acids.

How is the score calculated and how is it read?

The calculation is a ratio: the amount of digestible amino acid in 1 gram of dietary protein is divided by the amount of the same amino acid in 1 gram of reference protein, and the result is multiplied by a hundred. The guideline splits the resulting score into three bands: below 75 percent is low, 75-99 percent is good, and 100 percent and above is high protein quality. The egg and milk are shown as examples of the last band.

What sets the score is not a single food but the diet as a whole. In diets where the share of plant protein is high and the share of animal protein is low, the score can stay below 100 percent. This shows that the mixed diet does not meet the requirement for some of the essential amino acids.

Where does the gap in the diet in Türkiye come from?

The guideline also opens up the source of that 83 point country average. In Türkiye, 58 percent of total protein intake comes from plant foods, and 40 percent of that share is supplied by grains that are poor in lysine, above all by refined grain products.

The food that could close the lysine gap of grains is dried legumes, but their contribution to protein intake has been measured at only 3.7 percent. Animal foods rich in essential amino acids, meanwhile, are consumed rarely or in small amounts by most of the population. The way to raise the score is where these three numbers point: lowering the share of refined grains, raising the share of dried legumes, and pairing the two groups in the same meal.

Where does it turn into a mistake?

The most common confusion is equating quality with amount. High protein in 100 grams of a food does not show that it is of high quality; in dried legumes the protein value is high and the quality low. The reverse is true as well: a protein small in amount but complete in pattern makes a more efficient contribution to the daily calculation. So when looking at a food's protein value you have to look not only at the gram column but also at which group that protein completes.

The second confusion is mixing up two columns of the table. The value 1.04 g/kg is not a higher target; it is the cost of meeting the same target with a low quality pattern. When the pattern improves, that is when animal and plant sources are balanced, the need pulls back towards the first column. You can follow the amount side from the protein guide, and the food by food comparison from the protein tab of the nutrition values table.

This content has been prepared to support general nutrition literacy; it does not take the place of medical diagnosis or treatment and does not carry the character of personal nutrition advice. A personalized plan is prepared only by a qualified dietitian; if there is pregnancy, chronic illness, regular medication use, child nutrition or a clinical condition, supervision by a doctor and a dietitian is required before starting a plan.