Quality vs Quantity: how to assess the quality of proteins?
Proteins are one of the body’s essential macronutrients. They are perceived as beneficial for health and products with claims concerning protein are the second-fastest growth category in the food and drink sector. With so many new protein-enriched products on offer, it can be difficult for consumers to make sense of them. In this article we explain how protein quality can be assessed and what the differences are depending on its origin.
Role and composition of protein
Protein is a big family among the body’s essential macronutrients. It is made up of subunits: amino acids, 19 of which are used to build protein. Eleven of them can be made by the body whereas eight others must come from food: Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine (with the addition of Histidine in infants). They are said to be ‘essential’ because the body cannot produce them in sufficient quantities.
Proteins have an essential structural role (muscles, bones, etc.) and are also important for digestion, immune responses and transporting oxygen throughout the body. The body maintains a balance between their synthesis (anabolism) and breakdown (catabolism), and they need to be renewed constantly. A large part of protein comes from food (ANSES, 2019).
Two sources of protein are available: animal and plant. Animal sources: meat, fish, milk and dairy products, and eggs are protein-rich foods. As for plants, the most protein rich are oily seeds, legumes, and grains.
How to assess the quality of a protein?
Two criteria exist to determine the quality of a protein: its essential amino acid composition and its digestibility.
The richer the protein is in essential amino acids, the easier the body will be able to produce new proteins. If a single amino acid is missing, it will not be possible to produce the protein. However, if the amino acid intake is too high, the body will be unable to use them, and they will be lost through oxidation. This hinders the detoxification process and can have harmful consequences in the long term.
The second important criterion to determine the quality of protein is for it to be easily digestible. Digestibility is the ability to absorb ingested nitrogen (GAUDICHON, 2002): the higher it is, the more the compounds provided by the food will be used.
The chemical score reflects the quantity of the most limiting essential amino acid in relation to the reference protein profile. The limiting amino acid corresponds to the essential amino acid found in the smallest quantity in the test protein. (TESSIER, 2021)
In order to compare the quality of proteins from different origins, the Food and Agriculture Organization (FAO) developed the PDCAAS (Protein Digestibility Corrected Amino Acid Score). It weights the chemical score of the protein by its true nitrogen digestibility measured from GI losses. (TESSIER, 2021)
In 2013, the FAO recommended a second score, the DIAAS (Digestibility of Indispensable Amino Acids Score) to determine protein quality. (TESSIER, 2021)
Unlike the PDCAAS method, which assesses the faecal digestibility of proteins, the DIAAS incorporates the ileal digestibility of amino acids. Furthermore, the DIASS method allows for scores > 100% whereas as the PDCAAS is cut off at 100%. It can therefore be assumed that there may be health benefits associated with higher DIAAS scores.
Protein quality depending on origin
Using these assessment methods, it is possible to compare the quality of plant proteins with animal proteins. Plant proteins have high scores but may have insufficient levels of one or more essential amino acids, as shown by scores below 100% in the tables below.
Legumes are often low in methionine and cysteine, while lysine is generally limited in grains (HERTZLER, 2020).
Most animal protein sources are considered complete protein sources
Due to their PDCAAS very close to 100%, most animal protein sources are considered complete protein sources that meet essential amino acid requirements.
To go further: protein digestion speed
More than their composition and digestibility, the speed of protein absorption is now at the centre of scientific discussions. Indeed, the effects on health vary depending on the speed at which amino acids appear in the blood.
- Serum proteins are considered ‘rapid’ because they are rapidly released from the stomach and pass quickly into the intestine. There amino acids are absorbed over a short period (BOIRIE, 1997). It has been shown that the rapid arrival of amino acids including leucine into the bloodstream leads to an increase in muscle synthesis (JÄGER, 2017).
- In contrast, caseins are said to be ‘slow’ because they precipitate in the stomach. Their amino acids are released and absorbed more slowly over a period of up to several hours (BOIRIE, 1997). Thus, these proteins are best suited to meet the body’s needs during long periods of fasting, such as at night.
Dairy proteins are very interesting from a nutritional point of view
Dairy proteins are very interesting from a nutritional point of view: they combine a particularly well-balanced composition of essential amino acids with high digestibility (> 95%) (GAUDICHON, 2002), which gives them very high protein quality scores. Depending on their nutritional specificities, their use can be targeted according to the desired effects and the chosen target group.
ANSES. (2019). Les protéines : Définition, rôle dans l’organisme, sources alimentaires. Récupéré sur ANSES.
BOIRIE. (1997). Slow and fast dietary proteins differently modulate postprandial protein accretion. Physiology, 14930–14935.
GAUDICHON. (2002). Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements. Gastroenterology, 50–59.
HERTZLER. (2020). Plant proteins: assessing their nutritional quality and effects on health and physical function. Nutrients.
JÄGER. (2017). International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition.
RUTHERFURD. (2015). Protein Digestibility-Corrected Amino Acid Scores and Digestible Indispensable Amino Acid Scores Differentially Describe Protein Quality in Growing Male Rats. The Journal of Nutrition.
TESSIER. (2021). Les “dessous” du PDCAAS et du DIAAS, deux critères en apparence simples de qualité nutritionnelle des protéines. Cahiers de nutrition et de diététique, 102-110.
 Innova Database, 2015–2019