Dietary protein quality evaluation in human nutrition
Report of an FAO Expert Consultation, FAO Food and Nutrition Paper n.92
As the global population increases, it is more important than ever to define the amount and quality of protein required to meet human nutrition requirements and to describe the protein supplied by foods and mixed diets.
Proteins vary according to their origin (animal or plant), their amino acid composition (particularly their relative content of the essential amino acids) and their digestibility. High quality proteins are those that are readily digestible and contain the dietary essential (indispensable) amino acids in quantities that correspond to human requirements. For many years, bioassays, mainly in rats, was the method of choice to assess the nutritional value of proteins. This value was expressed in parameters such as protein efficiency ratio, net protein utilisation and biological value. In 1991, the Food and Agricultural Organisation (FAO) and World Health Organisation (WHO) adopted the method known as Protein Digestibility Corrected Amino Acid Score (PDCAAS). However, on the 15th February 2013, a report from a FAO Expert Consultation held in New Zealand in March 2011 recommended that the Digestible Indispensable Amino Acid Score (DIAAS) should replace the PDCAAS as the preferred method of measuring protein quality and describing the protein contribution of foods. This decision reflects advances in the analytical methods available to measure the quality and digestibility (bioavailability) of dietary proteins.
The report - Dietary protein quality evaluation in human nutrition, FAO Food and Nutrition Paper n.92 (published on 15 February 2013) - recommends that the method be adopted in Codex food labelling and claims guidelines. The FAO report also recommends future research to determine protein and amino acid needs that may be above the minimum requirements in certain circumstances.
The report describes how the new method, known as DIAAS (Digestible Indispensable Amino Acid Score), will overcome some of the short comings of the current PDCAAS method. PDCAAS estimates protein digestibility over the total gut (referred to as ‘faecal’ protein), i.e. it measures the disappearance of protein between the mouth and the end of the digestive tract and from this uses a single value of crude protein digestibility to correct the digestibility of individual indispensable amino acids. In contrast, the DIAAS method determines amino acid digestibility at the end of the small intestine (mouth to terminal ileum), which more accurately reflects the extent of amino acids absorption. Another key difference is that DIAAS considers the digestibility of individual amino acids in the protein, as opposed to the protein as a whole.
The FAO report highlights the benefit of treating dietary amino acids as individual nutrients when assessing protein quality. In the new methodology, ratios are derived for the digestibility of each dietary indispensable amino acid in the food, compared to a specified reference amino acid pattern (three patterns have been identified – one for infants up to 6 months, one for 6 months to 3 years, a third for all other age groups); the lowest ratio (expressed as a percentage) is taken as the DIAAS value. An example is provided in the FAO report on page 24. The FAO advises that DIAAS values above 100 should no longer be truncated (as currently happens with the PDCAAS method), except for single food sources and when determining the protein value of a mixed diet, as this penalises proteins with high concentrations of the indispensable amino acids, which are of particular interest from a food security perspective. A DIAAS over 100 for an individual food indicates the potential for the food to complement the protein contribution of a food with a lower DIAAS value (i.e. a food with a ‘limiting’ amino acid). Other characteristics of the PDCAAS method noted by the FAO are: it over-estimates the protein quality of products containing anti-nutritional factors; it does not adequately take into consideration the bioavailability of amino acids; and it overestimates the quality of proteins that are limiting in more than one amino acid.
One of the indispensable amino acids, lysine, can be modified as a result of the Maillard reaction that occurs during food processing, affecting its bioavailability. Because it measures the digestibility of individual amino acids, the DIAAS method provides a means of determining bioavailable lysine, excluding the transformed portion of the amino acid, thus providing an assessment of the true protein quality of the food as consumed.
As is often the case with new methodology, there is currently insufficient data on the ileal digestibility of amino acids to support an immediate move to the DIAAS method. The FAO calls for more data on the ileal digestibility of the protein in foods consumed by humans, determined in human and animal models, and more interspecies digestibility comparisons. This will enable identification of the preferred animal model for human digestion of proteins. A robust and validated DIAAS dataset for a wide range of foods will then be required before the method can be fully adopted.