Protein quality is determined on the basis of three characteristics, being: the quantity of protein in a foodstuff, the quantity of essential amino acids in the protein and the digestibility of the amino acids in the small intestine. Milk protein scores well for these items and therefore it is often regarded as a high-quality protein.
- The quantity of protein in a foodstuff
- The quantity of essential amino acids in the protein
- The digestibility of the amino acids in the small intestine.
The number of essential amino acids in the protein differs for each protein source (see table 1). Milk, eggs and beef contain a wider range of essential amino acids and there is a difference in the profile of essential amino acids within each protein food. (2) For an adequate intake of all the essential amino acids it is important to consume a combination of various protein sources each day. For example, the quantity of lysine is lower in the proteins in grains but higher in milk proteins. So for this reason a cheese sandwich is not a bad idea when considering the provision of a range of amino acids.
TABLE 1 Overview of essential amino acids in mg/g protein of various foodstuffs
Table 1 provides totals for the quantity of methionine and cysteine (the sulphurous amino acids). It is basically assumed that methionine is the essential amino acid, but the question whether the quantity of cysteine has any influence on the requirement for methionine is still under discussion. This also holds for phenylalanine (the essential amino acid) and tyrosine, the aromatic amino acids. (2) Both the sulphurous and the aromatic essential amino acids are interconvertible and can be metabolised from each other. However as neither can be synthesized de novo in humans this is why people often talk about eleven essential amino acids instead of nine.
Ileal protein digestibility
One of the factors used to determine the quality of the protein source is the digestibility of the amino acids in the small intestine (1). The digestibility of a protein is preferably measured at the end of the small intestine (terminal ileum). This is considered to be the best way to determine the bioavailability of the amino acids in the food protein for absorption and use in the body. It also allows us to properly compare various proteins that we take in from food. In fact, food may contain factors that influence the digestion process and so make the amino acids in the protein less or more available for absorption. An example of such anti-nutritional factors are the trypsine inhibitors in vegetable protein sources. (3)
However, determining the digestibility of amino acids in the small intestine (ileal digestibility) and routinely carrying this through is not easy. For this, animal models have been developed and studies have been conducted into the digestibility of the nitrogen present in human proteins. This is based on the difference measured between the quantity of nitrogen taken in (through the protein) and the quantity that is found in the faeces (faecal digestibility). The disadvantage of this method is that bacteria in the large intestine also metabolise nitrogen and so faecal nitrogen may not be a reliable measure of the actual digestibility of the amino acids. Besides, this method does not take into account the secretion of the body’s own (endogenous) proteins into the small intestine during the digestion process. Therefore measuring ileal protein digestibility is preferred. (3)
The nitrogen digestibility of milk protein and the digestibility of the individual essential amino acids in milk protein is high with an average of score of 95%. Other animal proteins and concentrated or purified vegetable proteins, such as soy and pea, have a digestibility score of over 90%. The body however experiences greater difficulty digesting other vegetable proteins. (1)
- Tomé, D. (2012). Criteria and markers for protein quality assessment – a review. British Journal of Nutrition, 2012; 108, S222–S229.
- FAO/WHO/UNU Expert Consultation (2007). Protein and amino acid requirements in human nutrition. World Health Organ Tech Rep Ser. 2007;(935):1-265.
- Schaafsma, G. (2012). Advantages and limitations of the protein digestibility-corrected amino acid score (PDCAAS) as a method for evaluating protein quality in human diets. British Journal of Nutrition, 2012; 108, S333–S336
- FAO (2013). Dietary protein quality evaluation in human nutrition. Report of an FAO Expert Consultation. Food and Agriculture Organisation of the United Nations. Rome, 2013.