Measures of protein quality for monogastric animals

Digestible protein figures are not entirely satisfactory measures of the value of a protein to an animal, because the efficiency with which the absorbed protein is used differs considerably from one source to another. In order to allow for such differences, methods for evaluating proteins, such as the protein efficiency ratio (PER), the net protein retention (NPR) and the gross protein value (GPV), which are based on the growth response of experimental animals to the protein under consideration, have been devised. 

Liveweight gains may not be related to protein stored, and a more accurate evaluation of a protein may be obtained by using the results of nitrogen balance experiments. In such experiments, the nitrogen consumed in the food is measured, 

Balance trials are susceptible to several sources of error  

This is a direct measure of the proportion of the food protein that can be utilised by the animal for synthesising body tissues and compounds, and may be defined as the proportion of the absorbed nitrogen that is retained by the body. A balance trial is conducted in which nitrogen intake and urinary and faecal excretions of nitrogen are measured, along with the endogenous fractions in these two materials. The biological value is then calculated as follows  

Adapted from Mitchell H H 1942 Journal of Biological Chemistry 58 873. 

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