Efficiency of utilisation of absorbed amino acids

The mixture of amino acids of dietary origin absorbed from the small tine (i.e. the truly digested amino acids) is utilised for the synthesis of tissue protein. The efficiency of this process, which depends upon composition of the mix relative to that of the protein to be synthesised, is ¬best represented by its true biological value. This will in turn depend upon the biological values of the digested undegraded dietary protein and digested microbial protein, and upon the relative proportions of each contributing to the mix. In addition, it will vary with the primary function which it is required. Microbial protein is thought to have a relative constant biological value of about 0.8, whereas that of dietary origin variable and characteristic of the foods making up the diet. Prediction of such dietary values is extremely difficult, since the biological values of the individual proteins are no guide to their value in combinations.
An alternative approach is to estimate the supply of essential amino acids made available to the tissues (i.e. those absorbed from the small intestine) and to relate this to the amino acid requirements of the animal. This approach needs information on the truly digestible amino acid content of the undegraded dietary and the microbial protein.
The essential amino acid content of ruminal microbial protein is fre¬quently claimed to be relatively constant. In fact, large differences in the amino acid composition of samples of microbial protein have been shown to exist
There is evidence that the essential amino acid composition of undegrad¬able dietary protein may differ significantly from that of the original dietary material and it has been suggested that estimates of its contribution to the amino acid supply should be based on the amino acid profile of the insol¬uble fraction of the dietary protein rather than that of the whole dietary protein. A comparison of the essential amino acid profiles of whole and insoluble proteins in some common foods.