Methods of comparing amino acid composition of proteins: application to undigested proteins in the preruminant calf

The digestive utilization of dietary proteins at different sites along the digestive tract and the effects of their ingestion on the function of the digestive tract (secretion of endogenous proteins, development of microflora) can be inferred from the proportions of dietary, endogenous or microbial proteins. A first approach was to examine the proportions of some characteristic amino acids. A more accurate interpretation could be made by studying the additional undigested protein provided by the ingestion of an 'experimental' diet whose composition was calculated, for example, with regard to the undigested protein obtained with a very digestible diet (milk in the calf). These interpretations were often insufficient and did not extract all of the possible information. Thus, more global methods with statistical criterions had to be used. Some allowed two proteins (correlation and linear regression, average relative difference, chi 2) to be compared. Others made it possible to take many proteins into consideration (factorial correspondence analysis). Finally, examples were given to seek the respective proportions of many proteins in a complex mixture or to estimate the most probable amino acid composition of the 'microbial + endogenous' proteins which escaped digestion in the small intestine. These various approaches have been discussed, with examples taken from our experimental results obtained in the preruminant calf, to show their advantages and limitations. In any case, the quality of the interpretation depends on the quality of the available results on the amino acid composition of endogenous, microbial and undigested dietary proteins.