True digestibility of protein

The microbial protein synthesised in the rumen may be protozoal or bac¬terial, the relative proportions depending upon conditions within that organ. Thus low rumen pH tends to reduce protozoal activity and stimu¬late that of certain bacteria. The mixture of bacterial and protozoal pro¬tein, along with dietary protein not degraded in the rumen, passes to the abomasum and small intestine. Here it is broken down to amino acids, which are then absorbed into the body. The digestibility of bacterial pro¬tein is lower (about 0.75) than that of the protozoal (about 0.90) and the overall digestibility of microbial protein will depend to some extent upon the rumen environment. However, protozoal protein constitutes some 5 to 15 per cent only of the total microbial protein flow from the rumen and its influence on the overall digestibility of microbial protein will be small. The composition of bacteria is variable. is an acceptable approximation.
About 15 per cent of the total nitrogen is in the form of nucleic acids, about 25 per cent is cell wall protein and the remainder is true protein. Available evidence indicates that the digestibility of nucleic acid nitrogen is of the order of 0.8-0.9, that of microbial true protein 0.85-0.9. It is commonly assumed that the protein associated with the cell walls is com¬pletely indigestible. Most estimates of the true digestibility of microbial protein are of the order of 0.85-0.87, which is higher than might be expected in view of the proportions of the fractions present in the pro¬tein. Although nucleic acids are highly digestible their nitrogen is of no use to the animal, as after absorption they are totally excreted in the urine.
The digestibility of the undegraded dietary protein is a characteristic of the protein mix in the food and may vary considerably from diet to diet. The true digestibility of the undegraded dietary protein will vary with the pro¬portion of the various protein fractions present. Thus amino acids, peptides, globulins, albumins and glutelins will be almost completely digested; pro¬lamins, proteins associated with the cell walls and denatured proteins will have digestibility values of about 0.8; the protein of Maillard products and nitrogen bound to lignin will be completely indigestible.
Digestibility has been shown to be inversely related to the content of acid detergent insoluble nitrogen (ADIN), which reflects that part of the food nitrogen which is closely bound to insoluble fibre. The digestible undegradable protein content (DUP) of a food is calculated thus:
DUP = 0.9 (undegradable protein - ADIN x 6.25)
This equation is based on the assumptions that ADIN is indigestible and that the digestible fraction has a true digestibility of 0.9.
In the case of foods such as maize gluten and some distillery and brewery by-products, which have been heat treated under moist conditions, Maillard-type reactions may occur, resulting in an increase the concentration of nitrogenous compounds insoluble in acid-detergent. Such 'acquired ADIN' does have a finite though low digestibility and the above equation is unreliable when used for such foods.