Plant proteins are found in high concentrations in foods such as legumes, nuts, oats and quinoa. Plant proteins are of interest in human nutrition because a large percentage of the World’s population derive their dietary protein intake mainly from cereals. Cereals are the main source of protein in developing countries, where they make up around 20 % of the daily protein intake. Cereals are problematic as a protein source because they are low in the essential amino acids lysine, and can also be low in threonine. The average Western diet contains around 100 mg·kg-1·d-1 of lysine, but research has shown that British vegetarian diets contain 44 mg·kg-1·d-1 and the diets of healthy Indian men contain between 39 and 53 mg·kg-1·d-1. Lysine requirement is estimated to be around 12 to 45 mg·kg-1·d-1 which suggests that lysine intake is insufficient in many parts of the World.
Wheat contains around 10 % protein by weight, and of these around 85 % are gluten proteins which can be classified as gliadins and glutenins. These proteins have a high content of praline and glutamine and are often referred to as prolamins because of this. However, the limiting factor in the use of wheat protein (as with all cereals) in the body is the lysine and threonine content, which is low. Traditional methods for measuring protein requirements are the biological value (BV) method and protein efficiency ratio (PER). More recently, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) has been used. Based on this method the current requirement is 15 to 58 mg of lysine per g protein. On this basis the current quality index for gluten and wheat protein is 26 and 40 %, respectively.
Research1 has looked at the nutritional value of wheat based plant protein to assess its digestibility and postprandial retention. Researchers fed subjects 136 grams of wheat in the form of toast, containing 24.6 grams of [15N]-labelled wheat protein. Samples of intestinal fluid, blood and urine were collected for the 8 hours postprandially. The results showed that the ileal digestibility of wheat nitrogen was 90.3 %, suggesting that 9.7 % of the wheat protein was not absorbed. Protein transferred to the deamination pool and lost through deamination was 24.7 % and this peaked at 8 hours. The urinary excretion of dietary nitrogen ([15N]) was therefore high. Researchers calculated the postprandial wheat protein retention as 66.1 %, based on 33.9 % losses (24.7 % from deamination and 9.7 % from ileal loses (not digested).
Postprandial utilisation of wheat based plant protein was measure at 66 % in this research. This is 89 % of the figure attained by milk protein (75 %) and higher than estimates based on the PDCAAS scores. Although lysine is the limiting amino acid in wheat protein, this can be compensated for in the diet by use of other plant protein surces that contain adequate lysine. Lysine is plentiful in many legumes and so combining sources of wheat protein with legumes provides the body with the recommended levels (15 to 58 mg per g protein) of lysine. While wheat protein has an ileal digestibility similar to other plant foods, as well as a lower postprandial biological value, its net potprandial protein utilisation is better than might be predicted from its lysine content using PDCAAS scores. This suggests that amino acid scores are problematic in assessing protein nutritional quality.
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